A Darth Vader-type villain for the melon industry is back this fall with a vengeance — and his lightsaber is attacking desert-grown cucurbits in California, Arizona and Mexico.
The black-hooded culprit is the cucurbit yellow stunting disorder virus (CYSDV) that attacks the Curcurbitaceae botanical family including melons, cucumbers, gourds, plus summer and winter squash.
Molecular biology laboratory tests have confirmed CYSDV in cucurbits in fall melons in California's Imperial Valley, across Arizona's melon growing region, and in Mexico.
CYSDV's sole vector is the B biotype of the sweet potato whitefly, Benicia tabaci. CYSDV is not transmitted in seed. The plant holds the virus for seven to nine days.
CYSDV symptoms develop first on older leaves and mimic water stress. Interveinal chlorosis streaks the leaves, which later turn bright yellow. The plant's internal transport system breaks down so the plant tries to save itself by dropping the older leaves. The plant's vigor is reduced causing less nourished fruit.
CYSDV first reared its ugly head in California and Arizona in fall 2006 melons. In the Yuma Valley, about two thirds of the melon crop was lost, according to Kurt Nolte, University of Arizona (UA) area Extension agent, Yuma County, Ariz.
In the spring 2007 melon crop, CYSDV started later in the growing season reducing crop damage. In the Yuma Valley this fall, plants just two weeks old without a canopy or even vining, showed signs of the disease. Laboratory tests conducted at the University of Arizona (UA) confirmed the disease.
“The bottom line is will we continue to be able to grow melons in Arizona?” asked Judith Brown, UA virologist and whitefly vector biologist, Tucson, Ariz. “It's a huge deal since our economy, country, food security, and sustainability are at stake.”
This (CYSDV) is leading to a “significant event,” Nolte said. “It's not just localized since it's been confirmed in two countries and two states. Last fall it was a disaster and it looks almost identical to what we saw then. It's pointing to a significant problem this fall.”
To help slow the virus spread from spring to fall melons in 2007, some Western melon growers this past summer participated in a host-free period.
“The host-free period that we tried to make happen was not successful,” Nolte said.
The host-free period was not a failure, Brown said, but it was a good start although complete participation didn't occur. Economics is what drives growers' ability to pull out or not, she said.
“It's a tough one — we're caught between a rock and a hard place on that,” Brown said. “What we're recommending is that we reduce or eliminate for at least a month in the summer and the winter any over-seasoning hosts.”
Brown conducted laboratory tests on about 60 Arizona samples from late July to mid-September and the majority came back positive for CYSDV.
In Arizona, CYSDV was confirmed in fall melons in the Wenden/Salome area (La Paz County), the Harquahala Valley, near the town of Maricopa (Pinal County), and in Roll, Wellton, Tacna, Yuma Valley, and Dome Valley in Yuma County, Nolte said.
Positive CYSDV was found in commercially grown watermelons, cantaloupes, and honeydews in Yuma County. The virus was also found at the UA research farm in cucumbers, gourds, and squash, but not in commercial production, Nolte said.
In the Imperial Valley, positive CYSDV findings were in cantaloupes scattered throughout the area in mid-September, said William Wintermantel, USDA-Agricultural Research Service research plant pathologist, Salinas, Calif., who conducted the initial CYSDV lab tests in California.
Samples were then sent to the University of California, Davis lab of Robert Gilbertson whose results confirmed each of Wintermantel's findings.
The virus has also been confirmed in Mexico around Hermosillo and Caborca.
While the Yuma County melon harvest generally starts the second week of October, it's hard to determine what the actual losses will be but it could be significant, Nolte said.
“There is some talk in our area here that the (future of the) fall melon industry is in jeopardy,” Nolte said. “Unless we find some germplasm for commercial varieties that are resistant to this virus, there are guys who are rather hesitant to plant if they know the virus will affect them.”
Such germplasm research is underway by Jim McCreight, USDA-ARS, Salinas, Calif., and by Kevin Crosby, Texas Agricultural Experiment Station, Weslaco, Texas.
The approach taken by a team of Western leaders including Brown, Wintermantel, Gilbertson, Extension experts, and others is trying to control the whitefly vector that in turn would control the virus. Even so, a few whiteflies feeding on a plant can cause the disease.
According to the UA's Brown, the fall virus findings in Arizona mean two things.
“First we have a widespread virus, and second, there are a group of producers and pest control advisors that are now recognizing the symptoms,” Brown said. “It's good news that the awareness has been raised. Hopefully, the next step will be people working together to develop a tighter host-free period next summer.”
The earlier the infection occurs in the plant then the more damage to the crop, Brown said. Some growers are taking immediate action by applying augmentive measures such as vitamins and foliar feeding in an effort to reduce plant stress.
“Yet the bottom line — is it economical? Will growers be able to produce a product that's the quality they're after, in the market window when they can sell it, and for the price to produce the crop?” Brown asked.
Time will tell the seriousness of the problem. Wintermantel said monitoring should continue.
“Based on what we saw in the late spring and this fall, it looks like CYSDV is here to stay,” Wintermantel said.
“We need to look at where the virus is persisting. We had a hard freeze last winter and the virus was still there in the spring. There are definitely hosts out there providing a conduit for moving CYSDV from one cropping season to the next. Whiteflies can carry CYSDV long distances, but there must be a reservoir out there that is harboring it in order for the whitefly to acquire the virus. We need to identify the reservoir host plants.”
Wintermantel encouraged melon industry leaders to continue working together to develop effective management strategies to make production more efficient and to reduce losses.
“We want growers to continue to be able to produce a profitable crop during the spring and fall melon seasons,” Wintermantel said.
Efforts are underway to secure federal funds for a western region integrated pest management project to look at weeds as sources this coming winter, and to continue the mapping of whiteflies and the virus to look at larger patterns. A request for funds last fall was not approved.
Those wishing to contribute financially to the CYSDV issue can send donations in care of the Yuma County Farm Bureau, 1573 Kuns Court, Yuma, Ariz. 85365.