The big question on growers and PCAs’ minds is could the event repeat itself during the 2011-2012 winter lettuce season?

“This is unlikely to occur at the same magnitude this coming winter since last year’s infection was caused by a rare ‘perfect storm’ of conditions,” Matheron said.

The ‘perfect storm’ included the unlikely mix of five consecutive nights of temperatures near or below freezing in late November combined with an extended period of morning leaf wetness for 14 consecutive days.

Other contributing factors included wet soil — common in irrigated lettuce, sclerotia fungi traditionally located in the top 2 inches of soil, rain or dew on the plant leaves, senescent plant tissue, and soil temperatures between 50-75 F.

Matheron discussed the event, which he termed an “epidemic,” during the 2011 Preseason Vegetable Workshop at the Yuma Agricultural Center (YAC) in late August. The UA-sponsored event was packed shoulder-to-shoulder with winter vegetable growers, PCAs, and other industry members.

“One sclerotium can produce copious amounts of spores which can infect many plants,” Matheron said. “The aerial production of spores creates a very noticeable increase in lettuce drop disease.”

Sclerotia of S. sclerotiorum range from 2 to 20 millimeters in size. S. minor sclerotia are .5 to 2 mm in size. The smaller S. minor produces 10 to 100 times more sclerotia than S. sclerotiorum, Matheron says.

Yuma lettuce grower John Boelts of Desert Premium Farms calls last winter’s S. sclerotiorum lettuce drop outbreak “voracious” – the worst aerial-borne sclerotia outbreak he had experienced in his 15-year vegetable career.

Boelts says the central and northern areas in the Yuma Valley were the hardest hit. Some large growers lost hundreds of acres of lettuce.

“There wasn’t any escaping it — even for the best of growers,” Boelts said. “It was a challenge to try to outguess it. The outbreak was devastating and challenging for those trying to put vegetables into a box to move to market.”

Boelts says aerial spore movement typically impacts a plant or two but not several hundred acres. Damage at Boelts’ own operation included about 10 acres in three fields of mostly romaine lettuce.

Pat Riley, a Yuma area-based pest control adviser with Green Valley Farm Supply, estimated crop losses at 20 percent to 80 percent in infected fields.

“It came up pretty quick,” Riley said. “The disease turns the plant to mush and there’s nothing left.”

Containing sclerotia in winter and summer lettuce is a challenge. After the lettuce crop is harvested, the field is normally disked which mixes sclerotia back into the soil for the next lettuce planting.

S. Sclerotiorum management tools include cultural, biological, and chemical efforts to target sclerotia control.

Cultural practices can include no action at all since a sclerotia population naturally decreases over time. Soil solarization and summer soil flooding for three to four weeks can speed up the breakdown of sclerotia.

“Field flooding during the summer when the soil temperature is about 90 F is a viable cultural practice to help rid the bank of viable sclerotia that may be present,” Matheron told the crowd.