Winter vegetable growers in Yuma County, Ariz., and Imperial County, Calif., question whether an epidemic of Sclerotinia sclerotiorum fungus-caused lettuce drop disease last season will repeat itself during the 2011-2012 season.

The disease destroyed about 3 percent of the iceberg and romaine lettuce crop in Yuma County with total losses estimated at $8 million. Losses were also found in cabbage and cauliflower.

The heaviest crop damage was reported in the Yuma Valley (Arizona) and the Bard-Winterhaven (California) areas. Losses in infected fields ranged from 25 percent to 100 percent. The lettuce drop outbreak occurred from just after Christmas through January.

The Yuma County-Imperial County area is the U.S. winter vegetable production capital where about 90 percent of the nation’s supply of vegetables is grown. Production shifts to this low desert production area in the winter months once the summer vegetable season ends in California’s Salinas Valley, the nation’s top vegetable production region.

Head and romaine lettuce are planted in the desert from September through December with harvest from November through early April.

Lettuce drop disease is caused by two related types of sclerotia-forming fungi in the soil – Sclerotinia minor and Sclerotinia sclerotiorum. The fungi typically remain in the soil and colonize the lower portion of the plant, including the roots, leading to progressive plant drooping, plant collapse, and eventually plant death.

Last season a “perfect storm” of weather and other conditions caused a rare heavy dispersion of S. sclerotiorum spores into the air which caught the vegetable industry by surprise. 

“The S. sclerotiorumfungus produced mushroom-like structures called apothecia,” said Mike Matheron, University of Arizona (UA) plant pathologist.  “These structures produced copious amounts of aerial-borne spores which were carried by the wind. When the spores landed on senescent tissue the lettuce drop disease was initiated on the plant.”

Only the S. sclerotiorum fungus, not S. minor, can produce spores releasable into the air.

An aerial spore-releasing event can potentially provide enough spores to infect and destroy an entire field of lettuce.