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Iris yellow spot virus poses threat to onions

If onion fields resemble hay pastures, growers should be suspicious of iris yellow spot virus.

“We saw some fields in the Lower Rio Grande Valley this year that looked like hay,” said Texas Extension specialist Juan Anciso at the Texas Produce Convention recently in San Antonio.

Anciso said the Texas Experiment Station at Weslaco will conduct field trials to identify management strategies for the disease, which can cause severe economic loss.

“We’ve requested support of $7 million over the next five years,” Anciso said. “The Senate approved $250,000 for next year.”

Anciso said research efforts will include plant host resistance as well as pesticide trials. Fungicide tests during the 2006 season indicated no significant difference between treatments but did show benefits up to 50 bags per acre with both copper and phosphoric acid products.

Anciso said growers should look for the hay effect as well as diamond-shaped lesions on onion leaves. Elongated white spots also indicate potential for the disease. “This year we saw some gold spots,” he said. “That’s not a classic symptom.”

He said gold or white spots on the edge of the leaf also indicate potential disease infection. “Also look for a green island inside the damaged area.”

He cautioned producers not to confuse iris yellow spot virus damage with thrips injury. ‘Onion thrips transmit the virus,” he said. “Western thrips do not.”

Typical thrips damage might include “silvering” the plant. “Heavy thrips damage, however, could be an indicator for the virus.”

Nationally, the virus has shown up in Georgia, New Mexico, Alabama, California, Idaho and Colorado.

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