Olivier Billoint

Dimitri Galayko
email: dimitri.galayko@isen.fr

Bernard Legrand

Andreas Kaiser

Architecture-level center frequency drift compensation method for micro-mechanical filters used in IF stages of wireless telecommunication receivers

A prototype of a 433.92 MHz RF receiver using a micro-mechanical resonator as channel-selection filter in the 2 nd IF stage has been realized (94.5 kHz center frequency, 20 Hz bandwidth). A novel approach is used for managing the temperature drift of the center frequency of the micro-mechanical filter. Instead of stabilizing the filter's center frequency by complex device-level and technological modifications (bias voltage tuning, mechanical or thermal compensation), we modified the architecture of the IF stage in order to continuously adapt the IF frequency to the filter's center frequency deviations. This is achieved by periodical real-time measurements of the filter's center frequency and by then generating the appropriate second LO frequency. The measurement of the center frequency is achieved by putting the filter in an oscillating closed loop. The measured relative matching error between the 2 nd IF frequency and the filter's center frequency is better than 0.005%.

INTRODUCTION.

The point of our work is to demonstrate that a micro-electro-mechanical resonator can be used as an IF bandpass filter in a RF receiver. Compared to discrete SAW filters, MEM filters fabrication technology is compatible with electronic IC technologies. Use of MEMS components in wireless communication systems could allow to design RF receivers completely integrated on silicon. Nevertheless the MEMS components have electrical characteristics quite different from their electronic discrete counterparts, and their use in wireless architectures often requires more than a simple electrical interface management (impedance and signal level adaptation, biasing, …etc). For an optimal integration of MEMS components in RF systems, architecture modifications can be useful and sometimes necessary. A typical example of such a situation is the case of a micro-mechanical filter. The center frequency of such filters depends strongly on the temperature (variation of -4ppm per degree). This instability can not be accepted for an IF filter with narrow band and needs to be compensated in some way.

It is known that the resonance frequency of mechanical resonators can be tuned with the bias voltage applied to its mobile part [START_REF] Nguyen | Micromechanical resonators for oscillators and filters[END_REF] or by mechanically compensating the temperature [START_REF] Hsu | Mechanically Temperature-Compensated Flexural-Mode Micromechanical Resonators[END_REF] and [START_REF] Nguyen | Microresonator Frequency Control and Stabilization Using an Integrated Micro Oven[END_REF]. This feature can be employed for the temperature stabilization of the filter. Nevertheless this method is hard to implement in a RF architecture because these tuning voltages are usually quite high (20-50 V) and difficult to generate and control precisely in mobile devices. Moreover, the bias voltage determines input and output impedances of the filter [START_REF] Nguyen | Micromechanical resonators for oscillators and filters[END_REF]. Thus modifying the bias voltage for temperature stabilization would affect the impedance matching of the filter. An architecture-level solution is proposed : instead of tuning the resonator's center frequency, the local oscillator's frequency is constantly corrected in order to get the downconverted carrier frequency to be equal to the actual center frequency of the filter. This method allows the use of micro-electro-mechanical filters built in industrially available simple technologies. In this way we transfer the temperature instability managing task from MEMS device level to architecture level, where the mastering of the technologies is much better. Since the center frequency of the filter changes with the temperature, this correction must be done in real-time during the use of the receiver.

This article describes a receiver that uses a micro-electro-mechanical filter as a second-IF channel selection filter. The goal is to demonstrate the use of such a filter as an IF filter and all related architecture issues rather than designing a commercial-use receiver compatible with existing communication standards.

We have used a well-known comb-drive resonator which properties have been described in numerous papers [START_REF] Nguyen | Micromechanical resonators for oscillators and filters[END_REF]. Principal characteristics of the used resonator are high selectivity (2 to 30 Hz bandwidth depending on the vacuum level), center frequency of 94.510 KHz (at ambient temperature), typical values of bias voltage of 40-60 V and a frequency temperature drift of -4 ppm/°. Its general view and biasing scheme in filtering mode are presented in fig. 1. The filter has been fabricated in a thick epi-poly technology provided by ST Microelectronics. Fig. 1. General view of a micro-mechanical comb-driven filter [START_REF] Nguyen | Micromechanical resonators for oscillators and filters[END_REF].

The overall architecture of the realized receiver prototype is presented in the fig. 2. The receiver works in the 433.92 MHz frequency band (ISM). Since the center frequency of the micro-electro-mechanical filter used as channel selection filter is quite low, we have decided to build a triple conversion super-heterodyne receiver and to use the mechanical filter in the second-IF stage. The first intermediate frequency has been chosen equal to 10.7 MHz.

The whole receiver architecture is presented on the fig. 2. The RF and 1 st IF part is implemented with a standard heterodyne receive module that won't be described in this paper. The frequency of the second local oscillator is constantly corrected according to the measured value of the filter's center frequency. The signal issued from the output of the filter can be downconverted to the baseband if mixed with a 3 rd LO signal, that must also be variable and track the measured center frequency of the 2 nd IF filter. The 3 rd IF blocs are not included in the prototype, since they are not necessary to demonstrate the principles of the frequency shift correction.

CENTER FREQUENCY MEASUREMENT.

There are several approaches to measure the center frequency of a bandpass filter. The classical approach is to excite the filter with a pulse signal and to analyze the output impulse response. Since the tone corresponding to the central frequency of the filter will dominate the output response, a measurement of this frequency is possible. This approach is used for the measurement of the center frequency of a Gm-C band-pass filter [START_REF] Yamazaki | An Accurate Center frequency Tuning scheme for 450-kHz CMOS Gm-C Bandpass Filters[END_REF]. A step signal is applied to the input and the frequency of output transient oscillations is measured. This method is difficult to apply to a comb-driven filter, because of its high quality factor. As the bandwidth is narrow, using a step input signal doesn't produce enough output signal since there is far too less power from the input signal concentrated in the filter's bandwidth. Finally, the best way to measure the micro-mechanical center frequency in this case is to make it oscillate by putting it into a positive feedback loop with high gain and 0° phase shift. In this case free oscillations will be produced at the ringing frequency of the filter. Moreover, if the gain is high enough, the oscillation's settling time diminishes and the output signal's amplitude rises quickly to a detectable level. Then, by means of a comparator, it is possible to convert the oscillations in a TTL signal, which can be computed by a micro-controller in order to measure its frequency. ) using a 32 bits floating point library (taken from an application note) in order to increase computing accuracy.

The frequency measurement is based on two simultaneous-running processes. The micro-controller increments a clock cycles counter while a constant number (N, which value depends on the accuracy needed) of oscillations cycles are counted. Once N oscillations cycles have been counted, the process stops, M is assigned the number of counted clock cycles and the measured frequency is computed with the following formula : 4. shows the two simultaneous-running processes that are used to make the frequency measurement. A rising edge of the comparator's (oscillator's) output is actually detected on the next rising edge of the reference clock. The delay between the two edges is unknown, as it depends on the oscillator frequency and phase relation with respect to the reference clock. This gives rise to a measurement uncertainty equivalent to one reference clock period (see Fig. 5.). Thus, taking into account this uncertainty, the actual frequency is comprised in an interval bounded by The maximum error being one clock cycle, the resulting error on the measured frequency can be bounded by :

T M N f measured 1 * = Fig.
max min f f f - = Δ ( ) 1 1 * 1 1 1 * 1 * 1 1 * - - =       - - =       - -       ≤ Δ M M T N M M T N T M N T M N f
At the end, we obtain :

( )

1 1 * - ≤ Δ M M T N f
For example, if a frequency of 94.5 kHz is measured, we have got the following configuration : It is important to notice that this error doesn't take into account the calculation approximations made by the micro-controller during multiplications and divisions. In order to know the effective value of f Δ , the following test has been performed (see Fig. 6.).

