'Dads' thins out a pesky fungus

Researchers work to reclaim prime onion- and garlic-growing acres

Tam Moore
Freelance Writer

Capital Press

August 24, 2007

TULELAKE, Calif. - In a plot not far from the University of California Intermountain Research and Extension Center office, a stand of onions grows behind the sign "Dads trial."

You might ask "whose dad?" Or wonder why most of the Klamath Basin 's onion farmers turned out in late June to visit the thrifty stand of processing onions.

What they and station Director Harry Carlson knew is that just one year ago that ground was lost to onions. The soil was laced with dormant seed-sized objects called "sclerotia," which carry white rot, a killer of all allium varieties, including onions, garlic and leeks.

After Carlson and his research crew sprayed a smelly sulfur substance on the ground when the plot's 2006 barley crop came off, the path opened to retaking the contaminated ground. That smelly spray is diallyl disulfide, named "dads" by growers and scientists. Incorporated into the soil, dads triggers white rot spore production when there aren't any onions around. The spores died without forming new sclerotia, breaking the disease cycle months before this spring's onion seeding.

Dads and a variety of other management treatments promise to stem the spread of white rot, allowing farmers to reclaim infested ground.

The other spray showing promise here is garlic juice.

California growers and handlers, after two years of a voluntary sanitation program, in 2005 launched a research marketing order that's paying for testing dads, garlic juice and complimentary fungicides.

White rot, Sclerotium cepivirum, is a problem worldwide. Onions and garlic are high-value crops grown in all Western states. California has about 75,000 acres of commercial onions and garlic planted this year, but about 13,000 acres of the best ground can't be used because of lingering white rot infestations.

Mike Davis, the UC plant pathologist specializing in California 's vegetable crops, described white rot as the worst disease of the onion family, worldwide. Sclerotia can lie dormant in soil for decades. Davis said just one of the tiny pellets in 10 liters of soil is enough to grow into a white rot hotspot. All it takes is the roots of an onion or one of its cousins sending out the activating chemical signal.

"There's been a lot of research worldwide for a number of years, with no silver bullet," Davis told the Tulelake growers.

But the combination of triggering spore production without onions present, in-furrow fungicide sprays and seed treatments promises to take back infected ground - if you can justify the cost.

The California Onion and Garlic Research Advisory Board is funded by assessments of 1.25 cents a hundredweight on onions, 2.5 cents cwt. on garlic.

Dads is a sample of its work. Bob Ehn, a veteran pest control adviser who once worked for a consortium of onion and garlic growers and handlers, is executive director of the new research advisory board. He also has his fingers on several hundred gallons of dads imported from China where it is used as a food flavoring.

The Chinese connection was discovered after a Louisiana chemical plant abandoned dads production. Davis said the U.S. firm estimated national demand would be 2,000 to 3,000 gallons a year, too small to justify cost of the chemical synthesis.

The imported dads costs $200 a gallon by the time it hits the dock at Long Beach , Calif. In Davis ' first experiments, carried out at infested growing areas in California and Nevada , the application rate was 1 gallon for every treated acre. Figure in the labor and equipment and you get an idea of what it might cost to thin sclerotia.

Carlson last week published data showing a half-gallon rate of dads killed 80.4 percent of sclerotia, compared with 84.7 percent kill for the 1-gallon rate. Davis is thinking of a split application, half rate when soil temperatures are right in the spring, and half rate again in the fall before soil temperatures drop.

"You could get 85 percent and then 85 percent of what's left," he speculated in answer to a grower question.

"But you won't get it all. There's a need to follow up with another treatment (of fungicide) at planting."

Davis described the two treatment methods as "very important tools" in reducing the pressure of white rot. Unchecked, it spreads by flooding, by leveling fields or seed bed formation, and even by the wind carrying scales of infected onion or garlic bulbs to distant fields.

Dads isn't the only agent that triggers sclerotia into action without a host. Next to that trial, Carlson planned to take down another barley plot and spray the ground with the garlic oil extract. In England scientists have tried both laboratory and field tests of a compost made primarily from onion processing waste. They found the compost worked well in peat soil, but didn't lower sclerotia counts in sandy soils. In Syria , scientists six years ago uncovered eight mutant plants that resist white rot, opening up possibility of genetically resistant commercial plants.

For the research board's Ehn, the answer comes down to grower management, starting with mapping actual areas where white rot killed part of a crop. That can lead to spot treatments with expensive material such as dads, and to paying attention so soil isn't spread farther into a field.

Ehn is leading crop management research on an infested area near Coalinga , Calif. He has asked California 's Department of Pesticide Regulation for a grant to support part of the costs when farmers use dads on their ground.

"But how do you use it without a management plan?" Ehn asked.

His hope is that an August agreement with University of California will launch more management experiments in several locations of the state.

For California growers, the help comes at a time when an estimated 1 million pounds a year of Chinese garlic is imported into the United States . Getting U.S. production costs down - which includes getting back on the most productive ground now lost to white rot - is part of the mandate given the Onion and Garlic Research Advisory Board.

Freelance writer Tam Moore is based in Medford , Ore. E-mail: moore.tam@gmail.com