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.