MEMS Filter

Frequency measurement LO2 corre ction LO2 A 10.7 MHz frequency is generated by a HP 33120A generator in order to allow the measurement of the MEMS's center frequency (assuming that the whole system's temperature and pressure are steady).

Because of jitter, 2 nd LO frequency synthesis precision and uncertainty about the test-frequency generator, the effective f Δ measured is ±5 Hz maximum.

SWITCHING BETWEEN THE MEASUREMENT AND RECEIVING MODES.

The 2 nd IF stage has two operating modes: receiving and measurement. The measurement mode has been described in the preceding section. The receiving mode is standard like in classic IF stage, the only difference is that the frequency of the local oscillator is updated at the beginning of every receiving phase.

During the receiving phase, the phase corrector and the amplifier used to measure the ringing frequency of the filter must not stay connected to the resonator in order to prevent any perturbation on the received signal. Thus, the switching between measurement and receiving modes is made by use of bilateral switches that allow a digitally programmable architecture and that can be driven by the micro-controller. Fig. 7. shows the schematic of the complete 2 nd IF stage and Fig. 8. the different elements used in each operating mode. As the theoretical error on frequency measurements is ±1 Hz around the filter's center frequency (equal to 94.5 kHz in our case), the second oscillator's frequency must be generated with at least a precision of ±1 Hz around 10.6 MHz.

Precise frequency synthesis is usually achieved with a PLL which can generate variable frequencies by means of a multi-bit programmable divider. However, in our case, because of the accuracy needed in the frequency variations (at least 1 Hz step around a 10.6 MHz frequency, i.e a precision of 0.00001%) and due to the high number of bits needed to control the output frequency, a PLL architecture is not a good solution. We chose to generate the second oscillator's frequency with a digital component from Analog Device called DDS (Direct Digital Synthesis) that allows a step of 11 mHz for 0 to 15 MHz frequencies and which can be controlled with 3 pins of the micro-controller (serial data, clock and frequency update).

MICRO-ELECTRO-MECHANICAL FILTER INTERFACE.

The mechanical filter bloc is realized on a printed circuit board separate from the main receiver board since for experimental operation the filter has to be placed in a vacuum chamber. The schematics of the filter board is presented in the fig. 9.

A capacitive coupling between the input and output electrodes of micro-mechanical filter reduces the attenuation at out-of-band frequencies [START_REF] Rantakari | Reducing the Effect of Parasitic Capacitance on MEMS measurements[END_REF]. In order to maximize the out-of-band attenuation, we have used the method described in [START_REF] Rantakari | Reducing the Effect of Parasitic Capacitance on MEMS measurements[END_REF]. The parasitic coupling current present in both resonators is canceled, only the motional signal of the biased resonator is present at the output.

A heating element is placed near the biased resonator to adjust approximately the operating temperature of the system during the test process.

EXPERIMENTAL RESULTS.

To test the receiver, the filter has been placed in the vacuum chamber with 3 Torr air pressure, so the -3db bandwidth was 20 Hz. To test the receiver, we have used an amplitude-modulated signal with 5 Hz modulation frequency and 50% depth. The receiver test has shown a 0.005% matching precision between the 2 nd LO frequency and the filter's center frequency. To test the reliability of the IF stage operation, we have heated the filter in the way that his center frequency was shifted by 400Hz (4% of the central frequency). To better observe the 2 nd IF correction, we have set the time interval between measurements (and so between 2 nd IF corrections) equal to 8 s, in order to have a big frequency deviation between two measurement (10-30 Hz depending of the heating rate). The amplitude of the AM signal at the filter output decreased considerably between two measurements, since the filter center frequency was changing and the signal became out of band. After each measurement the amplitude jumped back to the value corresponding to the filter transmission at the resonance frequency. The measurement phase time is about 143 ms. Although it seems to be high, we should take into consideration that in real RF systems the center frequency of the micro-electro-mechanical filter would be much higher (aimed values are about several hundred megahertz), so the measurement time will be considerably decreased.

CONCLUSION.

The achieved receiver prototype demonstrates a way of use of micro-electro-mechanical filters in an IF stage and a possible solution for such a fundamental problem of micro-electro-mechanical filters as the temperature frequency drift. The prototype filter has a center frequency at 94.5 kHz, but the demonstrated principle can be applied to filters operating at much higher frequencies.

The case of a micro-electro-mechanical resonator has shown that sometimes architectural-level solutions are more efficient and easier to implement than technology level or than modifications on the device.

REFERENCES.

Fig. 2 .

 2 Fig.2. Overall receiver architecture.

Fig. 3 .

 3 Fig. 3. Operation of the 2 nd IF stage in the measurement phase. The micro-controller used is a PIC16F74 from Microchip, operating at 20 MHz (which means a clock frequency of 5 MHz, i.e s T 7 10 . 2 - =

Fig. 4 .

 4 Fig. 4. Principle of measurement.

Fig. 5 .

 5 Fig. 5. Illustration of the measurement uncertainty.

Fig

  Fig. 6. Measurement protocole.

Fig. 7 .

 7 Fig. 7. Complete schematics of the 2 nd IF stage.

Fig. 8 . 4 .

 84 Fig. 8. Measuring and filtering phase detailed diagram.

Fig. 9 .

 9 Fig. 9. Compensation of parasitic input-output coupling in the micro-mechanical filter.

  Fig. 10. General overview of the complete RF receiver.


C Desbiez

The never-ending story of cucurbits and viruses

Keywords: epidemiology, viral disease, emergence, transmission, evolution, control, emergence

Cucurbits are widely grown worldwide in diverse agroecosystems. Viruses represent one of the major threats for cucurbit production. More than 90 viruses have been described infecting cucurbits in natural conditions. Their prevalence and agronomic impact are highly contrasted, and the major problems at the worldwide level are related to circa 10 virus species, both « classic »i.e known for decades-or recently emerged on a large scale. Among the « old » mosaic-inducing viruses, aphid-transmitted viruses (potyviruses, cucumber mosaic virus) remain agronomically important, particularly in temperate climates. Despite their long-term prevalence and apparent stability, introductions of new strains have been observed in several countries in the last years, probably via commercial exchanges of plant material. Some of these introduced strains have rapidly replaced pre-existing virus populations, sometimes resulting in an increased economic impact. In Mediterranean and tropical climates, as well as in heated greenhouses in temperate regions, whitefly-transmitted viruses (begomoviruses, ipomoviruses, criniviruses) now constitute major problems. Their rapid emergence can be explained by the combination of changes in climate and cultural practices that favour their natural vectors, and the increased exchanges of plant material that made possible their long-distance spread. Thrips-transmitted viruses are present in Asia and Latin America and can be locally very damaging. Viruses transmitted by seeds or by contact are also easily disseminated worldwide and represent important constraints for exchanges of plants or genetic material. Accurate and regular reevaluation of the complex viral pathosystem infecting cucurbits in different countries, adaptation of cultural practices and development of resistant cultivars contribute to the continuous arm race against the introductions and local evolution of virus populations.

INTRODUCTION

Cucurbits are among the most important vegetable crops worldwide. Among the circa 800 species described, the major crops are melon (Cucumis melo), cucumber (C. sativus), watermelon (Citrullus lanatus) and different squashes (Cucurbita pepo, C. moschata, C. maxima). Other cucurbits (bitter gourd, chayote) are also grown in some parts of the world (http://www.fao.org/faostat/). Mature or immature cucurbit fruits are used for human or animal consumption. Seeds are used as snack or for oil production. Some cucurbits are grown for ornamental use (colocynths), as sponges (Luffa acutangula) or as containers or music instruments [START_REF] Lecoq | Cucurbit viruses: the classics and the emerging[END_REF][START_REF] Romay | Cucurbit crops and their viral diseases in Latin America and the Caribbean Islands: a review[END_REF].

