A plant virus identified for the 1st time last fall [2006] on Arizona and Sonora melon and squash crops has the potential to cause severe damage on upcoming crops. Cucurbit yellow stunting disorder virus, CYSDV, can infect members of the botanical family Cucurbitaceae, including all types of melons, summer and winter squash, pumpkins, gourds, and cucumbers. Severe commercial damage occurred in 2006 on melons in southern Arizona, and on melons and squash in Sonora, Mexico.

"What I observed in the cucurbit crop in Mexico was astounding -- 100 percent infection and extremely severe symptoms in watermelon, honeydew, cantaloupe, spaghetti squash, acorn and kabocha squash, and zucchini," says Judith K. Brown, virologist and whitefly vector biologist in the College of Agriculture and Life Sciences at The University of Arizona (UA). She isolated and identified the virus from plant samples submitted to her by growers both in Yuma, Arizona, and Sonora, Mexico.

CYSDV symptoms develop 1st on older leaves and mimic water stress.

Interveinal chlorosis -- a yellowing between the veins -- streaks the leaves, which later turn bright yellow. Small green spots develop on the leaves of certain varieties. As the plant's internal transport system breaks down, it tries to save itself by dropping older leaves.

Without enough leaves, the plant's vigor is reduced and it can't support and nourish the fruit.

"The fruits are smaller, not as sweet, and don't ship or store as well," Brown says. "Plants do not produce the expected yields, and the quality is reduced. Last September [2006], growers in Caborca, Mexico, and in Yuma said they didn't get the size or the sugar content. No shipper is going to take a fruit that's not ripe."

Milas Russell, who grows melons in Yuma and in Imperial Valley, California, reported losing nearly 60 percent of his Yuma cantaloupe and honeydew crop last fall [2006]. Some of the plant samples Brown analyzed came from Russell's fields.

Like the recently identified Tomato yellow leaf curl virus, TYLCV, Cucurbit yellow stunting disorder virus is transmitted by the B and Q biotypes of the sweet potato whitefly, _Bemisia tabaci_. Whiteflies feed on leaves and transfer the viruses through their saliva.

However, the 2 viruses belong to entirely different families and thus infect different plant species. Brown says more information is needed on the extent of the CYSDV host range.

CYSDV was 1st identified in cucumber and melon crops in the Middle East more than 15 years ago and in cucumber and melon plantings in Spain about 10 years ago. Brown and others identified the virus in Central America and in the Rio Grande Valley, Texas, in 2003-2004.

The Arizona and Mexico infections are the 1st to be documented in squash and watermelon crops.

Growers started reporting the 1st virus symptoms in Yuma and Imperial in September [2006], and Brown's lab identified the virus in October 2006. The Agricultural College at the University of Sonora also contacted Brown and sent photos of symptoms in watermelon, cantaloupe, and squash to identify.

"There was one report of symptoms in the Phoenix area but samples were not received from those farms so we are not certain that the virus was present there," Brown says. "It seemed that every crop planted over a 6-week period early in the fall was infected in succession. And this wasn't even a heavy year for whitefly. Beginning with the plantings in August [2006], the symptoms hit every 3 weeks over the entire region in a wave that seemed to move from Caborca to Yuma and Imperial Valley."

Disease incidence appeared to vary depending on the time of planting, with the early-season fields in Mexico experiencing approximately 60 to 80 percent infection, and mid- to late-season plantings at 100 percent, Brown says. All of the symptomatic plants in Mexico were heavily infested with the whitefly.

Brown says a virus like CYSDV doesn't move across state or country lines without assistance. It has to be moved either in infected plants (seedlings) or by whiteflies on plants infected or that are migrating between locations.

"A source of infection that cannot be ruled out is the potential for introductions resulting from the movement of plants between states, countries, even regions," she says. "Through these practices, we are moving increasing numbers of exotic viruses and vectors, initially associated with introductions through international trade. This year [2007], we are contending with 2 exotic viruses at the same time -- the Cucurbit yellow stunting disorder virus, and the Tomato yellow leaf curl virus."

Control is difficult because no chemical or biological controls currently exist for either of the viruses. Stepping up water and fertilizer and early season insecticide applications to reduce vector populations may help, but these are expensive practices, compromising the producers' ability to grow a sustainable crop, according to Brown.

"We don't know if the virus infects wild cucurbit or other uncultivated hosts -- it may be symptomless in some plants while causing symptoms in others," she says. "The wider the range, the harder it is to control the virus."

Brown suggests that growers buy virus-free transplants or start their own from seed, and consider maintaining a host-free season in the summer by withholding plantings. Coordination is now under way between producers in cucurbit growing areas. Growers from Arizona, California, and Mexico formed a research committee in January [2007] to survey fields through the spring and summer to determine virus carry-through from previous seasons. Brown's virus diagnostic lab at the UA will analyze plant samples submitted from throughout southern Arizona and Sonora, Mexico. To create a badly needed host-free period

-- the only practical solution to controlling the disease when resistant varieties are not available -- growers will decide how long to delay planting a crop and when an infected crop should be removed.

The status of the disease and the whitefly populations on Mexico's west coast is highly significant because winds can blow whiteflies carrying the virus from south to north, and when they reach Sonora the winds move them northward into Arizona and California," Brown says. "That's why we need to work together. It's all one region. Our interest is in solving the problem on a regional basis."

