White rot control continues to be the biggest challenge facing the research-driven marketing order for California garlic and onion producers.

“We’re a small group, but we’re committed,” said California Garlic and Onion Research Advisory Board Chairman and Fresno County, Calif., producer Kevin Lehar at the organization’s annual meeting last month in Tulare, Calif.

California produces 91 percent of U.S. garlic production. Fresno, Kings and Kern are the top garlic producing counties with 17,731 acres planted last year. Onion acreage is split between fresh market and processing with the total at nearly 49,000 acres statewide.

With the future of the small yet high-value garlic and onion industries threatened by the spread of the white rot fungal pathogen Sclerotinia cepivorum in the San Joaquin Valley, growers, handlers and processors established the marketing order to develop a white rot management program.

White rot is a soilborne fungus that strikes both garlic and onions practically around the world. Its reproductive structures, tiny, round, and poppy seed-size, appear on the bulb and base plate and spread through soil. It is now throughout SJV garlic-growing areas, brought in chiefly by infected cloves on harvesting equipment and bins. It can also be spread by sheep and goat grazing and, for short distances, by running water.

Lehar said the management program for white rot encourages inspection of all garlic seed fields, sanitation of field bins and equipment and minimizing equipment movement from known infected fields to clean fields. The plan also includes restriction of planting in areas known to have white rot, reporting all fields with white rot infestations, establishment of a garlic seed certification program and developing methods to reduce soil populations of white rot.

“We have to be our own police; we can’t wait for state or federal action,” Lehar said.

Citing field trials in the Tulelake area in Northern California, University of California, Davis graduate student Allison Ferry said a combination of Diallyl Disulfide (DADS) and a fungicide significantly reduced white rot pathogens in soil samples. In a 500-kilogram soil sample, white rot pathogens went from 120 to 24. If there is high disease pressure, Ferry noted, DADS is particularly effective.

Diallyl disulphide and related sulfides are the active ingredients in the end-use product DADS fungicide, which is based on a natural metabolite of garlic. DADS suppresses white rot disease on onion and other bulb vegetables, by reducing the level of Sclerotium cepivorum inoculum in the soil in the absence of a host crop. A biostimulant, DADS triggers growth of the white rot pathogen; then in the absence of onions or garlic for a carbohydrate source, it dies.

DADS cost very high

The dilemma with using DADS, according to advisory board technical director Bob Ehn, is that the product is extremely expensive. Applying DADS to an entire field costs $200 per acre.  As a result, few growers use it.

"It is only made in limited quantities twice a year due to low demand," says Ehn. There is an effort to secure grant funding to study the use of GPS to use variable rate applications to target only infected areas of a field.

Although white rot 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. 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. Mild winters encourage infections.

White rot is triggered by the characteristic hydrogen sulfide odor detected from onion or garlic fields after a rain. 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 seed.

Farmers cannot kill white rot. They must learn to manage it.

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.

Current research Ferry is conducting includes use of mass spectrometry to see concentration and movement of the fungicide Folicur in Tulelake clay soils and sandy soils. She is also comparing fungicide levels in plant roots, bulbs and leaves. Ferry said knowing how the fungicide acts in various soil types and how it correlates to the amount of disease in the field may allow for reducing the number of applications.

Biotech approaches

There are some biotech approaches to allium white rot reported New Zealand plant geneticist Colin Eady. Eady, who has been working on white rot resistant allium varieties, said that in theory they can be developed, but regulations on where and how biotech varieties are grown have hindered field trials.

Imperial County entomology farm advisor Eric Natwick said onion thrips must be managed because the pest vectors iris yellow spotted virus (IYSV). The disease reduces yield, bulb size and causes storage problems in garlic and onions.

Desert onion and garlic growers first experienced the virus problem seven years ago.

Infested fields have a silvery appearance, Natwick said.

Natwick said control options for the disease-vectoring thrips include biological control with natural predators. However, the predators may not be high enough in numbers to control onion thrips until later in the season — when the damage has already been done. Avoiding planting allium crops downwind of small grains — which attract thrips — can also help. Host plant resistance is years away from development, he added.

Cultural practices that can help control thrips include use of sprinklers which wash the thrips from the plants. Water droplets also inhibit thrips movement. Burying cull piles and removal of volunteer onions also can slow the spread.

Insecticide sprays can be effective, but chemicals must be rotated to avoid resistance. Natwick advised checking labels to make sure products are approved by the California Department of Pesticide Regulation.  Sampling five plants in four areas of a field is recommended. If more than 30 thrips per plant are found, fields should be treated, he said. That number can be lower for younger plants or higher for older plants. Timing of applications is critical to knocking down thrips populations. Sprays are most effective when weather is cooler. Use of surfactants is also recommended to get the material down into the leaves.

Tom Turini, a UCCE farm advisor in Fresno County, said IYSV has been found there, but at low levels.  In a 2011 field trial with processing onions, Turini said insecticide applications began when there were five to 10 thrips per plant detected. The materials used did reduce the thrips population in the fields, but he said there were no yield improvements.

In the Klamath Basin where the growing season is shorter, UCCE Siskiyou County Director Steve Orloff said growers need to protect their onion crops from thrips for eight weeks. His trials involved both chemigation and sprays with systemic and contact materials.

Garlic growers are still experiencing periodic outbreaks of Rust, fungal disease that devastated many fields in the 1990s, said UCCE farm advisor Steve Koike from Monterey County. Since those first Rust infestations, Koike said effective fungicides were registered and the disease has been managed. What is new are changes in the biology of Rust. Outbreaks of Rust in the 1990s did not affect leeks, but in the last few years Rust has been found in that crop. Koike said a 2011 trial found that Quilt Xcel at 21 ounces or Quadris at 12 ounces worked in Rust suppression. Rust is still a production factor in allium crops and no tolerant cultivars have been developed. Rust management guidelines have been initiated, Koike added.