Whereas many wild cucurbits are perennial, cultivated ones are mostly annual. A great variety of types and cultivars are grown in relation to local conditions and consumer's tastes. Cucurbits are grown in very diverse agroecosystems, from traditional rain fed or irrigated gardens to sophisticated soilless glasshouses.

Cucurbits production is threatened by several pests and diseases. Insects or nematodes induce direct damages to the crops, and can also transmit viruses. Viruses represent challenging problems as they can impair both the yield and the quality of the crops, and no curative methods are available. Viruses rarely cause plant death, but they can induce different symptoms belonging mostly to three categories:

-mosaics, i.e. patterns of discoloration on the leaves or fruits, often associated with deformations and size reduction. Fruit quality can be altered with low sugar content, and sometimes marbling and hardening of the flesh -necrosis affecting some organs (necrotic spots on leaves or fruits) or the whole plant, sometimes leading to general wilt and death -yellowing of the older leaves that also become brittle. Fruit production can be reduced but the quality of the fruits is not obviously affected.

Mixed infection of several viruses in a single plant are common and can result in more severe symptoms.

Cucurbit-infecting viruses represent a highly dynamic system. Several novel virus species, and/or viruses that were present on other hosts but are now found to infect cucurbits, are described each year, as well as new locations or new hosts for known viruses. Many of these viruses have a potential or acknowledged agronomic impact. Until recently, descriptions of new viruses included a characterization of their symptomatology, host range, and transmission properties. With the development of high-throughput sequencing, some viruses are known only by their molecular properties without any information on their importance in plants. Completely asymptomatic viruses that could be commensal or even beneficial to plants are thus described, but the risks of their evolving to become deleterious are unknown.

APHID-TRANSMITTED VIRUSES: OLD VIRUSES, NEW STRAINS, PERSISTING PROBLEMS

Aphids constitute important agronomic problems in themselves. They induce direct damages to crops by their sap-sucking feeding: plant weakening, leaf crispation in response to saliva toxicity and photosynthesis reduction. Their feeding behaviour makes them particularly efficient virus vectors. For some virus genera, virus acquisition and transmission take place during short probing punctures of the aphid stylet in the epidermal cells of the plants (non-persistent transmission). In that case, the transmission is not very specific, many aphid species can transmit a given virus. The virus remains in the stylet of the aphid, its retention lasts only minutes to hours and the aphid remains viruliferous for less than one day. Other virus species are persistently transmitted: the aphid acquires the virus after several hours of feeding in the phloem of an infected plant. The virus transits through the insect gut and the haemolymph, then migrates to the salivary glands where it can be released to a new plant when the aphid feeds on the phloem of another healthy susceptible plant. In that case, the transmission is more specific, only one or a few aphid species can transmit a given virus. The aphid can remain viruliferous all its life [START_REF] Ng | Transmission of plant viruses by aphid vectors[END_REF].

In cucurbits, viruses from the genera Cucumovirus and Potyvirus are non-persistently aphidtransmitted, whereas viruses from the genus Polerovirus are persistently-transmitted.

Cucumber mosaic virus (CMV)

Cucumber mosaic virus (CMV, genus Cucumovirus) has small icosahedral particles of circa 29 nm in diameter. CMV is among the first plant viruses described, more than one century ago. Its genome consists of 3 positive-stranded RNAs, encapsidated separately. CMV has one of the broadest host ranges for a plant virus, it infects more than 1000 species, both dicots and monocots. The symptoms on cucurbits usually consist in mosaics and deformations on leaves and fruits. CMV is efficiently transmitted non-persistently by more than 60 aphid species. Interestingly, CMVinfected plants attract aphids by the emission of volatiles, but aphids perform poorly on infected plants and tend to emigrate quickly. This manipulation of aphid behaviour can increase virus spread [START_REF] Mauck | Deceptive chemical signals induced by a plant virus attract insect vectors to inferior hosts[END_REF]. CMV is seed-transmitted in some hosts, but seed transmission in cucurbits seems to be rare [START_REF] Jacquemond | Cucumber mosaic virus[END_REF][START_REF] Lecoq | Cucurbit viruses: the classics and the emerging[END_REF].

Two major subgroups of CMV, namely groups I and II, have been defined based on biological, serological and molecular properties. Strains from group I were also called heat-resistant, contrary to the heat-sensitive group II [START_REF] Jacquemond | Cucumber mosaic virus[END_REF]. Within group I, two major subgroups are acknowledged, namely IA and IB. Group IA is widespread throughout the world; group IB was mostly present in Asia until recently. Reassortants, i.e. virus isolates whose RNAs belong to different groups or subgroups, and recombinants -presenting genetic exchanges within one RNAhave been observed [START_REF] Jacquemond | Cucumber mosaic virus[END_REF][START_REF] Ohshima | Temporal analysis of reassortment and molecular evolution of Cucumber mosaic virus: Extra clues from its segmented genome[END_REF]. Reassortants can present fitness advantages in some hosts and become predominant in local populations [START_REF] Ben Tamarzizt | Cucumber mosaic virus populations in Tunisia peppers are mainly composed of virus reassortants with resistance-breaking properties[END_REF]. Evolutionary analyses suggested that current CMV populations are the descendants of a single ancestor existing 1600-2600 years ago but that the population mostly radiated 300-550 years ago, and that reassortant populations were locally confined and less than 250 years old [START_REF] Ohshima | Temporal analysis of reassortment and molecular evolution of Cucumber mosaic virus: Extra clues from its segmented genome[END_REF].

Potyviruses

At least 20 potyviruses infect cucurbits (Table 1). Three of them, zucchini yellow mosaic virus, papaya ringspot virus and watermelon mosaic virus, are particularly important worldwide.

Potyviruses have a single-stranded monopartite genome of about 10 kb, encapsidated in filamentous flexuous particles about 700 nm long. Viruses in the family Potyviridae, including potyviruses, induce in their infected hosts typical pinwheel-like cytoplasmic inclusions. Potyviruses are non-persistently transmitted by numerous aphid species. They can also be transmitted mechanically by pruning tools or by contact between plants.

Zucchini yellow mosaic virus (ZYMV)

Zucchini yellow mosaic virus still represents the challenging case of a very successful emerging virus. ZYMV was first observed in a garden in Italy in 1973, and the first severe epidemics took place in France in 1978 [START_REF] Desbiez | Zucchini yellow mosaic virus[END_REF]. Within a few years, ZYMV caused severe epidemics in almost all cucurbit-growing areas worldwide [START_REF] Desbiez | Zucchini yellow mosaic virus[END_REF]. ZYMV symptoms are usually very severe, and early infections can lead to complete crop losses. ZYMV presents a high biological variability in symptomatology, transmission and ability to overcome resistances. ZYMV has a narrow host range outside cucurbits. In areas where cucurbits are not grown year-round, ZYMV can be maintained in wild annual or perennial cucurbits. The few wild cucurbits present in Europe are not susceptible to ZYMV, and the sources of primary inoculum in epidemics remain a key question. Non-cucurbit hosts including weeds and ornamentals can constitute virus reservoirs (Lecoq et al., 2014) but their prevalence in the vicinity of the crops can be low. In temperate areas, ZYMV epidemics are often irregular, probably in relation to a low probability of primary infections [START_REF] Lecoq | Molecular epidemiology of Zucchini yellow mosaic virus in France: an historical overview[END_REF]Lecoq et al., 2014). Studies on ZYMV molecular diversity have revealed three molecular groups, namely A, B, and C without host specificity. Groups B and C are restricted geographically whereas group A contains worldwide isolates including those of the first descriptions in France and Italy. Within group A, 4 to 6 subgroups were defined [START_REF] Lecoq | Viruses of cucurbit crops in the Mediterranean region: an ever-changing picture[END_REF]. Molecular analyses have allowed in some cases to track the origins of ZYMV strains emerging in new areas. Even in a single location, strain replacement or "switches" have been described (Lecoq et al., 2014), suggesting that distant sources of infection, besides local reservoirs, play an important role in ZYMV epidemics. Natural spread by aphids takes place mostly at a very local scale (Lecoq et al., 2014). Long-distance virus dissemination can happen through infected fruits or plants [START_REF] Lecoq | Potential involvement of melon fruit in the long distance dissemination of cucurbit potyviruses[END_REF]. Seed transmission of ZYMV has long remained controversial, with estimations ranging from 0 to 18%. Using a sensitive RT-PCR detection method, vertical transmission was observed in Cucurbita pepo subsp. texana at a 1.6% rate [START_REF] Simmons | Experimental Verification of Seed Transmission of Zucchini yellow mosaic virus[END_REF]. Positive plants were symptomless and had low virus titre, but they could be sources for further horizontal transmission, either by aphids or mechanically [START_REF] Simmons | Zucchini yellow mosaic virus (ZYMV, Potyvirus): Vertical transmission, seed infection and cryptic infections[END_REF]. Seed transmission may thus have contributed to the rapid worldwide spread of ZYMV, and to the still ongoing frequent exchanges between viral populations.

