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New Virus Plagues Desert Melons

A new whitefly-transmitted virus identified as Cucurbit Yellow Stunting Disorder Virus hit the fall melon crop for the first time this year in Arizona and may also account for some crop damage in the Imperial Valley, though it was still being confirmed there. The virus caught growers and PCAs by surprise not only because it is new, but also because it was so widespread in Arizona.

“There has been a pretty widespread incidence of this virus occurring in this area,” said Mike Matheron, University of Arizona research plant pathologist at the Yuma Agricultural Center. “Every melon field was affected to some degree or another.”

The virus, which was first identified in the U.S. on melons in West Texas, was confirmed in Arizona after widespread damage this season. DNA testing was still underway to confirm its presence in the Imperial Valley, where fall melon growers also reported unusual symptoms and some crop damage.

The virus is transmitted by whitefly after feeding. Older leaves on infected plants turn yellow, starting with a splotchy light green mottling that progresses later into completely yellow leaves.

“As the disease progresses, more and more of the leaves get this yellow color. Initially if you look at the field, it looks like a yellow strip down each row that in time gets wider as more leaves become infected,” Matheron said.

In its native Middle East and in Texas, where the disease is also confirmed, the Yellow Stunting Disorder Virus leads to yield reductions and reduced quality as a result of lower sugar content. Arizona growers are reporting similar losses.

“One grower in Arizona told me that if his melon plant normally puts out four melons, the virus-infected one would put out three,” he said. “But you could get even further reductions in marketable melons because infected melons don’t reach size or have lower sugars. It all depends on how early the plant or field got the disease.”

The virus is closely related to lettuce infectious yellows virus that lettuce and then melon growers first spotted years ago. That virus is no longer considered a pest in Arizona because the whitefly that transmitted the virus is no longer the predominant whitefly species in Arizona. Matheron said the yellow stunting disorder virus appears to be exclusive to cucurbits.

He said good sanitation will be key to helping prevent a re-infestation of the virus next year. He suggests growers rapidly disk their fields after harvest and manage resprouts diligently.

“We can hope that next year the conditions that brought this about won’t be the same, as virus intensity does go up and down from year to year. But one of the important things for virus diseases in general is you don’t want to carry over infected plants from year to year,” he said. “Fall melon plantings right now should finish up in another month and if all those plants are destroyed by spring we will have broken the cycle.”

Matheron said managing whitefly populations will also help, although controlling the vector helps reduce but not eliminate the disease because the vector must feed on plants before applied materials can kill it.

“Application of insecticides is a secondary way of managing the disease,” he said. “It’s better to somehow kill the whitefly before it has a chance to feed on plants.”

Researchers this year will continue studying the biology and origin of the virus and its vector and looking at other potential hosts to provide more information to growers and PCAs for next season.

“Time will tell if it’s going to be a perennial problem of the same intensity,” Matheron said.

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