Foreign competition, disease and escalating production costs have taken a substantial toll on domestic garlic and onion crops, but the final shot has not yet been fired.

UC researchers are fighting back. White rot (Sclerotium cepivorum) is one of the most devastating diseases on allium crops (onions and garlic) in California — particularly for Salinas Valley producers.

“The incidence of white rot in the Salinas Valley is part of the reason most of the state’s garlic production is now in the San Joaquin Valley,” says Mike Davis, UC Extension plant pathologist.

Even that production is in jeopardy due to white rot, according to Robert Ehn, technical manager for the California Garlic and Onion Research Advisory Board.

“White rot has caused our garlic production to shift from the Salinas/Santa Clara Valleys to the West Side,” he says. “We have practiced avoidance by identifying white rot fields and then never planting an allium crop in those fields again. But, we’re beginning to run out of prime allium land on the West Side.”

UC researchers have been wrestling with the white rot problem for years. One of the most promising developments has been research on the use of diallele disulfide (DADS). A derivative of garlic, DADS triggers germination of white rot sclerotia in the soil before a new allium crop is planted. Under a non-host crop, the compound greatly reduces surviving white rot sclerotia that survive to infect a later-planted allium crop.

Recently, the California Department of Pesticide Regulation funded a two-year study to further evaluate the usefulness of DADS in commercial allium crops.

“The study will start this fall in Fresno and Modoc counties,” Davis says. “It will also include materials not labeled for this exact use. Maxim and Folicur are the fungicides with the most promise at this point, and they will be evaluated along with DADS.”

The material has been very promising in past research trials, according to Davis. “It’s very effective,” he says. “It kills about 95 percent of the sclerotia.”