Watermelon mosaic virus

Watermelon mosaic virus (WMV) is present worldwide in temperate and Mediterranean areas but is almost absent on cucurbits in tropical areas, without definitive explanation. WMV infects a wide range of cultivated and ornamental hosts as diverse as pea, orchids and ginseng, as well as numerous weeds that can constitute reservoirs. In cucurbits, WMV induces vein banding and mosaic symptoms milder than those of ZYMV, even if some strains cause severe leaf deformations. "Watermelon mosaic disease" has been observed for more than 50 years. Biological and serological studies revealed the presence of two viral entities named WMV-1 and WMV-2. WMV-1 was shown to be a strain of papaya ringspot virus (PRSV, see below) and WMV-2 was renamed WMV. WMV is closely related molecularly to soybean mosaic virus (SMV) but is an interspecific recombinant in the 5' region of the genome between SMV and another legume-infecting potyvirus, bean common mosaic virus [START_REF] Desbiez | The nucleotide sequence of Watermelon mosaic virus (WMV, Potyvirus) reveals interspecific recombination between two related potyviruses in the 5' part of the genome[END_REF]. Despite its low biological variability, WMV molecular diversity is high. The first studies based on coat proteincoding regions revealed the presence of three molecular groups showing some correlation with the geographic origin of the isolates: In France and other European countries, Group1 strains present for more than 40 years have been replaced within a decade by Group3 ones probably introduced in the mid-1990s [START_REF] Desbiez | Emergence of new strains of Watermelon mosaic virus in South-Eastern France: evidence for limited spread but rapid local population shift[END_REF]. During the few years of coexistence of the two types of strains, mixed infections were frequent and intraspecific recombinants were detected but they did not emerge in the populations (Desbiez et al., 2011a). The increase of the number of sequences available from worldwide isolates now shows a very complex situation with evidence for numerous long-distance exchanges. WMV has not been described as seed-borne in cucurbits. Long-distance virus spread could be the result of worldwide trade of vegetatively propagated hosts, including orchids or other ornamentals, and/or of cucurbit seedlings, plants or fruits.

Papaya ringspot virus (PRSV) and the « PRSV cluster »

Papaya ringspot virus (PRSV, formerly WMV-1) induces mosaics and deformations in cucurbit leaves and fruits. After it was considered as a distinct virus from WMV (formerly WMV-2) it was shown to belong to the same species as a virus previously described on papaya. P (papaya) strains of PRSV infect papaya but have a low infectivity on cucurbits, contrary to W (watermelon) strains that efficiently infect cucurbits but do not infect papaya. Phylogenetic analyses suggest that P isolates derive from W isolates, and that the switch took place frequently during the evolution of PRSV. Indeed, a single mutation in the major protease of the virus (NIa-Pro) is sufficient for the phenotypic change between the P and W phenotypes [START_REF] Chen | A single amino acid of NIaPro of Papaya ringspot virus determines host specificity for infection of papaya[END_REF]. Besides PRSV, several viruses closely related biologically, serologically and molecularly constitute a "cluster" of cucurbit-infecting viruses: Moroccan watermelon mosaic virus, Algerian watermelon mosaic virus, zucchini shoestring virus, zucchini tigre mosaic virus, Sudan watermelon mosaic virus, wild melon vein-banding virus, zucchini yellow fleck virus. They are geographically restricted to a few countries, mostly in Africa and Middle East, but MWMV is now emerging in the Mediterranean Basin [START_REF] Desbiez | New species in the papaya ringspot virus cluster: Insights into the evolution of the PRSV lineage[END_REF] Other potyviruses infect cucurbits. They are not particularly related serologically or molecularly between each other and with ZYMV, WMV or the PRSV cluster (Table 1). Clover yellow vein virus and turnip mosaic have a broad host range and are present worldwide but are only minor viruses on cucurbits. The other viruses are more cucurbit-restricted and have a limited geographic distribution even if they can be locally important [START_REF] Lecoq | Cucurbit viruses: the classics and the emerging[END_REF][START_REF] Perotto | Identification and characterization of a new potyvirus infecting cucurbits[END_REF].

Poleroviruses

Poleroviruses are persistently transmitted by aphids. They are phloem-restricted and are not mechanically transmissible. In cucurbits, poleroviruses usually induce yellowing symptoms on the older leaves that become thickened and brittle. They affect the yield of infected plants but have usually no impact on fruit quality. Poleroviruses have single-stranded positive-stranded RNA genomes of 5.6 kb, encapsidated in icosahedral particles of circa 25 nm diameter. The first cucurbit polerovirus described in 1992 in France, cucurbit aphid-borne yellows virus (CABYV), was present there at least 10 years before but had long been unnoticed [START_REF] Lecoq | A new yellowing disease of cucurbits caused by a luteovirus, cucurbit aphid-borne yellows virus[END_REF]. Besides cucurbits, CABYV infects many weeds that are usually symptomless. CABYV is present worldwide except in some islands. Two main molecular groups have been observed, one containing Asian isolates, the other isolates from America, Africa and Europe [START_REF] Kassem | Genetic Diversity and Potential Vectors and Reservoirs of Cucurbit aphid-borne yellows virus in Southeastern Spain[END_REF].

Besides CABYV, at least five poleroviruses inducing similar symptoms have later been described on cucurbits [START_REF] Knierim | Molecular diversity of poleroviruses infecting cucurbit crops in four countries reveals the presence of members of six distinct species[END_REF] (Table 1). Recombinants between CABYV and melon aphid-borne yellows virus have been observed [START_REF] Knierim | Molecular identification of three distinct Polerovirus species and a recombinant Cucurbit aphid-borne yellows virus strain infecting cucurbit crops in Taiwan[END_REF]. Except pepo aphid-borne yellows virus, all these viruses are present so far in Eastern Asia only (Table 1). No seed transmission was observed for cucurbit-infecting poleroviruses.

WHITEFLY-TRANSMITTED VIRUSES: EMERGING VECTORS, EMERGING VIRUSES

Whereas aphid-transmitted viruses remain important in temperate countries, whiteflytransmitted ones have emerged worldwide in the last decades. The greenhouse whitefly Trialeurodes vaporariorum and the tobacco whitefly Bemisia tabaci are the most important vectors. B. tabaci is now considered as a species complex including over 40 species [START_REF] Elfekih | Standardized molecular diagnostic tool for the identification of cryptic species within the Bemisia tabaci complex[END_REF]. The MEAM1 and MED biotypes (formerly B and Q, respectively) have emerged throughout the world thanks to their high fitness and frequent resistance to insecticides. The most important viruses transmitted by whiteflies on cucurbits belong to the genera Begomovirus, Ipomovirus and Crinivirus.

