A plant virus identified for the first time last fall on Arizona and Sonora (Mexico) 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. “What I observed in the cucurbit crop in Mexico was astounding - one-hundred 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.
CYSDV symptoms develop first 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, 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, Calif., reported losing nearly 60 percent of his Yuma cantaloupe and honeydew crop last fall. 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, Bemicia tabaci. Whiteflies feed on leaves and transfer the viruses through their saliva.
However, the two 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.
Growers started reporting the first virus symptoms in Yuma and Imperial in September, 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.
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, we are contending with two 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 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.