About three years have passed since researchers first found Cucurbit Yellow Stunting Disorder Virus (CYSDV) in melon fields across the low desert-growing areas of California, Arizona and Sonora, Mexico.

YELLOWED AND brittle leaves are symptoms of CYSDV in melons grown in the low desert in California, Arizona and Sonora, Mexico.

Since the discovery researchers have worked to develop best CYSDV management guidelines for growers using learned knowledge on the virus, vector and host plants.

The virus, originally found in the Middle East, is vectored by the whitefly, Bemisia tabaci-B biotype.

CYSDV symptoms first appear as yellowing and brittleness in older melon leaves followed by a breakdown in the plant’s nutritional transport system and the loss of vigor. The end result is smaller-sized melons with reduced brix content.

Researchers from the University of Arizona, the University of California, U.S. Department of Agriculture, and the University of Sonora (Mexico) have worked closely to develop strategies to allow growers to remain in melon production despite the virus while maintaining profitability.

“Field sanitation including the removal of area wide overwintering plants and weeds and the plant residue following harvest are the most effective ways to minimize CYSDV in desert melon production,” said John Palumbo, entomologist with the University of Arizona (UA).

Palumbo has conducted CYSDV-melon trials at the Yuma Agricultural Center (YAC) in Yuma, Ariz. since 2007. He shared his knowledge and findings during a summer workshop held at the UA Cooperative Extension (CE) office in Phoenix, Ariz.

According to Palumbo, the whitefly and CYSDV have a number of host plants in common: the Cucurbitaceae plant family including melons, summer and winter squash, pumpkins, gourds, and cucumbers; plus winter romaine lettuce and several major summer weeds.

A voluntary, 25 day host-free period in Yuma County in 2007 and 2008 included a disk down after the spring melon harvest. The coordinated efforts reduced the whitefly’s ability to acquire, retain, and transmit the virus, and consequently reduced the incidence of virus in melon fields.

Growing melons is all about location, location, location. Whenever possible, Palumbo suggests planting fall melons away from cotton and alfalfa which are major whitefly hosts. Steering clear of orchards and residential subdivisions is also encouraged.

The diligent control of adult whitefly populations can reduce late-season buildup in spring melons and summer crops while minimizing the whitefly migration into fall melons. The proper use and timing of insecticides on seedling melons is critical to reduce the whitefly population down the road and ultimately, the incidence of CYSDV.

Fabric row covers can effectively prevent whiteflies from transmitting CYSDV during stand establishment and provide a good shelter for young melon plants until first bloom. In a 2007 YAC trial on whitefly colonization and CYSDV incidence, melons were planted under lightweight spun, bonded polyester fabric row covers including the mid-bed trench, floating, and tunnel types. The row covers were removed after pre-bloom, bloom, and post-bloom.

“The longer we kept the row covers over the plants the less incidence of CYSDV occurred,” Palumbo said. “The floating covers provided the best protection, particularly when used in conjunction with insecticides.”

Trials on the effectiveness of foliar and soil-applied insecticides for adult whitefly control and virus reduction showed positive results.

Of the foliar sprays on the market, the combination of Capture (bifenthrin) and Thionex (endosulfan) provided the best knockdown/residual control for up to 11 to 14 days between applications, Palumbo said.

In a 2008 trial, spring melons were intentionally planted late to encourage whitefly buildup.

“The compounds providing the best whitefly control in the trial were Fulfill in combination with Thionex along with the experimental product Pyrifluquinazon,” Palumbo said. “Three sprays of the products at eight to 14 day intervals provided 60 percent to 65 percent whitefly reduction.”

Pyrifluquinazon, also known as NAI-0101, is not yet commercially available to growers.

Fulfill and Pyrifluquinazon alter a whitefly’s feeding behavior.

In a just completed summer trial initiated after the first leaf stage in melons, Pyrifluquinazon and Venom applied separately, plus the combinations of Pyrifluquinazon-Venom and Capture-Thionex, provided a 50 percent-plus adult whitefly reduction.

Fulfill applied separately did not significantly reduce the virus.

Most melons grown in Yuma County are furrow irrigated. Some watermelons are drip irrigated.

In a summer 2008 trial six soil-applied insecticides were applied in the seed line about 3 inches below the seed.

“The combination of Coragen and Venom was the most effective soil treatment, but not any better than Venom used alone,” Palumbo said.

In the same trial, insecticides were also applied as in-furrow sprays, covering the seed during the planting operation. Overhead sprinklers were used to incorporate the materials downward in the soil.

“We saw no benefit from in-furrow sprays from any insecticide,” Palumbo said.

In a fall trial last year, Coragen, Venom, Venom-Coragen, and Durivo provided a 50 percent to 65 percent reduction in whiteflies and CYSDV compared to the untreated plot.

Palumbo said, “In all of the soil insecticide trials I’ve conducted to date, Venom probably has performed the most consistently and has the best fit against the virus and whitefly management in fall melons. The Venom label allows a foliar or soil application, but not both.”

Neonicotinoid seed treatments including Supresto and Crusier used alone do not offer significant whitefly control or suppression of CYSDV symptoms, Palumbo says.

Of the melon varieties on the market, Palumbo says the Loredo and Loredo-type varieties appear to be the most tolerant to CYSDV, but still require assistance from insecticides to make quality melons.

Drip chemigation is popular in Arizona’s La Paz and Maricopa counties. The UA’s first drip chemigation trial was conducted with and without row covers.

Palumbo says the foliar use of Venom by itself eliminated 60 percent to 65 percent of the virus. Venom used with row covers performed even better.

The bottom line is CYSDV management with chemicals and cultural practices from seedling emergence and to about first bloom offers good control.

While insecticides are expensive and cut into growers’ profit margins, Palumbo says some insecticides are very effective against CYSDV and the whitefly vector.

Early and aggressive insecticide applications can help produce high-quality melons in CYSDV infested fields with higher brix levels.

Palumbo is convinced insecticide use is more effective in commercial fields compared to his smaller experimental trials.

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