Begomoviruses

Begomoviruses are single-stranded DNA viruses. Their genome consists of one or two circular DNAs (DNA-A and DNA-B) of 2.6 kb each, encapsidated in particles presenting a very typical double-icosahedral shape, giving its name to the Geminiviridae family (gemini = twin). Begomoviruses are persistently transmitted by whiteflies from the Bemisia tabaci complex. Virus acquisition takes place within several hours of insect feeding, then the insects remain viruliferous for their whole life. There is generally no replication in the vector nor transovarian transmission to the offspring. Some begomoviruses are also transmitted by other whiteflies including T. vaporariorum [START_REF] Sangeetha | A distinct seed-transmissible strain of tomato leaf curl New Delhi virus infecting Chayote in India[END_REF].

Begomoviruses belong to two molecular clades, « Old World » (OW) and « New World » (NW). The OW clade is considered as basal in begomovirus evolution. It contains both monopartite and bipartite viruses, whereas NW viruses are bipartite [START_REF] Nawaz-Ul-Rehman | Evolution of geminiviruses and their satellites[END_REF]. Cucurbit-infecting begomoviruses are found both in the OW and NW clades and are mostly bipartite [START_REF] Romay | Cucurbit crops and their viral diseases in Latin America and the Caribbean Islands: a review[END_REF] (Table 1). Begomoviruses can be associated with satellite molecules that are ssDNA molecules of circa 1.3 kb, dependent on their helper virus to complete their cycle. They do not encode any coat protein and are encapsidated in the capsid of their helper virus. Betasatellites usually increase the severity of viral infections and contribute to overcome resistance genes [START_REF] Zhou | Advances in Understanding Begomovirus Satellites[END_REF]. Alphasatellites, mostly associated with betasatellites, usually have no obvious effect on their helper virus [START_REF] Zhou | Advances in Understanding Begomovirus Satellites[END_REF], but they can also contribute to reduce betasatellite accumulation and symptom severity [START_REF] Idris | An unusual alphasatellite associated with monopartite begomoviruses attenuates symptoms and reduces betasatellite accumulation[END_REF]. Satellites were thought to be associated with monopartite OW begomoviruses only, but atypical alphasatellites were found with some bipartite NW ones including melon chlorotic mosaic virus (MeCMV) [START_REF] Romay | Association of an atypical alphasatellite with a bipartite New World begomovirus[END_REF]. Beta-and alphasatellites are also associated with some OW bipartite begomoviruses, particularly tomato leaf curl New Delhi virus (ToLCNDV, see below)

Before 1998, only two begomoviruses were described on cucurbits: the OW watermelon chlorotic stunt virus (WmCSV) in the Red Sea region, and the NW squash leaf curl virus (SLCuV) in Central and Northern America [START_REF] Lecoq | Viruses of cucurbit crops in the Mediterranean region: an ever-changing picture[END_REF], Since then, the emergence of B. tabaci MEAM1 and MED has prompted that of the viruses they transmit. Better knowledge of virus variability has also led to the characterization of new viruses and the distinction of members in species complexes, as for SLCuV and squash mild leaf curl virus that constitute different species. Asian squash-infecting begomoviruses are distinct from SLCuV and SqMLCV and constitute at least four species (Table 1) (https://talk.ictvonline.org/taxonomy/). Nowadays at least 18 begomovirus species are described on cucurbits, with contrasted geographic range and agronomic impact (Table 1). Limited long-distance exchanges have taken place: the NW virus SqLCV was introduced in the 2000s in Israel, Egypt and Jordan [START_REF] Lecoq | Viruses of cucurbit crops in the Mediterranean region: an ever-changing picture[END_REF]. The OW virus ToLCNDV was first described in India but is currently spreading in the Mediterranean Basin and Asia. ToLCNDV isolates from the Mediterrenean Basin are very homogeneous molecularly, suggesting a unique dissemination event [START_REF] Zaidi | Tomato leaf curl New Delhi virus: a widespread bipartite begomovirus in the territory of monopartite begomoviruses[END_REF][START_REF] Juarez | Natural Hosts and Genetic Diversity of the Emerging Tomato Leaf Curl New Delhi Virus in Spain[END_REF]. The host range of ToLCNDV includes tomato as a major host, but also 43 other species including cucurbits [START_REF] Zaidi | Tomato leaf curl New Delhi virus: a widespread bipartite begomovirus in the territory of monopartite begomoviruses[END_REF]. The emerging Mediterranean Basin isolates are highly adapted to cucurbits but do not perform well on tomato even if whitefly transmission between tomato and cucurbits is possible [START_REF] Fortes | A Novel Strain of Tomato Leaf Curl New Delhi Virus Has Spread to the Mediterranean Basin[END_REF]. In its country of origin, ToLCNDV is frequently associated with satellites with a low specificity (it can replicate several different satellites). DNA-A of ToLCNDV can also trans-replicate DNA-B of other begomoviruses, what may contribute to its great adaptability and epidemiological success. No satellite was associated with the emerging Mediterranean Basin isolates so far. ToLCNDV has not yet emerged in America and Eastern Asia.

Ipomoviruses

Ipomoviruses, like potyviruses, are members of the family Potyviridae. Contrary to potyviruses they are not aphid-borne but are transmitted by whiteflies in a semi-persistent way: acquisition and transmission happen in a few hours. The viruses do not multiply in the whiteflies and are not transmitted to the offspring. Three ipomoviruses infect cucurbits: cucumber vein yellowing virus (CVYV), squash vein yellowing virus (SqVYV) and coccinia mottle virus (CocMoV). They share a similar genomic organization, differing from that of potyviruses in the 5' part of the genome [START_REF] Dombrovsky | Ipomovirus -an atypical genus in the family Potyviridae transmitted by whiteflies[END_REF]. Ipomoviruses induce vein yellowing and mosaic symptoms on cucurbit leaves. Synergism in co-infection with other viruses can result in enhanced symptoms and important losses [START_REF] Gil-Salas | Co-infection with Cucumber vein yellowing virus and Cucurbit yellow stunting disorder virus leading to synergism in cucumber[END_REF]. CVYV was first described in Israel in 1960, and has since been observed throughout the Middle East and Mediterranean Basin. It is now emerging in southern Europe in relation to high populations of B. tabaci MEAM1 and MED. Phylogenetic analyses suggested that CVYV originated in the Middle East [START_REF] Velasco | Genetic variation and evolutionary forces shaping Cucumber vein yellowing virus populations: risk of emergence of virulent isolates in Europe[END_REF], but highly divergent and variable CVYV isolates were also observed in Sudan, indicating long-term presence of the virus in Sub-Saharan Africa. The strains now emerging in the Mediterranean Basin are more closely related to the Middle East isolates than to the Sudanese ones [START_REF] Desbiez | Evidence for long-term prevalence of cucumber vein yellowing virus in Sudan and genetic variation of the virus in Sudan and the Mediterranean Basin[END_REF]. CocMoV, has been described in Sudan on the wild perennial cucurbit Coccinia grandis. Although it readily infects cultivated cucurbits in laboratory conditions, it has never been observed on crops. This may be related to a low transmissibility by the local vectors or absence of crops in the vicinity of the infected coccinia. It is an example of a minor virus that might emerge if it is introduced in a favourable environment [START_REF] Desbiez | Characterization of a new cucurbit-infecting ipomovirus from Sudan[END_REF]. SqVYV was first described in Florida [START_REF] Adkins | Identification and characterization of a novel whitefly-transmitted member of the family Potyviridae isolated from cucurbits in Florida[END_REF] and later in Puerto Rico and California. Its host range is mostly limited to cucurbits, the most severe symptoms being observed in squash and in watermelon, where it induces vine decline and fruit rot. It was found in 2014 in Israel [START_REF] Reingold | Watermelon Fruit Rot Disease in Israel is Caused by a Distinct Squash vein yellowing virus (SqVYV) Strain[END_REF] and in Iran (Abkhoo, direct GenBank submission). The high molecular divergence between isolates from America, Israel and Iran suggest that the presence of the virus in these areas is not related to direct and recent genetic exchanges.

