New research results indicate that cultural practices including temperature and soil moisture management between lettuce crops can provide up to 100 percent destruction of lettuce drop disease.
Mike Matheron, a University of Arizona Extension plant pathologist based at the Yuma Agricultural Center, Yuma, Ariz., shared his findings with growers, pest control advisers, and others during the 2008 Preseason Vegetable Workshop in Yuma sponsored by Cooperative Extension.
Lettuce drop is a fungal disease that costs growers in the low-desert lettuce production areas of Yuma County, Ariz., and the Bard-Winterhaven area of Imperial County, Calif., up to about $2.5 million annually depending on the crop value, Matheron says. Losses per acre range from 1 percent to 5 percent of the crop, and even higher during wet years. A 50 percent to 100 percent crop loss can occur in individual fields.
Lettuce drop disease is caused by two fungal pathogens, Sclerotinia minor and S. sclerotiorum, sometimes called minor and major respectively. The sclerotia of the S. minor pathogen are smaller than the S. sclerotiorum. The pathogens cause brown decay in lettuce that destroys the plant crown tissue causing the plant to wilt and collapse and become unharvestable.
“To control lettuce drop, you must neutralize the Sclerotinia sclerotia,” Matheron says. “The ways to accomplish this are to prevent sclerotia from germinating and to destroy the sclerotia.”
Lettuce drop affects head, romaine, and leaf lettuces. The S. sclerotiorum pathogen occasionally affects cauliflower and cabbage while S. minor has been found in broccoli, but these are minor hosts in the low desert compared to lettuce.
Over the last two decades Matheron has studied the efficacy of fungicides currently on the market and products awaiting registration. While fungicides provide some sclerotia control, Matheron is convinced that cultural practices can eliminate most sclerotia.
“Managing cultural conditions including hot summer temperatures combined with some irrigation to wet the soil can achieve up to a 100 percent reduction of sclerotia,” Matheron says. “If you don't have live sclerotia, you won't have any disease. Basically you get 100 percent disease control.”
Soil temperature trials were conducted for eight weeks in 2007-2008 in fields between lettuce crops. Matheron buried packets of S. minor and S. sclerotiorum up to 2 inches deep in the furrow.
Irrigation every one to two weeks kept the soil moist. Soil temperatures in separate plots averaged 79 degrees and 90 degrees. Then 300 sclerotia were collected from the trials for laboratory analysis.
In soil with S. minor sclerotia, germination was reduced in the soil averaging 79 degrees. Soil at the 90-degree average reduced germination by 50 percent.
“The sclerotia weren't able to germinate under higher temperatures,” Matheron says. “At six to eight weeks, some sclerotia even disintegrated.”
In tests with S. sclerotiorum, living sclerotia numbers were reduced over several weeks, but took longer to kill the pathogen due to its larger size. After eight weeks nearly 100 percent of the sclerotia were dead.
“Think of sclerotia as seeds,” Matheron says. “The sclerotia are pathogen seeds and we're trying to reduce sclerotia that are capable of germination and causing lettuce drop. The more you destroy the fewer are available to cause lettuce drop.”
Trials focusing on soil moisture included irrigated and non-irrigated ground. Irrigated soil was watered every one to two weeks and the mean soil temperature was 90 degrees. Sclerotia packets were placed in the furrow.
Results indicated a higher S. minor destruction rate in six to eight weeks in the irrigated soil. In non-irrigated tests, germination was also reduced, yet Matheron was unsure if the sclerotia were permanently damaged or in dormancy. Trials with S. sclerotiorum showed similar results to the S. minor findings.
Matheron conducted experiments with flooded soil. In three trials, sclerotia were buried 0, 4, and 8 inches deep in soil-filled buckets for a short period of time. Water was maintained 1 inch above the soil line. The mean soil temp from 0 to 8 inches deep ranged from 86 to 92 degrees.
After one week, most S. minor sclerotia were dead and all were destroyed after two weeks. S. sclerotiorum were hardier because of the larger size. After one week 20 percent had germinated, but all were dead after two to four weeks.
Effective control of lettuce drop disease using cultural techniques requires other management responsibilities. “Even a field sanitized with S. minor can face pathogen reintroduction,” Matheron says. “If someone walks into a field containing sclerotia and then walks in a sclerotia-free field, the second field can be re-infested.”
Matheron also shared results from trials over the last four years on the efficacy of fungicides for lettuce drop control. He evaluated the products Botran (Gowan Co.), Contans (Advan), Endura (BASF), Rovral (Bayer CropScience), Serenade (AgraQuest), and Switch (Syngenta). He also tested products awaiting registration including Fluazinam, LEM 17, and Polyoxin-D.
Results on current fungicides showed the best S. minor control was about 50 percent with Endura, about 45 percent with Rovral, and just above 40 percent with Botran. For S. sclerotiorum, Contans provided near 50 percent control, about 45 percent with Rovral, and Endura controlled about 35 percent. Each value is the mean from four trials with two applications of each product per trial.
In efforts to achieve even higher lettuce drop control with fungicides, Matheron mixed full rates of two products and applied them at thinning and later in the season. Applying Endura and Rovral achieved a near 80 percent control of S. minor and 65 percent control of S. sclerotiorum.
In the trials of unregistered fungicides, Fluazinam provided 75 percent control of S. minor and slightly less for S. sclerotiorum. LEM 17 controlled about two-thirds of S. minor sclerotia and about one-third of the S. sclerotiorum. Control with Polyoxin-D was less than half for S. minor and under 20 percent for S. sclerotiorum.
Matheron is confident that improved cultural practices combined with fungicide use can greatly reduce lettuce drop. “The widespread adoption of a summer-flooding treatment of fields infested with sclerotinia would greatly reduce the incidence of lettuce drop. Coupled with the care of not reintroducing the pathogen to those fields, this cultural treatment could be an excellent means of minimizing this disease. Fungicides could be used in fields that are not suitable for flooding.”
Currently there are no lettuce drop disease-resistant, commercially-acceptable lettuce varieties on the market, Matheron says.