Management of iris yellow spot virus (IYSV), a global disease of onions and other allium crops spreading through California and other Western states, eludes any single practice or material and calls for an integrated approach until adequate plant resistance can be developed.
That’s the latest word from researcher Hanu R. Pappu, associate professor of plant pathology at Washington State University at Pullman. Pappu has been collaborating with other plant pathologists and industry groups in the Pacific Northwest, Colorado, Utah, and California.
Although scheduled to make his report at the California Garlic and Onion Research Advisory Board’s symposium in Tulare, Pappu was unable to attend the gathering due to an airport closure. His findings were presented by Eric Natwick, University of California Cooperative Extension director for Imperial County.
IYSV, one of the tospovirus group, has a broad host range, including onions, garlic, leeks, chives, and several ornamentals and weeds, Natwick explained.
Its symptoms are yellow- to straw-colored, diamond-shaped lesions on leaves and scapes. Infections have been mistaken for herbicide burn. Late in the season infected plants lodge and plant vigor and bulb size are reduced.
It was first discovered in iris in the Netherlands, hence its name. Known in Oregon and Idaho onion fields since the early 1990s, it was first identified in California in 2002.
Plant pathologists are struggling to learn more about how to detect the virus, which is thought to have multiple strains. Natwick noted that it first appeared in Southern California about six years ago with some variation of the basic symptoms and was not immediately a cause for concern.
“Since then it’s been a developing disease problem and we need to be very aware of it and the importance to the industry,” he said.
IYSV is vectored mainly by onion thrips and perhaps by other thrips species, and Natwick, an entomologist specializing in thrips management, said the disease survives in “green bridge” situations of overlapping green and processing onion crops.
Volunteer onions, weed hosts such as puncture vine, kochia, or prickly lettuce along roadways, and onion culls also harbor the virus. Additional weed hosts are being identified. Adult thrips can move it from outside a field or larvae can acquire it and distribute infection within the field.
“Dealing with IYSV can be a complicated process, and if you suspect you have a problem get professional help from your farm advisor who can relay it on to the university plant pathologists for identification,” Natwick said.
“Field sanitation is very important, and remember that the crop, like a human, may have a more severe infection if it is stressed.”
Important too is crop rotation, in cooperation with neighbors, to discourage a buildup of the virus. “If you plant onions downwind from a small grains crop, you can count on getting a bunch of thrips as that crop matures. “Work for a healthy, uniform stand; separate bulb and seed crops, and keep a distance between susceptible crops and fields with a history of IYSV,” Natwick added.
For the time being, there does not appear to be great difference in resistance to the virus between varieties, but plant breeders are working on that goal, he said. New developments are posted on the Web site Alliumnet.com.
Turning to his own presentation on management of thrips and IYSV, Natwick noted that thrips species abound and only about 1 percent of the known 5,000 species of thrips are considered pests. Due to their tiny size, positive identification requires use of a powerful hand lens, if not a microscope.
In onions and garlic, the major concern is onion thrips and western flower thrips, which occur simultaneously in various ratios. Onion thrips are most important in these crops.
“They have an extensive host range, and this leads to lots of problems. Since they are pests to so many crops, too much insecticide use can cause resistance.”
Key to a management program is identification of the problem species and an understanding of the biology of the pest, along with the extent of economic injury and monitoring to determine a treatment threshold.
“You want to treat before they reach an economic injury level. But you also want to consider biological controls, host plant resistance, and cultural controls,” Natwick said.
Among natural enemies of thrips are minute pirate bugs, predaceous mites, and lacewings, although these do not build up sufficient numbers to prevent crop injury.
In the absence of complete resistance to thrips or IYSV, where possible, varieties having any marginal tolerance of the pests should be used. Transplants that are thrips- and virus-free should be used.
In addition to field and roadside sanitation to remove hosts, cultural management practices include use of sprinkler irrigation and straw mulch for some suppression of thrips.
Natwick said knowing the biology and life cycle of the target pest is important, and a rule-of-thumb to remember is immature stages are generally easier to control than adults.
Insecticides such as Lannate, Vydate, Entrust, Radiant, Mustang, Warrior, and azadirachtin are effective. During hot weather, application during the early morning or the evening when the thrips are more active is recommended.
Rotation of classes of chemistry aids in preventing insecticide resistance, and use of surfactants helps materials reach hidden larvae.
In an account of highlights of the board during the past year, Chairman Kevin Lehar said the state marketing order has been approved for continuation for another five years of operation.
Lehar said the main purposes of the board are to maintain California’s leadership in onions and garlic and to deal with white rot in the crops. The board, composed of growers and handlers, has an annual research budget of $100,000 aimed at controlling white rot and other pests.
Lori Berger, executive director of the California Specialty Crops Council, in updating the symposium on regulatory issues, said crop nutrients will be subject to increasing attention.
The Tulare-based council represents 20 crops, ranging from onions and garlic to fruit, vegetables, cut flowers, and honeybees.
“Use of fertilizers is going to be scrutinized as much as pesticide use, and the bad thing is there is very little data to work with,” she warned.
While the garlic and onion industry is making progress with new pesticide registrations to deal with key pests, she said, the industry’s production challenges are being conveyed in cooperation with the council to state and federal regulatory agencies. A chief element of this approach is on-farm tours for agency officials.
Also important are the council’s continuing efforts to respond to retailers’ questions about growing practices and social benefits of specialty crops, Berger added.