For more information, contact Judith K. Brown at 520-621-1402 or at <jbrown@ag.arizona.edu br>

[Byline: Susan McGinley]

--

Dick Hamilton
Former Plant Disease Moderator
Canada
<rihamilto@shaw.ca>

[Cucurbit yellowing diseases caused by whitefly transmitted _Closterovirus_ are of great economic importance in different areas of the world. In the more recent epidemic events it has been observed that the yellowing diseases are induced by Cucurbit yellow stunting disorder virus (CYSDV). At present CYSDV has been detected and causes problems in Spain, Portugal, Morocco, Arab Emirates, and North America. In Spain, the yellowing symptoms caused by CYSDV are frequently observed in 100 percent of the plants in the affected greenhouse, resulting in great economic losses due to the important reduction of the crop yield.

The cucurbit family includes a number of valuable crop species (melon, cucumber, squash/pumpkin, watermelon). Much of this review is concerned with transgenic resistance to viruses, shown to be the major application of biotechnology in the cucurbit family. Progress made with the production of transgenic cucurbit crops is discussed.

Published data on field tests of transgenic cucurbits are reviewed, showing that much progress has been made with multiple virus-resistant cucurbit crops, which can be productive without chemical control of insect virus vectors. Modes of virus resistance in transgenic cucurbits are discussed, as is the bio-safety of such crops.

For the 1st time, a detailed analysis has been made of worldwide and US field test applications for cucurbit crops. Worldwide, most field test applications were for melon (54 percent), followed by squash (32 percent). Worldwide, most field test applications were for virus resistance (84 percent), and most applications (77 percent) were in the USA. Two transgenic multiple virus-resistant squash crops have been deregulated (released for sale). Additionally, the analysis shows that there are transgenic multiple virus-resistant crops in all major cucurbit species already available, for which several different companies have applied for field tests. This would imply that such crops are ready to be marketed should conditions permit, which would have an impact worldwide in reduction of ecological damage due to chemical control of the insect viral vectors.

Symptoms of interveinal chlorosis (yellowing) are commonly observed in melon or cucumber plants grown in greenhouses in the southeastern coast of Spain. The causal agent was shown to be cucurbit yellow stunting disorder virus (CYSDV). The virus was demonstrated to be transmitted specifically by the tobacco whitefly (_Bemisia tabaci_

[Bt]) and was retained by the vector for at least 7 days. Its experimental host range was restricted to members of the family Cucurbitaceae. Filamentous, flexuous virus particles typical of closteroviruses were observed in infected plants. The length distribution of the virus particles showed 2 peaks at 825 to 850 nm

(nanometers) and 875 to 900 nm. Analysis of double-stranded (ds) RNA extracts revealed 2 major dsRNA species of approximately 8 and 9 kbp (kilo-base pair). Random cDNA (complementary or copy DNA) cloning of viral dsRNA was performed, and a virus-specific cDNA clone (p410) of

557 nucleotides that hybridized with the smaller of the 2 viral dsRNA species was identified. Computer-assisted analysis showed that the deduced amino acid sequence of p410 was significantly similar to the HSP7O homologs of the closteroviruses and showed greater similarity to the HSP7O homolog of the _B. tabaci_-transmitted lettuce infectious yellows closterovirus (LIYV) than to the HSP70 homologs of the aphid-transmitted closteroviruses. The data suggest that CYSDV is a member of a newly recognized subgroup of closteroviruses with bipartite genomes exemplified by LIYV.

In a survey during 2000-2002 to determine the identity and prevalence of viruses affecting cucurbit crops in Cyprus, 2993 samples of cucumber, zucchini, melon, and watermelon were collected from the 5 major cucurbit-growing areas in Cyprus. Zucchini yellow mosaic virus (ZYMV), Papaya ringspot virus type W (PRSV-W), Watermelon mosaic virus (WMV), Cucurbit aphid-borne yellows virus (CABYV), Cucumber mosaic virus (CMV), and Squash mosaic virus (SqMV) were detected by enzyme-linked immunosorbent assay (ELISA), and Cucurbit yellow stunting disorder virus (CYSDV), Beet pseudo-yellows virus (BPYV) and Cucumber vein yellowing virus (CVYV) by reverse transcription polymerase chain reaction (RT-PCR).

ZYMV was the most prevalent virus of cucurbits in Cyprus with an overall incidence of 45 percent. PRSV-W, CABYV and WMV were detected in 20.8 percent, 20.8 percent and 7.8 percent of the samples tested, respectively. CYSDV was detected in most greenhouse cucumber samples with yellowing symptoms (88.1 percent), whereas BPYV and CVYV were found in only 2.4 percent and 9.5 percent, respectively, of samples.

CMV and SqMV were not detected in any cucurbitaceous crop during this survey.

Considering the economic importance of the disease caused by CYSDV, the design of efficient strategies to control the disease is required. Undoubtedly, the best option to control CYSDV would be the use of cultivars carrying genetic resistance against the virus.

However, only one source of natural genetic resistance to CYSDV has been described already, and it is in a melon very different from cultivated varieties. It would be very interesting to have other sources of genetic resistance against CYSDV not only in melon, but also in other cucurbits of economic importance affected by CYSDV.

The introduction of genetic resistance against CYSDV in plants susceptible to the virus has been accomplished by means of plant transformation with foreign genes (transgenes) capable of conferring this character. Thus, the technology developed consist of a method to generate genetic resistance against CYSDV in plants previously susceptible to the virus, by means of plant transformation with transgenes containing a sequence with an inverted repeated of a gene of the genome of the virus itself. The type of plant that could be used with this technology includes any plant susceptible of CYSDV infection, as a cucurbit for example melon, watermelon, or cucumber.

The plants obtained with the technology developed will have improved genetic resistance against CYSDV, and therefore represent an important improvement of traditional plants as their use will allow increasing the yields of the crops reducing the economic losses caused by CYSDV.

Arizona and Sonora can be found on the map at <Click HERE>.

- CopyEd.MJ]

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