Walking along the tiers of beds and trays in mushroom houses this spring, Santa Clara County growers found green mold stretched across empty patches where the round, white tops of button mushrooms should have been.

A new strain of an old disease is threatening Santa Clara County's number one agricultural product. Button mushrooms, Agaricus bisporus — valued at $30 million to $35 million a year for the county — have been infected with Trichoderma harzianum Th-4, according to Santa Clara County farm advisor María de la Fuente. She anticipates T. harzianum Th-4 will reduce this year's mushroom yields by 25%.

T. harzianum Th-4, is a very aggressive new strain of an old disease caused by a fungus. Green mold, the old disease, spreads slowly and is usually associated with improperly processed compost. The new strain has appeared even in properly processed compost and proven to be severe and fast-spreading in the mushroom industry in Canada and Pennsylvania, says de la Fuente. Pennsylvania's yields fell nearly 50% during its first year of infection, she said.

In early April, a Santa Clara County grower noticed the rapidly spreading mold and called de la Fuente. CDFA's Plant Pest Diagnostic Center in Sacramento isolated the aggressive strain from the samples she sent. The results were confirmed by highly specific lab tests at two other internationally recognized institutions. By June, the grower had suffered a 40% yield loss.

“Recently, I sampled several farms in the Santa Clara County area, others in Monterey, Santa Cruz and San Mateo counties, and found that they all have the disease,” she says.

After 2 to 5 weeks of an apparently normal start, sections of a tray can turn green with Trichoderma spores. Initially a few isolated tray sections are infected, but soon it becomes epidemic, commonly infecting 30% of a mushroom house, and sometimes making cultivation cycles of the entire house unprofitable.

The problem can be exacerbated by mites, another common pest, but they aren't the only ones spreading Trichoderma spores. Wind-car-ried dust particles, insects, contaminated hands, clothing and tools of workers can contribute to the spread of spores.

Mushroom growers are covering patches of T. harzianum Th-4 with salt to stop the spread of the mold.

Another strain of T. harzianum is used in agriculture as a biocontrol agent to kill soilborne pathogens. De la Fuente says, “I think that the scientific community needs to look at whether this widely used strain of Trichoderma harzianum is not mutating or adapting and causing this disease on mushrooms.” However the source of the problem has not been determined.

De la Fuente suspects the new disease arrived last year Santa Clara County because production yields dropped from 145 tons per acre in 1995 to 133 tons per acre in 1996.

California growers may be granted a special use of the fungicide benlate (Benomyl) to treat the planting material, called spawn, to stem the spread of the disease. Currently use of the fungicide on mushrooms is restricted to application after the crop is established. To ease this restriction, de la Fuente recently requested the California Department of Pesticide Regulation's endorsement for the petition prepared for the Environmental Protection Agency (EPA) for a Special Local Need Registration Section 24(c) of benlate to treat the spawn before planting. EPA has granted a similar permit to the state of Pennsylvania. One problem with using this fungicide is that there is a high risk of killing the mushroom, itself a fungus. To reduce the risk, she will advise growers to treat the spawn right before planting and to make sure they don't overdose.

“The potential loss for California mushroom farmers because of this disease is sizable,” de la Fuente says, “and a benlate spawn treatment will have a significant impact in reducing this threat as it did for Pennsylvania growers.”

—Ed.