Six samples taken in three Winter Garden fields in Southwest Texas late last spring indicate a virus may have become a significant problem in onions.

Though no wide-sampling efforts were conducted, initial testing indicated iris yellow spot virus, which forms lesions on leaves and causes the tips to die back. However, the enzyme-linked immunosorbent assay (ELISA) test -- the only test used to date -- can sometimes return false positives, said Dr. Mark Black, a Texas Cooperative Extension plant pathologist in Uvalde.

Further complicating diagnosis is its range of symptoms, Black said. "The symptoms of iris yellow spot virus are not always diagnostic, and more often there is overlap with early symptoms (and symptoms during aggressive fungicide use) of about four fungal diseases of onion foliage," he said.

The virus has plagued many Western onion-growing states, but has not previously been diagnosed in Texas.
Black is working with Dr. Tom Isakeit, a College Station-based Extension pathologist, to pinpoint the cause. Only six samples were tested from the Winter Garden at the end of the growing season. However, plant material was not of quality to reproduce the virus in the laboratory by the time ELISA tests were received, Black said.

During an August grower meeting, Black told producers Texas has had problems with two other closely-related viruses -- tomato spotted wilt virus and impatiens necrotic spot virus. Tomato spotted wilt virus has been found in peanut, tomato, pepper and greenhouse-grown ornamentals. Impatiens necrotic spot virus has been mostly in greenhouse-grown ornamentals.

"Hopefully, experiences with these virus relatives will enable us to move forward on definitive diagnosis and developing control strategies in onions," Black said.

Though samples were taken in only a few fields, consensus is building among onion growers that a funded research initiative is needed since iris yellow spot virus has become a major problem in many onion-growing states.

"Industrywide, it's a No. 1 concern," said Dr. Juan Anciso, Extension vegetable specialist in Weslaco. "I was talking to the onion growers association, and they say it's high priority to initiate funding."

Most recently, the virus has been discovered in Arizona, California, Nevada, Oregon, Utah, Washington and Georgia.

Iris yellow spot virus previously has not been recognized as a major problem in Texas, pathologists say. But because of vague symptoms, it may have been overlooked for a few years, as was the case in the other states.
The virus is transmitted by onion thrips, which are small, slender insects with fringed or feather-like wings. Thrips can also use weeds as hosts, Isakeit said. High thrip populations could also increase the potential for spreading the virus.

Isakeit said when the virus infects the plant, "The onions' leaves will get a diamond-shaped pattern."

Flecking and elongated yellow lesions may occur instead, Black noted.

Thrips already are a growers' enemy since they feed on the leaves of onions and have become resistant to many pesticides. Lannate is the only effective commercial product left for producers, Anciso said.

"That's not good if there's only one out there," he said. "They (the thrips) are becoming more tolerant to it."
Producers can also control weeds and volunteer onions, which "can serve as thrip hosts (and perhaps virus hosts)," Isakeit said.

Increasing the number of plants per acre has reduced the disease in other states. Sanitation methods such as destroying cull piles and unharvested bulbs that could carry the virus over the summer are also recommended.
Though onions in the Lower Rio Grande Valley were not tested for iris yellow spot virus this spring, "thrip populations were high, and in retrospect, symptoms were within the range reported for iris yellow spot virus," Black said.

Some onion varieties may not be as susceptible to the virus as others, Isakeit said, so "it's a good idea to start variety testing to see if there are any differences."