The fungus causing lettuce drop causes a rot that usually begins on the stem near the soil surface and a water-soaked area appears; it can spread downward until roots decay and can spread upward. The pathogen rapidly ascends the stalk, killing the leaves in succession until it reaches the heart of the lettuce plant. Layers of collapsed leaves lie flat on the soil surface after infection. Inner leaves are invaded completely by the fungus, which reduces the head to a wet, slimy mass. The entire plant may collapse in less than two days.
“Lettuce drop is one of the most destructive diseases of lettuce and has been reported in all lettuce-growing regions of the world. In the USA, the disease regularly occurs in the two primary lettuce-producing states of Arizona and California. Yield losses vary from 1% to nearly 75% depending on conditions, but under ideal disease conditions an entire field may be lost. The disease is caused by two closely related soilborne fungi, Sclerotinia sclerotiorum (Lib.) de Bary and S. minor Jagger.
Sclerotia of S. sclerotiorum can survive up to seven years and their longevity is affected by location of sclerotia within the soil profile, duration of burial and soil temperature and moisture.
Commercially acceptable cultivars with adequate levels of resistance are not currently available. Thus, current management strategies for lettuce drop rely primarily on fungicides such as iprodione and boscalid.”
Authors: Chitrampalam, P., and B. M. Pryor.
Affiliation: Department of Plant Sciences, University of Arizona, Tucson
Title: Population density and spatial pattern of sclerotia of Sclerotinia sclerotiorum in desert lettuce production fields.
Source: Canadian Journal of Plant Pathology. 2013. 35:494-502 Available: http://www.tandfonline.com/doi/abs/10.1080/07060661.2013.841758#.U35Ck_ldV1Y
Fresh market lettuce production in the desert growing areas of Southern California and Arizona is a billion dollar industry and the region annually produces >95% of the leafy vegetables consumed in the U.S. in the fall and winter months. Consumers desire lettuce without any blemishes or insect damage. Consumer standards result in the annual use of insecticides on the lettuce crop as described by Arizona entomologists John Palumbo and Steve Castle…
“In desert vegetable production systems, growers have been delivering high-quality safe produce to the fresh market for decades, and this has been accomplished almost exclusively through the use of insecticides.”
“…western lettuce growers and consultants have reported that chemical control is the only effective IPM tactic available for the control of most major insect pests. Naturally occurring biotic control agents are simply not capable of providing the level of crop protection necessary for meeting the marketing demands for fresh produce. … Because of the short time these crops are in the field, minor feeding activity may render the product unmarketable because of high consumer standards.”
“More recently, the fresh produce industry has experienced significant growth in the value-added market, where lettuce and other leafy greens are prepared and sold as fresh-cut lettuce packs and ready-to-eat, bagged salad mixes. The growth of this industry has also resulted in higher cosmetic standards for leafy vegetable crops, often to the point where virtually no insect contaminants or feeding blemishes are tolerated.”
Authors: John C. Palumbo and Steve J. Castle
Affiliation: University of Arizona Department of Entomology
Title: IPM for fresh-market lettuce production in the desert southwest: the produce paradox
Publication: Pest Management Science (2009) 65:1311-1320.