AIDS of allium crops: Garlic industry develops white rot control program

The California garlic industry is developing a four-year routine it hopes will manage white rot, a disease considered the “AIDS of allium crops” throughout the world.

Bob Ehn of the California Onion and Garlic Research Committee (COGRC) talked about the disease and measures to bridle it during the recent Kern County Vegetable Crops Meeting at Bakersfield.

COGRC, formerly the American Dehydrated Onion and Garlic Association (ADOGA), is composed of ConAgra, Sensient Dehydrated Flavors, De Francesco and Sons, Christopher Ranch, and Empire Foods.

COGRC's objective to see that garlic production and processing meet both government and industry standards. It operates with funds contributed by processors.

White-rot pressure has risen in recent years in the West Side of the San Joaquin Valley as acreage migrated there from the Salinas Valley. Ehn said garlic has become an important rotational crop and land free of white rot and suited for production of dehydrator garlic has become scarce.

White rot, Sclerotium cepivorum, is a soilborne fungus that strikes both garlic and onions practically around the world. Its reproductive structures, tiny, round, and poppy seed sized, appear on the bulb and base plate and spread through soil.

Long soil life

Although its inoculum declines year-by-year in the absence of allium crops, enough to regenerate an infestation, or strike, can persist in soil for as long as 40 years, and Ehn said that's the big problem in managing it.

It moves unseen at a rapid rate through roots. Damage is not observed until it reaches the base plate of a plant. Death of the plant comes soon afterward. It appears more in garlic because planting times coincide with the end of its overwintering phase. Soil temperatures of 58 to 60 degrees are ideal for it to spread. The mild winter of 2002 and, so far, that of 2003, have encouraged infections.

White rot is triggered by the characteristic hydrogen sulfide odor detected from onion or garlic fields after a rain, and COGRC sees the offensive odor as a key to management of the pathogen.

It is now throughout SJV garlic-growing areas, brought in chiefly by infected “seed,” or actually, cloves, on harvesting equipment and bins. It can also be spread by grazing of sheep or goats and, for short distances, by running water.

Ehn noted that if there were a way to plant with true seed (as onions are) instead of vegetatively with cloves, perhaps spread of the disease could be controlled more successfully in garlic. White rot readily attaches to garlic crop residue and cloves for seeding, whereas few diseases are spread by true seed.

Industry alarmed

The industry, according to Ehn, is alarmed because recent infestations in the SJV were not three or four plants as seen in the past but two- to three-acre patches that spread rapidly throughout a field.

“You don't kill white rot,” he said. “We have it and the only thing we can do is learn to manage it.” COGRC has developed procedures to confine infestations. At the moment, they inspect all fields, and strikes are mapped by GPS and recorded. Known infestations are spot treated, and only seed from certified, disease-free fields is used for planting.

White rot closed down garlic in the Tulelake area, where most of the California industry seed once was produced. Seed is now grown in white-rot-free fields in Shasta County, Nevada, and Oregon.

The privately funded research effort by OGRC and earlier by ADOGA, he said, has spent more than $100,000 since 1986 in trying to come up with ways to control white rot.

Plant pathologists at the University of California and University of Oregon have assisted with practical methods, and the Nevada Onion and Garlic Council has also cooperated.

Tricking disease

One element in the management effort is the use of garlic powder, not considered a pesticide and available for use, to “trick” the disease into first germinating in the absence of a crop and then dying off without a host. The applied garlic powder acts by emitting the hydrogen sulfide odor to stimulate the disease just like a crop would.

“The problem,” Ehn added, “has been getting the garlic powder to the correct depth in the soil. It is hard to work with, but it has reduced populations almost to the extent that methyl bromide did.”

Another compound showing some promise is a petroleum distillation product, diallyl disulfide, also known as DADS or Alliup, which also tricks white-rot sclerotia into germinating with the garlic odor. It is used in Australia and New Zealand but is not presently available in the U.S.

“The problem with it is it only does one thing on one crop. All the onion and garlic acres that have white rot are not nearly enough for a chemical company here to consider producing it.”

Among other controls considered are methyl bromide or metham sodium spot treatments. However, they would have to be teamed with other techniques for any appreciable gain. Seed and in-furrow treatments with Folicur and Switch also show some promise. Neither compound is registered for garlic, but both are in IR-4 trials to pursue registrations.

Management proposal

The proposed management regime to minimize soil populations would be: Year 1: identify fields with white rot. If the disease is detected in the fall, spot-treat with metham sodium through sprinklers to kill the white-rot sclerotia on the soil surface. Year 2: rogue out all volunteer garlic plants to remove disease inoculum. Year 3: apply garlic powder. Year 4: apply garlic powder again prior to planting.

“Then we may be able to rotate back to garlic, if we can use in-furrow applications of Folicur and Switch. From an economic standpoint, we don't know how that will work in terms of grower utility.”

California garlic posted 39,000 harvested acres for 2000. In the last decade, in addition to the increasing white rot problem, the industry has been stricken by multiple issues: potential loss of existing crop protection materials, the difficulty in registering new materials, and imports of Chinese garlic, estimated at 50,000 tons in 1999.

Imported Chinese product, competing for the same market window as domestic supplies but selling at a third to a half what California growers expect, has caused hardship in the California industry.

Typically, about 60 percent of California's annual garlic crops of some 6 million pounds has been dehydrated, with 20 percent to 25 percent sold fresh and the remainder used for seed.

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