FRESNO COUNTY, Calif. - More than 10,000 acres on the West Side of California’s San Joaquin Valley have “Do Not Plant Garlic and Onions” labels plastered on them due to the spread of white rot.
White rot is caused by a soil borne fungus, and once a field is infected garlic and onions can no longer survive there, according to Bob Ehn, technical manager of the California Onion and Garlic Research Committee.
This red-lined acreage represents 71 infected fields in what Ehn calls the “heartland” of the best garlic production area of the valley around Huron and Five Points, Calif.
“We cannot keep dodging the white rot bullet by avoidance. At some point we have to wake up and try to manage this beast,” said Ehn.
To that end, Shannon Mueller, a University of California Cooperative Extension Fresno County farm advisor, will host an April 29 field day in a heavily infested white rot field where she is testing fungicides for white rot control.
Next Thursday’s field day will begin at 10 a.m., and participants can obtain directions to a gathering area from Mueller.
“Because of the field’s location we need to reduce traffic into the area. We will be gathering at a nearby central location to carpool to the site,” said Mueller.
The farm advisor can be contacted directly at 559/456-7261 for directions. Mueller’s e-mail is [email protected] 
White rot fungus probably was introduced into the San Joaquin Valley from the Salinas, Calif. area on infected seed, said Ehn. It was first identified in the San Joaquin in the early 1990s.
Once infected, the sclerotia can lay dormant in the field for at least 20 years and maybe longer. When garlic or onions are grown on infected soil, the fungus can go down the germination tube of garlic or onions and cause white rot into the bulb of the plant.
It is not just a California problem. Ehn said it is in most garlic growing areas of the world.
It can wipe out an entire garlic or onion field, especially if infected dirt is ragged around by a land plane or other tillage equipment. It can also spread rapidly in irrigation water or by equipment moving from an infected to a clean field.
“We have been putting a lot of research dollars into trying to find a way to manage it,” said Ehn.
One line of defense is the use of a biostimulant that can be used the season before a garlic or onion field is planted. According to Ehn, this biostimulant “wakes up” the sclerotia into believing there is a food source in the field. When the fungus cannot find a garlic or onion host, it expends its energy and may die.
“This can offer 85 to 90 percent control, but we have to find an in-season chemical control technique that will offer control,” said Ehn.
And he believes that may be at hand with two fungicides, Folicur from Bayer CropScience and Switch from Syngenta.
“Shannon’s trial is to look at chemical control only. There has been no biostimulant in her trial we will be seeing on April 29,” said Ehn.
“This is the first time we have looked at chemical control on the West Side of the San Joaquin Valley,” he added.