Adrenaline rushes through John Palumbo’s veins each fall as he peers through the scope atop his Winchester .308 rifle in the hunt for deer in the Arizona desert and elk in the High Country region.
Palumbo’s hunting persona mirrors his passion as a University of Arizona (UA) research scientist and Extension specialist. Palumbo is on the hunt for solutions for pests and diseases which threaten the economic viability of low-desert crop growers.
Cucurbit yellow stunting disorder virus (CYSDV) was first identified in desert melons in fall of 2006. The virus wiped out up to two thirds of the crop in Southern California and Arizona fields. A third of the melon crop was lost in 2008 and 2009.
“I attribute these melon losses largely to CYSDV,” said Palumbo who is based at the Yuma Agricultural Center (YAC), Yuma, Ariz.
Finding solutions to CYSDV is in the crosshairs of Palumbo and other associates. CYSDV attacks the Curcurbitaceae botanical family including melons, cucumbers, gourds, plus summer and winter squash.
Initial CYSDV symptoms include subtle interveinal mottling of the older leaves followed by yellow interveinal chlorosis. Chlorotic spots appear 10-14 days after infection along the leaf veins.
The virus lowers photosynthesis in the melon plant which reduces the plant’s ability to develop leaf chlorophyll. Other results include reduced plant growth and nutrient update, smaller fruit, and lower brix content.
The sole vector of CYSDV is the B biotype of the sweetpotato whitefly, Bemicia tabaci. CYSDV is not transmitted in seed.
CYSDV primarily targets fall melons in desert fields. About 40 percent of the desert fall cantaloupe and honeydew crops are infested with CYSDV on average. The virus attacks watermelons, but not as severely. Ironically, less than 1 percent of spring cantaloupes and honeydews become infested.
Desert spring melons are planted beginning in January and harvest is complete by early July. Fall melons are planted in August with a November harvest date.
Some moderately-tolerant CYSDV varieties are available. Palumbo believes the long-term solution to solving the CYSDV puzzle is through advanced plant breeding. Breeding research is underway at the USDA’s Agricultural Research Service facility in Salinas, Calif., under the leadership of James McCreight.
For now, Palumbo says the best way to control CYSDV is through management tools. These include vector and host plant management, field sanitation, crop isolation, and aggressive adult whitefly insecticidal control.
Palumbo discussed management strategies with growers, pest control advisers, and other industry members during a Progressive Farmers seminar in Blythe, Calif., sponsored by the University of California Cooperative Extension, Riverside County.
CYSDV infection is dependent on the feeding habits of the sweetpotato whitefly. Laboratory research suggests the insect must feed for about 18 hours to successfully pick up the virus. The insect can carry and transmit the virus up to nine days.
The identification of host plants is crucial to sweetpotato whitefly management. Initial findings linked cucurbits (including wild plants) and winter lettuce as major hosts. Further research tied alfalfa and common weeds as reservoir hosts.
Although cotton has not been identified as a CYSDV host, it is an important host crop for whiteflies between spring and fall melon crops.
Alfalfa is commonly grown with melons on the California side of the Colorado River (Imperial and Riverside counties). Cotton, alfalfa and melons are grown in southern Arizona. The Yuma-Imperial Valley area is the winter vegetable capital of the nation.
About 20 weeds host the sweetpotato whitefly and CYSDV, including alkali mallow and Wright’s groundcherry, all widely found in the Yuma area.
“The only way to successfully create a host-free environment for melons is to convince growers to quit growing alfalfa and lettuce and remove host weeds,” Palumbo said. “That is not going to happen. Cultural management of CYSDV is difficult given the crops and production practices common in the desert.”
Plant isolation is another good management tool. Palumbo recommends planting fall melons away from whitefly host areas including the previously mentioned host crops, urban areas with backyard gardens, and weedy orchards.
A cultural practice that helps reduce CYSDV is field sanitation. This includes the timely elimination of plant residue after harvest, plus the prompt removal of overwintering volunteer plants and weeds between the spring and fall melon crops.
A successful, but expensive, practice to reduce the whitefly-CYSDV threat is to grow melons under lightweight fabric row covers. Applied during the planting operation, the covers reduce the time that CYSDV-carrying whitefly adults can feed on the melons prior to bloom. Once the plants begin to flower, the covers must be removed to allow for pollination.
“The longer sweetpotato whiteflies are kept off the plants the better,” Palumbo said. “This delays the incidence of CYSDV.”
Palumbo referenced a Yuma-area grower who successfully grows fall melons under fabric row covers.
“Row covers are expensive, but the practice helps shield the plants from whiteflies and the virus. Most growers don’t want to mess with row covers,” Palumbo acknowledged. “Yet the difference in plants in covered melon blocks versus uncovered blocks is like night and day.”
The row cover can be mechanically installed following the planting operation over a mid-bed trench, but must be removed by hand.
Another effective and expensive method of adult whitefly control in melons is insecticides. Insecticidal control starts with the soil systemic product Venom (dinotefuron) applied as a subsurface injection below the seedline or via drip irrigation. Five foliar-based insecticide applications follow over the top during the stand establishment.
“The reason for the soil insecticide is to protect the plant as it emerges from the ground,” Palumbo said. “The overlay with the foliar knockdown products provides additional protection. This acts as a ‘chemical row cover.’ Following this program has shown to eliminate whitefly adults and the next generation of nymphs.”
Row covers and insecticides act as a one-two punch to prevent whitefly establishment on small seedlings.
“The longer whiteflies are kept from feeding and transmitting the virus during stand establishment pre-bloom then the less the virus will express itself near and at harvest,” Palumbo said.
Palumbo conducted insecticide trials at the YAC last year where he compared existing and experimental insecticides on young melon plants. The results showed that rotations of tank-mixed products including Lannate, Vydate, Fulfill, Brigade, and Thionex provided significant CYSDV reduction when applied after the Venom soil treatment.
The experimental products included pyrifluquinazon, cyazypyr, and an unidentified product Palumbo calls UAEXP_30; all tested with and without Venom applied at planting. “These three products have real potential,” Palumbo said. “Pyrifluquinazon is extremely effective on immature and adult whiteflies. Cyazypyr was also active as a foliar adulticide.”
These products could be commercially available in several years. Palumbo could not share the results of the “UAEXP_30” product, other than the product warrants further study.