Iris yellow spot virus (IYSV), a serious disease of onions in Southern California and the Pacific Northwest, is something San Joaquin Valley growers should be watching for, says Tom Turini, Fresno County farm advisor.

Speaking at the California Garlic and Onion Symposium in Tulare, Turini recommended growers be attentive especially to removing volunteer onions and related wild plants and making sure their onion transplants are virus-free.

A member of the tospovirus group, IYSV is transmitted by onion thrips, and its symptoms are usually diamond-shaped, yellowish lesions on leaves and scapes of onion. Infected plants tend to lodge late in the season, and plant vigor and bulb size are reduced.

The virus goes to most alliums, including iris and related ornamentals, as well as weeds such as jimsonweed, tobacco, and redroot pigweed. Garlic was previously not thought to be a host, but findings by Washington State University scientists in 2008 indicated otherwise.

Infections are usually most severe near edges of fields, and sources are often bulbs left in the field, in addition to host weeds.

Turini, at his present assignment for the past two years, previously spent eight years with Cooperative Extension in Imperial County, where he became familiar with IYSV, mostly in onion-seed fields.

“I’ve also seen it on the west side of Fresno County, but it’s typically not been as great a production issue in California as it is in Idaho, Oregon, Washington, and Colorado,” he said.

He added, however, that it caused major losses in the Antelope Valley in late varieties during 2005-06, and in 2008, it was found in the Sacramento Valley.

While no immunity to the pathogen has been reported, he said “there have been many reports over the years of different susceptibilities to the virus among different varieties.

“Waxy coverings on leaves and plant structures may play a part in relative susceptibilities to thrips and to the virus.”

For the moment, Turini said, the best control in the SJV is elimination of volunteers and planting of virus-free material in transplant fields.

Detailing his experience with IYSV and its thrips vectors, Eric Natwick, UC Cooperative Extension director for Imperial County, said management of the insects is a major front but is difficult because of their small size and secretive behavior.

“The larvae, particularly the first instar that acquires the virus, get into leaf shoots where they are difficult to control,” he said.

He stressed that his work with Turini several years ago showed that spraying for thrips can lower the amount of IYSV-infected plants, but it has no effect on the disease severity in infected plants.

Based on what is now known, he said, “You can’t spray your way out of an IYSV problem.”

Natwick warned that IYSV could potentially become a problem in onions in the SJV, but growers have been coping with it in the southern desert, except for the outbreak in the high desert.

Although several thrips species exist in the state, the two major species are western flower thrips and onion thrips, and the latter can cause significant damage, even if they are not carrying the virus, in onions, either in the field or in storage.

“The most important thing we can do is make accurate identification of the thrips species. There’s not much we can do for biological control in onions, but we also don’t want to ignore it,” Natwick said.

Establishment of natural enemies is typically difficult on an annual crop, and if they are present, the crop damage is already done by the time they build sufficient numbers to counter the pest species.

Guidelines for identification are available at www.biocontrol.ucr.edu. Basically, with adequate experience and magnification, a person can distinguish formations of thrips body hairs to identify species.

Better results can be had by cultural controls, including use of sprinkler irrigation, avoiding planting near hosts such as small grains, use of thrips-free transplants, and field sanitation to remove culls and other crop residue. Maintaining good soil fertility and adequate soil moisture will avoid plant stress.

If insecticides are used, timing is extremely important, and he recommended spraying during the cooler hours of the early morning or evening when thrips are more active.

Use of an oil or surfactant to move a contact material into crevices of the plant will improve the effect.

“None of the materials really work that well, but Lannate, Vydate, Success, Radiant, Mustang, or Warrior are more efficacious than others. Remember to use only products registered in California, and regardless of which works best, rotate classes of insecticides, as indicated on labels, to manage resistance,” he reminded.

Natwick noted that while only onion thrips transmit IYSV, western flower thrips can damage the crop, and high numbers of either can cause losses. More thrips can be tolerated later in the season.

To determine whether to spray, he suggested random sampling of five plants from four separate areas of a field, taking care to pull apart the plants to find all the thrips.

“We don’t have any hard-and-fast thresholds, other than about 30 thrips per plant at mid-season. For younger plants you might lower it a bit. Nearer to harvest you can raise it a bit.”

Natwick said he plans to investigate atriplex as a potential weed host used by IYSV to bridge between onion crops in the Imperial Valley.

Yellow nutsedge is a tough weed pest of onions, but Michelle LeStrange, Tulare County farm advisor, said recent trials signal a breakthrough with use of the two mitosis inhibitor herbicides, Outlook and Dual Magnum, against the tuberous weed.

She noted that although nutsedge can be culturally controlled by crops such as corn that shade it out, alliums do not provide enough shade to compete with it.

The field research was done 2007 and 2008 by farm advisors Richard Smith of Monterey County and Oleg Daugovish of Ventura County.

The two products have no postemergence activity and must be applied preemergence at the second true leaf stage of the crop. They delay sprouting of the weed’s nut-like tubers and can cause them to die off.

However, the Monterey County trials showed that if the nutsedge has emerged by the second true leaf stage, burning back the nutsedge with 7-7-0-7 fertilizer provides an opportunity for Outlook to work.

“There may be a yield reduction from the caustic action of the fertilizer, but it is much less than letting the nutsedge go uncontrolled,” Smith said after his trials.