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. 

Repeat?

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.

Fungicides

Matheron has tested traditional fungicides and biofungicides in separate YAC trials. The findings suggest traditional fungicides are best applied directly to the plant bed immediately after thinning and cultivation and at the rosette stage to prevent sclerotia germination. The highest level of traditional fungicide effectiveness in the trial was about 50 percent.

A one-year YAC trial evaluated eight biofungicide products. These biofungicides when applied alone, except for the product Contans, were equivalent in performance. All of the tested biofungicides, except for Contans, were less effective than the conventional product Endura. Contans and Endura were the most effective products in the trial for Sclerotinia drop.

“Applied fungicides, traditional and natural, should be applied as a barrier on top of the soil surface to reduce the fungal infection of leaves,” Matheron said.

The extreme S. Sclerotiorum outbreak was rare and it remains unknown if and when a combination of weather and other conditions could occur again.

Growers and PCAs question whether fungicides should be applied each crop cycle as insurance against a sporadic threat. Production cost margins are slim in vegetable production. Extra control costs can mean the difference between profit and loss.

“We are on tens to hundreds of dollars per acre margins,” Boelts said. “It’s a tough call. You can’t easily give up profit to prevent a disease which may or not occur.”

cblake@farmpress.com