Decisions by desert cucurbit growers on participation in a voluntary host-free period this summer in the West and Mexico, will likely impact the severity of a relatively new cucurbit virus in fall 2007 cucurbit crops.
The menace is the cucurbit yellow stunting disorder virus (CYSDV) that infects the Cucurbitaceae botanical family including melons, cucumbers, gourds, and summer and winter squash. The sole vector is the B biotype of the sweet potato whitefly, Bemicia tabaci. CYSDV is not transmitted in seed. The plant holds the virus for seven to nine days.
CYSDV symptoms mimic water stress and occur first on the plant's older leaves. Then interveinal chlorosis, a yellowing between the leaves, streaks the leaves that later turn bright yellow. Older leaves drop as the plant's internal transport system breaks down.
While CYSDV-resistant varieties are not yet available, germplasm research in the pipeline shows great promise.
CYSDV researchers, cucurbit growers, pest control advisors, and others huddled at a workshop in Yuma, Ariz., in late July, to strategize on the identification, spread, and management of CYSDV in the West and Sonora, Mexico.
CYSDV was first confirmed in the desert cucurbit growing areas of Imperial Valley, Calif., Yuma Valley, Ariz., and Sonora, Mexico, in the fall 2006 melon crop.
“In early fall 2006 melons, there was virtually no virus reported,” said Milas Russell, Jr., a melon grower in the Yuma and Imperial valleys, and president of Sandstone Marketing Inc., Yuma. “Ninety days later there was 100 percent infection of the crop from Niland, Calif., to the Tacna, Ariz., area. How did that happen? The volunteer melons and late summer commercial melons were suspects.”
“The only thing growers can do is destroy volunteer plants when they emerge to create a gap,” Russell said.“We need to encourage our neighbors to do the same.” Russell grows cantaloupes, honeydews, and mixed melons.
As part of the voluntary host period, Russell plowed down his melon fields following the 2007 spring melon harvest and prior to the suggested July 15 plow-down date in Arizona. The hope was to keep cucurbit fields clean until Aug. 15. Yet, due to a tight crop production window, Russell planned to first water his fall crop in early August.
While CYSDV infected 100 percent of Russell's 2006 fall melons, 15 percent of his spring 2007 crop had CYSDV. The spring virus didn't reduce melon quality or yield, Russell said. The CYSDV was located sporadically in his fields and around the edges.
An estimated 10 growers produce melons in the Yuma and Imperial valleys.
Laboratory tests confirmed CYSDV in 2006 fall melon plant samples in California and Arizona, plus melons and squash grown in Sonora, Mexico.
CYSDV typically devastates cucurbits by reducing fruit size and sugar content, plus shortening the product's shelf life.
Eric Natwick, University of California Imperial County Cooperative Extension director, Holtville, Calif., was instrumental in two CYSDV symptom surveys conducted in spring 2007, in the Imperial Valley.
The first survey was an early season examination of commercial melons, plus melons planted at the Desert Research Extension Center in Holtville. Of the 50 collected samples, all tested negative for CYSDV.
The second survey, in late June, consisted of three teams including: Natwick; Robert Gilbertson, University of California, Davis (UCD) plant pathologist; William Wintermantel, USDA-Agricultural Research Service plant pathologist, Salinas, Calif., and the Imperial County ag commissioner's office.
They trekked through every watermelon and cantaloupe field in Imperial County in search of CYSDV symptoms. One plant sample tested CYSDV-positive in independent tests at Gilbertson's UCD lab and Wintermantel's ARS lab. The infected plant material was from a field northeast of Holtville.
A survey of 2007 fall melons in Imperial County is planned.
During the host-free period this summer, many Western and Mexican melon growers planned to plow down fields to reduce and prevent CYSDV spread into the fall 2007 crop.
CYSDV has a Middle East pedigree dating back to 1985. By the early 90s, CYSDV was a serious problem in cucurbit production in the Middle East and Mediterranean regions. The virus moved into West Texas and southern Mexico from 1999 to 2002.
University of Arizona virologist and whitefly vector biologist Judith Brown, Tucson, Ariz., is focused on finding CYSDV answers. She confirmed CYSDV in the Yuma Valley in fall 2006 in her Tucson lab. Brown also confirmed the first 2007 spring CYSDV positive on April 28 in Somerton, Ariz., located south of Yuma. In addition to the Yuma Valley, the virus has been confirmed in the Harquahala Valley and Maricopa County.
The whitefly is an excellent CYSDV vector that makes management of the virus difficult, Brown said. She advocated the summer host-free period to remove virus hosts from the ground for an extended period of time.
“Growers should remove volunteer plants, clean out the fields after harvest, and have an amount of time to bake this virus out — about 6 to 8 weeks is recommended. That's tough for growers to tolerate as they juggle planting and harvesting schedules.”
Wild hosts can likely serve as reservoirs for the virus to overseason in the summer and winter. Yuma County Cooperative Extension agent Kurt Nolte and Yuma Agricultural Center research/extension entomologist John Palumbo are monitoring whitefly populations near commercial fields in the Yuma Valley.
The goal is to understand the interrelationship between the virus and the vector. Thirty-one yellow sticky traps were placed in melon fields and are checked weekly.
“In the fall melon crop, we will also experiment with several management techniques, including row covers and new pesticides to help mediate the virus problem,” Nolte said.
About 90 percent of the watermelons in the Yuma Valley are transplanted while most cantaloupes are direct seeded, Nolte said.
A potential culprit of the virus could be infected seedlings, Brown said. “If plants are infected, chances are they are coming from out of state, not from nurseries here.”
To learn even more about CYSDV, Robert Gilbertson of UCD is developing a specialized lab test, a polymerase chain reaction or PCR, to detect CYSDV in whiteflies.
In June 2007, UCD released a CSYDV brochure available on the Statewide Integrated Pest Management Program website at www.ipm.ucdavis.edu. Gilbertson, Wintermantel, Natwick, UC Cooperative Extension farm advisor Tom Turini, and several others authored the brochure. Turini is a former UC farm advisor in Imperial County who is now based in Fresno, Calif.
Until resistant plant varieties are available, Gilbertson offered growers suggestions to help manage CYSDV in melons:
Select the most vigorous and well adapted varieties;
When using transplants, use pathogen-free, whitefly-free transplants;
Treat prior to planting with neonicotinoids to manage whiteflies in the field;
Plant immediately after a host-free period if a period is conducted;
Don't plant in old established fields;
After the season, sanitation is important — pull up the plastic and plow the fields under. Institute a host-free period in between fall melons and spring melons. This can clearly knock down the amount of virus in the ecosystem.
Brown is optimistic about future CYSDV-resistant varieties coming online. In the meantime, she encouraged growers to consider row covers and longer host-free periods. She repeatedly asked that suspect plant samples be sent to her UA lab for analysis.
Growers cannot stop the virus from spreading, Wintermantel said. “Your goal is to keep CYSDV from quickly spreading. If you can minimize the numbers of whiteflies, then you'll reduce the spread. That's the best advice I can give you until there is a resistant variety out there.”
Milas Russell and others said available funds for CYSDV research are limited. He encouraged growers to contribute $2 per acre toward an overall goal of $5,000 to $10,000. Contributions can be mailed to: Yuma County Farm Bureau, 1129 S. Arizona Avenue, Yuma, Ariz. 85364.