Criniviruses

Criniviruses are single-stranded bipartite RNA viruses that have thin filamentous flexuous particles. Four criniviruses infect cucurbits: beet pseudo-yellows virus (BPYV), lettuce infectious yellows virus (LIYV), cucurbit yellow stunting disorder virus (CYSDV) and cucurbit chlorotic yellows virus. (CCYV) The symptoms they induce on cucurbits are indistinguishable visually, and very similar to those of poleroviruses: interveinal yellowing of older leaves, sometimes spreading to almost all the plant for the most susceptible cultivars.

BPYV, the first whitefly-transmitted crinivirus described, has a wide host range including ornamentals, crops and weeds. Originally found in California, it is now present worldwide, probably in relation to frequent exchanges of susceptible hosts including vegetatively propagated ones. In temperate areas, BPYV is often found in greenhouses where the conditions are favourable for its vector T. vaporariorum.

LIYV was found in 1981 in the desert regions of California and Arizona [START_REF] Wisler | Ecology and epidemiology of whitefly-transmitted closteroviruses[END_REF] where it caused important damages on cucurbits but also on lettuce and sugar beet. LIYV is well transmitted by the A biotype of B. tabaci but is very inefficiently transmitted by the MED and MEAM1 biotypes. When MEAM1 replaced the A biotype in the USA in the 1980s, LIYV completely disappeared but it was later replaced by viruses efficiently transmitted by MEAM1, particularly CYSDV [START_REF] Wintermantel | Emergence and epidemiology of Cucurbit yellow stunting disorder virus in the American Desert Southwest, and development of host plant resistance in melon[END_REF].

CYSDV and CCYV are transmitted by the same vector B. tabaci MEAM1 and MED and their dispersion patterns show some similarities. CYSDV was first described in the United Arab Emirates in 1982. It has spread to the whole Middle East and Mediterranean Basin in the 1980s-1990s, and is also present in Sub-Saharan Africa [START_REF] Mohammed | Identification and phylogenetic analysis of common pumpkin viruses in Sudan[END_REF]. Its host range among crops is mostly restricted to cucurbits, but it infects numerous weeds that can constitute reservoirs [START_REF] Orfanidou | Criniviruses associated with cucurbit yellows disease in Greece and Cyprus: an ever-changing scene[END_REF]. It was detected in North America in 2000 and has now become an agronomic problem in that region, in relation to very high vector populations [START_REF] Wintermantel | Emergence and epidemiology of Cucurbit yellow stunting disorder virus in the American Desert Southwest, and development of host plant resistance in melon[END_REF]. CCYV is also present in the Middle East and Sub-Saharan Africa and is now spreading in the Mediterranean Basin [START_REF] Orfanidou | Criniviruses associated with cucurbit yellows disease in Greece and Cyprus: an ever-changing scene[END_REF] and North America [START_REF] Wintermantel | First Report of Cucurbit Chlorotic Yellows Virus Infecting Melon in the New World[END_REF], but contrary to CYSDV it was first described in Japan [START_REF] Okuda | Host Range and Complete Genome Sequence of Cucurbit chlorotic yellows virus, a New Member of the Genus[END_REF] and is present in China and Taiwan.

Molecular diversity within each crinivirus species is very limited, and makes it difficult to estimate genetic exchanges between distant areas [START_REF] Orfanidou | Criniviruses associated with cucurbit yellows disease in Greece and Cyprus: an ever-changing scene[END_REF]. For CYSDV, two clades have been described, "Eastern" (Arabia, Sudan, Iran) vs. "Western" (Mediterranean Basin). For CCYV, Asian and Mediterranean isolates were very similar, suggesting recent exchanges [START_REF] Orfanidou | Criniviruses associated with cucurbit yellows disease in Greece and Cyprus: an ever-changing scene[END_REF].

THRIPS-TRANSMITTED VIRUSES: GEOGRAPHICALLY RESTRICTED BUT LOCALLY IMPORTANT

Six viruses in the genus Tospovirus, order bunyavirale, infect cucurbits. Tospoviruses are transmitted by thrips in a persistent multiplicative way. The viruses are acquired by larvae and transmitted by the adults, and they multiply in their vector. Indeed, many members of the bunyavirale are insect-infecting viruses and tospoviruses supposedly evolved from an insectinfecting ancestor [START_REF] Marklewitz | Evolutionary and phenotypic analysis of live virus isolates suggests arthropod origin of a pathogenic RNA virus family[END_REF]. Tospoviruses have a tripartite RNA genome, encapsidated in quasi-spherical enveloped particles 75 to 100 nm in diameter. Their symptoms on cucurbits are frequently very severe, including mosaics and dwarfing but also necrosis that can lead to plant death. The most common tospovirus wordwide is tomato spotted wilt virus (TSWV). It is a major pathogen on tomato and a wide range of crops and ornamentals, but on cucurbits it has only been observed occasionally in several regions of the world [START_REF] Romay | Cucurbit crops and their viral diseases in Latin America and the Caribbean Islands: a review[END_REF]. The other cucurbit-infecting tospoviruses are geographically restricted and present a much narrower host range than TSWV but they can be locally important. Watermelon bud necrosis virus, watermelon silver mottle virus, and melon yellow spot virus (MYSV) are present in different countries from Asia (India, China, Thailand, Japan) whereas melon severe mosaic virus and zucchini lethal chlorosis virus were found in Latin and South America (Mexico, Brazil) (Table 1). No tospovirus has been observed inducing important epidemics so far in Europe, Africa and North America. However, the Asian MYSV was recently observed in Ecuador [START_REF] Quito-Avila | Detection and occurrence of melon yellow spot virus in Ecuador: an emerging threat to cucurbit production in the region[END_REF], indicating that the geographic structure of cucurbit tospoviruses is changing and long-distance spread can take place, probably through the trade of susceptible hosts or the accidental spread of viruliferous thrips. Since these viruses are very damaging and difficult to control, particular care should be taken to avoid their accidental long-distance dissemination.

CONTACT, -SEED-AND SOIL-TRANSMITTED VIRUSES Tobamoviruses

Tobamoviruses were the first plant viruses studied. Their high stability in and outside plants contributes to their efficient dissemination and the difficulty to control them. Tobamoviruses have a ssRNA genome of ~6.5 kb, encapsidated in rigid rod-shaped particles 300 nm long. They do not have a specific biotic vector, but they are easily transmitted by contact and remain infectious for a long time on surfaces, in plant debris or in water. They are also seedtransmitted [START_REF] Dombrovsky | Seed Transmission of Tobamoviruses: Aspects of Global Disease Distribution[END_REF]. Contrary to other plant viruses, tobamoviruses seem to have undergone long-term co-evolution with their hosts as they constitute three clusters associated with asteroid, rosid or caryophyllid angiosperms. The most important tobamovirus on cucurbits is cucumber green mottle mosaic virus (CGMMV). CGMMV induces more or less severe leaf and fruit mottling, particularly in cucumber and watermelon [START_REF] Dombrovsky | Seed Transmission of Tobamoviruses: Aspects of Global Disease Distribution[END_REF]. Since its first description in 1935 in England; its distribution to additional countries increased slowly between 1935and 1985, faster between 1986[START_REF] Fukuhara | The wide distribution of endornaviruses, large double-stranded RNA replicons with plasmid-like properties[END_REF], and rapidly between 2007and 2016(Dombrovsky et al., 2017)). It is now present on all continents except in South America. Seed transmission must have contributed to its rapid spread, as well as its ability to rapidly colonize weeds close to infected crops, making eradication efforts useless [START_REF] Dombrovsky | Seed Transmission of Tobamoviruses: Aspects of Global Disease Distribution[END_REF]. Phylogenetic analyses indicated the presence of distinct CGMMV clades correlated with geographic origin of the isolates, showing a distinction between East Asia and European isolates [START_REF] Dombrovsky | Seed Transmission of Tobamoviruses: Aspects of Global Disease Distribution[END_REF], what does not appear consistent with a rapid worldwide dissemination through seed transmission. However phylogenetic signal is limited by the low genetic diversity of the virus.

