The California garlic and onion industry is ramping up its campaign against white rot, a devastating, worldwide, fungal disease often called the “AIDS” of allium crops.
No single practice is completely successful for control, and plant pathologists say a combination of chemicals and other actions is the only solution.
While new approaches are being developed and evaluated, the industry has been planting disease-free seed, avoiding planting in infested fields, removing infested plants, and cleaning harvesting and hauling equipment.
White rot, Sclerotium cepivorum, common in the Gilroy area since 1950, infected more than 14,000 acres (over 90 known fields) in the San Joaquin Valley between 1994 and 2007. It also occurs in desert, coastal and far Northern California counties.
Its tiny sclerotia, no larger than poppy seeds, invade an allium crop with seed, transplants, water, tillage, harvesting, animals, or wind. An infection of as few as 20 plants can explode across an entire field after disking or cultivation.
The disease, which also occurs throughout allium production areas across the U.S. and around the world, strikes only those crops, destroying the plant when it enters roots, stem plates, or bulbs.
Once established, it can inflict yield losses of 50 percent to 80 percent and remains in the field indefinitely, preventing replanting to any allium species.
The economic loss from the disease, including direct crop losses, removal of acreage from allium production, costs of research, and other expenses, has not been calculated.
The California Garlic and Onion Research Advisory Board (CGORAB), established in 2005 as a state marketing order largely to combat white rot, recently convened a symposium of growers, handlers, processors, and university researchers. They met in Tulare to exchange the latest research developments against the disease.
Bob Ehn, chief executive officer and technical manager of CGORAB, said they have allocated $100,000 from industry assessments for the current year for research on white rot, garlic rust, and iris yellow spot virus.
A new grant of $40,000, split between this and next year, has come from the California Department of Pesticide Regulation. It is earmarked for demonstrations of biostimulants on an anticipated 500 acres of white-rot infested ground in the San Joaquin Valley and the Tulelake area.
Nevada's garlic and onion industry has also contributed $10,000 to the research effort. Additional research support is being contributed by USDA-ARS at the Parlier, Calif. station.
Ehn said Bayer CropScience's Folicur fungicide, which has performed well for several years of limited registration in controlling white rot and other allium diseases, is anticipated to have full registration by the EPA in May and by the California Department of Pesticide Regulation 60 to 90 days later. It is proposed in a two-pronged effort: Folicur at fall planting, following biostimulants applied the preceding spring.
Short-term research approaches to solving the disease problem include biostimulants, seed treatments, in-furrow fungicide treatments, and soil solarization after flood irrigation. Evaluation of existing varieties and breeding for new germplasm for resistance to the disease is a long-term research effort.
On biostimulants, Mike Davis, plant pathologist at the University of California, Davis, has been researching “DADS,” or diallyl disulfide, a compound that when applied in the spring or fall mimics the odor of garlic plants and causes premature germination of white rot sclerotia before the next crop is planted.
The compound, combined with field sanitation, destroys 95 percent of the sclerotia, but the remainder is sufficient to set off an infestation that would prevent the field from being profitable, Davis said.
“But I'm a fan of DADS,” he added. “With that 95 percent we can really slow down the spread of white rot. So we need to combine it with another strategy.”
Related studies with garlic juice as a biostimulant through irrigation water have been done by Fred Crowe, emeritus plant pathologist with Oregon State University at Madras, an area where garlic seed crops for California are grown.
Crowe told the symposium he set out to verify the concept of biostimulants so other researchers could hopefully refine the basics for more success. His work began before the availability of DADS.
Although his data was very preliminary and trials need to be repeated, Crowe said he saw some response from the treatments and believes the concept might be worth further investigation, perhaps with drip irrigation.
Harry Carlson, Modoc County farm advisor, working in cooperation with Crowe and Davis, said he saw up to 85 percent control of heavy white rot in onions with Folicur, applied in-furrow at planting and followed up with broadcast applications.
He said the material represents “a breakthrough” for white rot control in dehydrator onions in the Tulelake area. There were reports of suspected phytotoxicity from cooperating growers, and he said additional evaluations with it and other products are needed.
He speculated that the control was caused by a layer of fungicide preventing the disease from reaching the root system.
Folicur in-furrow, plus Maxim, a registered Syngenta fungicide as a pelletized seed coating, gave 89 percent control in Carlson's other trials. Endura, an unregistered BASF fungicide, gave 80 percent control at high rates.
Discussing the bioengineering approach was Colin Eady, plant geneticist at the New Zealand Crop and Research Institute at Lincoln. Eady has an international reputation in onion biotechnology and is credited with development of a “tear-free” onion trait.
He has been investigating placement of white rot resistant genes from other species into onions since 2002. Noting that onions and garlic are more difficult to work with and thus lag behind other crops in genetic transfers, he said his approach is to incorporate genes for enzymes that neutralize the harmful compounds in white rot.
His current work is promising, he said, and “is about 100 times improved over previous garlic transformation systems.” Even so, expectations are that it could take as much as 10 years before resistant material is available for commercial use.
CGORC is funded by mandatory assessments, based on harvested tonnage and shared equally between growers and handlers. The board of directors consists of six grower members and six handler members, plus alternates and ex-officio members.
Members include fresh, dehydrated, processed, and seed segments from onion and garlic production areas of California's southern desert, San Joaquin Valley, and Tulelake area.