At least five other tobamoviruses infect cucurbits (Table 1). They are mostly restricted to Eastern Asia so far (Table 1), even if their seed transmissibility makes them potential threats worldwide.

Other seed-borne viruses

Besides tobamoviruses, several viruses are transmitted through seed (Table 1), either via external contamination of the tegument or more rarely through infection of the embryo. Squash mosaic virus (genus Comovirus) is transmitted by several beetle species, mechanically by pruning operations, and by seeds, what is probably the means by which it has spread throughout the world [START_REF] Lecoq | Cucurbit viruses: the classics and the emerging[END_REF]. It can reach the embryo and multiply during germination, leading to rapid systemic infection of the plantlets [START_REF] Lecoq | Control of cucurbit viruses[END_REF]. Melon rugose mosaic virus (genus Tymovirus) is seed transmitted in melon [START_REF] Mahgoub | Melon rugose mosaic virus: characterization of an isolate from Sudan and seed transmission in melon[END_REF]; however it has not yet spread worldwide and is restricted so far to the Red Sea region.

Melon necrotic spot (MNSV, genus Carmovirus) is seed-transmitted in the presence of the soil fungus Olpidium bornovanus. The virus is carried in the seed, released in the soil during germination, and introduced in the plant by fungal zoospores. It is also transmitted directly from plant to plant by zoospores. O. bornovanus being a very resistant and ubiquitous fungus, vectorassisted seed transmission must have contributed to the worldwide spread of MNSV.

Traditional seed transmissibility tests include serological tests on seeds, what often greatly overestimate the real seed transmission, and observation of symptoms on plantlets after germination tests [START_REF] Dombrovsky | Seed Transmission of Tobamoviruses: Aspects of Global Disease Distribution[END_REF]. However, in some cases plants infected from seed are asymptomatic and have very low virus titres that can be detected only by molecular methods [START_REF] Simmons | Zucchini yellow mosaic virus (ZYMV, Potyvirus): Vertical transmission, seed infection and cryptic infections[END_REF]. Using molecular approaches, seed transmission was observed for viruses considered as non-seed-transmissible, including begomoviruses [START_REF] Mabvakure | Ongoing geographical spread of Tomato yellow leaf curl virus[END_REF][START_REF] Sangeetha | A distinct seed-transmissible strain of tomato leaf curl New Delhi virus infecting Chayote in India[END_REF]. The agronomic impact of cryptic infections after seed transmission is not well known, but since in some cases vector transmission to new plants has been demonstrated from asymptomatic plants infected from seeds, these plants can constitute primary inoculum to start epidemics. The importance of seed transmission for long-distance spread of plant viruses has probably long been underestimated, and particular care should be taken during seed trade.

Soil-borne viruses

Besides MNSV, some viruses are soil-transmitted, either by fungi or by nematodes. Several nepoviruses, transmitted by soil nematodes, infect cucurbits (Table 1). They generally have a broad host range outside cucurbits, including annual and perennial hosts. Each species is transmitted by a few specific nematode species, and some are also transmitted by seed. They are usually geographically restricted, but tobacco ringspot virus, endemic in cucurbits in America, has been reported on other crops in different areas where it probably spread through dissemination of infected plant material. Viruses transmitted by soil fungi include MNSV and other viruses in the same family Tombusviridae. They usually have a limited geographic distribution and their agronomic impact is low [START_REF] Lecoq | Cucurbit viruses: the classics and the emerging[END_REF]

MINOR AND PARTIALLY CHARACTERIZED VIRUSES

Some viruses representing the unique cucurbit-infecting species in their genus have been known for long, but have never been observed outside the area where they were first described. It is the case for Ourmia melon virus, observed only in the North-West of Iran, the carlavirus melon yellowing-associated virus in Brazil, the torradovirus squash chlorotic leaf spot and the sobemovirus snake melon asteroid mosaic virus found only in Sudan (Table 1). Some of them may have a larger geographic range but may have been undetected because no accurate diagnostic was established. For highly damaging and/or widespread viruses, researches are usually undertaken to characterize their biological and molecular properties and develop serological or molecular diagnostic tools, but it is not always the case for minor viruses. On the other hand, the development of untargeted molecular diagnostic by high-throughput sequencing (HTS) has revealed viruses that were widespread but latent, both in crops and in wild plants [START_REF] Prendeville | Virus infections in wild plants are both frequent and often unapparent[END_REF]. Endornaviruses, for instance, are only transmitted vertically and do not induce symptoms in their hosts [START_REF] Fukuhara | The wide distribution of endornaviruses, large double-stranded RNA replicons with plasmid-like properties[END_REF][START_REF] Sabanadzovic | Cucumis melo endornavirus: Genome organization, host range and co-divergence with the host[END_REF]. They represent some of the lately discovered commensal or even symbiotic viruses that have long been unnoticed because only viruses causing damages on crops were studied [START_REF] Prendeville | Virus infections in wild plants are both frequent and often unapparent[END_REF][START_REF] Roossinck | Plant virus metagenomics: biodiversity and ecology[END_REF]. The use of HTS for diagnostic is a very powerful tool for virus characterization, but it should not be disconnected from biology. Indeed, many recent descriptions of new viruses rely only on their sequence properties, without information on their symptomatology, host range or transmission properties, what makes it difficult to estimate the risks of dissemination and potential agronomic impact of these viruses.

CONTROL OF VIRUSES

Despite the high diversity of plant viruses and of their modes of dissemination, control measures against these pathogens are to a large extent common to many of them [START_REF] Lecoq | Control of cucurbit viruses[END_REF]. There is no curative chemical treatment against plant viruses. Virus control relies mostly on cultural practices and the use of host genetic resistance when it is available.

Prophylaxis and good cultural practices involve limitation of virus sources and virus vectors in the crops and their vicinity. Weeding and the removal of volunteers before planting contribute to reduce the primary inoculum in crops. Overlapping crops should also be avoided, to limit the sources of infection to the young crops. Seeds can be produced in regions where seed-borne viruses are not prevalent, or mother plants may be grown under protected conditions. Serological or molecular testing of seed lots, and destruction of the positive ones, are important components of virus containment. For stable seed-borne viruses like CGMMV, seed treatment with chemicals or heat can reduce or abolish seed transmission. Long-distance movements of plant material should be limited to reduce the risks of virus spread.

In crops, plastic or unwoven covers can protect young plants from viruliferous insects, but the covers have to be removed to allow pollination. Plastic mulches contribute to the limitation of weeds and also have a repellent effect on aphids and whiteflies, at least until plant growth covers most of their surface. Disinfection of grafting and pruning tools with trisodium phosphate or skimmed milk is recommended [START_REF] Coutts | Zucchini yellow mosaic virus: Contact Transmission, Stability on Surfaces, and Inactivation with Disinfectants[END_REF][START_REF] Lecoq | Control of cucurbit viruses[END_REF]. Control of vectors in crops can be more easily achieved in greenhouse conditions, with the use of insect-proof netting, than in the field. Insecticide treatment or biological control of the vectors may contribute to reducing virus spread, particularly for persistently-transmitted viruses, but their efficiency against non-persistently transmitted viruses is limited or null.

Cross-protection, i.e. the use of a mild strain to protect plants against a severe strain of the same virus, has been used successfully against ZYMV. Attenuated strains have been obtained for several other viruses but have not been broadly used [START_REF] Lecoq | Control of cucurbit viruses[END_REF]. One limitation of cross-protection is its high specificity [START_REF] Lecoq | Control of cucurbit viruses[END_REF].

When they are available, resistant cultivars constitute the easiest and most environmentally-friendly way to control viruses. Resistance traits have been searched in the biodiversity of cultivated species and their wild relatives since the beginning of the 20 th Century, both by private breeding companies and research institutes [START_REF] Lecoq | Control of cucurbit viruses[END_REF]. Many commercial cultivars of cucurbits are resistant or tolerant to viruses. Most resistances used by breeders rely on one or a few major genes, as they were the most easily introgressed in agronomically interesting cultivars. Resistance factors can target any step of the virus cycle with very different mechanisms. Dominant resistance factors are involved in pathogen recognition of the virus, leading to a signalling pathway that results in induction of defence mechanisms. In the case of recessive resistance as well as non-host resistance, a plant factor required by the virus for its cycle is absent or modified so that the interaction with virus components cannot take place and the virus cannot complete its cycle [START_REF] Lecoq | Control of cucurbit viruses[END_REF]. Pathogen-derived resistance has also been used commercially against ZYMV, PRSV and CMV in squash in the USA [START_REF] Lecoq | Control of cucurbit viruses[END_REF].

One limitation of the use of resistance genes is the availability of resistance factors against each virus. Introducing these factors in high-standard cultivars is costly and time-consuming and it is difficult to keep pace with the increasing number of viruses. Resistance genes corresponding to factors important for viral infectivity can constitute good candidates for broad-spectrum virus resistance. For instance, melon and cucumber plants mutated or silenced in a translation initiation factor required for potyvirus infection presented broad virus resistance [START_REF] Chandrasekaran | Development of broad virus resistance in non-transgenic cucumber using CRISPR/Cas9 technology[END_REF][START_REF] Rodriguez-Hernandez | Melon RNA interference (RNAi) lines silenced for Cm-eIF4E show broad virus resistance[END_REF]. However, plant viruses have developed counter-defence mechanisms and can in some cases evolve rapidly by mutation or recombination to overcome host resistance (Desbiez et al., 2011b;[START_REF] Harrison | Virus variation in relation to resistance-breaking in plants[END_REF]. The selection of putatively durable resistance early in the selection scheme, the optimized deployment of resistances in agricultural landscapes and the selection of multigenic, partial and broad-spectrum resistances or tolerances could contribute to preserve the long-term efficiency of genetic virus control [START_REF] Acosta-Leal | Advances in Plant Virus Evolution: Translating Evolutionary Insights into Better Disease Management[END_REF].

CONCLUSIONS

The number of virus species described as infecting cucurbits is still rapidly increasing: About 20 species were described in 1980 [START_REF] Lovisolo | Virus and viroid diseases of cucurbits[END_REF]), 35 in 1998[START_REF] Lecoq | Cucurbit viruses: the classics and the emerging[END_REF], 59 in 2012 [START_REF] Lecoq | Viruses of cucurbit crops in the Mediterranean region: an ever-changing picture[END_REF] and more than 90 in 2019 (Table 1). Among the "new" viruses described, some are endemic whereas others are present worldwide. Aphid-borne viruses still constitute major problems in temperate climates, whereas whitefly-transmitted ones are emerging in Mediterranean/tropical areas wherever the invasive MED and MEAM1 B. tabaci are present. The high increase in host connectivity due to global trade, as well as climatic changes involving changes in hosts and vectors, play important roles in the rapidly changing situation of plant viruses [START_REF] Elena | Evolution and emergence of plant viruses[END_REF]. Why some viruses emerge while others remain unimportant is partially understood. Intrinsic properties like seed transmissibility and a large host range ought to favour virus emergence, but it is not always the case (Table 1). Local ecological conditions, as well as the chance of introduction of minor viruses in the international trade, can influence the dissemination and potential emergence of plant viruses. Particular care should be taken when exchanging genetic material, and good practices involving seed disinfection and insect control should always be applied to limit the impact of viral diseases on cucurbits.

Table 1 .

 1 Viruses infecting cucurbits and their biological properties. The most important or currently emerging viruses worldwide are indicated in bold. Viruses in italics are not yet acknowledged by the International Committee for the Taxonomy of Viruses (ICTV)

	Genus and species	Means of spread	First description	Geographic distribution	Cucurbits	Host range Outside cucurbits
		Begomovirus
	Melon chlorotic mosaic virus (MeCMV)	Whitefly	2004	Venezuela	narrow	narrow
	Tomato leaf curl new Delhi virus (ToLCNDV)	Whitefly	1948	Indian Peninsula, Mediterranean Basin,	wide	wide
					Indonesia, Thailand
	Squash leaf curl virus (SLCuV)	Whitefly	1981	USA, Israel, Jordan, Egypt	wide	narrow
	Squash mild leaf curl virus (SMLCuV)	Whitefly	1978	USA, Jordan	narrow	narrow
	Watermelon chlorotic stunt virus (WmCSV)	Whitefly	1994	Iran, Yemen, Sudan, Israel	intermediate	narrow
	Cucurbit leaf crumple virus (CuLCrV)	Whitefly	2000	USA, Mexico	intermediate	narrow
	Melon chlorotic leaf curl virus (WCLCuV)	Whitefly	2004	Nicaragua, Costa Rica, Guatemala	wide	wide
	Telfairia golden mosaic virus (TelGMV)	Whitefly	2016	Cameroon	unknown	unknown
	Coccinia mosaic Tamil Nadu virus (CMTNDV)	Whitefly	2016	India	unknown	unknown
	Chayote yellow mosaic virus (ChaYMV)	Whitefly	2016	Nigeria, Benin, Togo	narrow	unknown
	Tomato leaf curl Palampur virus (ToLCPaV)	Whitefly	2007	India, Pakistan, Iran	wide	wide
	Tomato yellow leaf curl-Mild virus (TYLCV-Mld)	Whitefly	1964	Jordan, Latin America (on cucurbits)	narrow	intermediate
	Luffa yellow mosaic virus (LYMV)	Whitefly	2003	Vietnam	unknown	unknown
	Squash leaf curl China virus (SqLCCNV)	Whitefly	1998	Southern Asia, India	wide	unknown
	Squash leaf curl Philippines virus (SLCuPhV)	Whitefly	2003	Philippines	unknown	unknown
	Squash leaf curl Yunnan virus (SLCuYnV)	Whitefly	2003	China	unknown	unknown
	Pumpkin yellow mosaic virus (PuYMV)	Whitefly	2001	Malaysia	unknown	unknown
		Curtovirus
	Beet curly top virus (BCTV)	Leafhopper	1909	Worldwide	wide	wide
		Clecrusatellite
	Melon chlorotic mosaic alphasatellite (MeCMA)	whitefly	2010	Venezuela	narrow	narrow
		Betasatellite
	Momordica yellow mosaic betasatellite (MomYMB)	whitefly	2016	Benin	(momordica,	unknown
						chayote)
		Alphacryptovirus
	Citrullus lanatus cryptic virus (CLCV)	Vertical	2013	Israel	unknown	unknown
		Alphaendornavirus
	Cucumis melo alphaendornavirus (CmEV)	Vertical t	2016	Worldwide	intermediate	unknown
	Lagenaria siceraria alphaendornavirus (LsEV)	Vertical	2014	China

ACKNOWLEDGEMENTS

We thank Dr H. Lecoq for critical review of the manuscript Literature cited