Fungicides Keep Lettuce from Turning Slimy


Lettuce Drop

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[4]:494-502 Available:

New Fungicides Take the Worry out of Controlling Mildew on Lettuce


Lettuce Downy Mildew

Downy mildew is a common fungus in most lettuce growing regions, especially during cool, moist weather.  Spores can be blown long distances.  Under favorable conditions, downy mildew is a very explosive disease, capable of appearing at high incidence in a field overnight.  When spores land on lettuce foliage, they germinate and can penetrate the lettuce leaf within three hours.  Lettuce is susceptible at all growth stages to the downy mildew pathogen.  Following penetration and establishment in the leaf, fruiting stalks grow through the leaves and branch repeatedly producing several spores on each tip, resulting in a whitish mat of millions of spores on each plant.  Affected tissues turn brown.  The fungus can penetrate to leaves internal to the wrapper leaves.  Relatively low levels of infection can downgrade a crop, cause significant trimming losses at harvest and promote decay by bacterial organisms during postharvest transport and storage.  During transit, lesions become soft and slimy as secondary decay organisms gain entrance through the tissues infected with the downy mildew fungus. High levels of disease can render a crop unmarketable.

“Incited by the obligate parasite Bremia lactucae, downy mildew is one of the most devastating diseases of lettuce worldwide. Attempts to manage this fungal disease using host-plant resistance have frequently failed due to the development of new races of the pathogen. Therefore, chemical control is of the utmost importance in humid areas where environmental conditions are very favorable for disease development.

Since the year 2000, a number of new fungicides targeting the Oomycetes, the class of fungi to which downy mildew belongs, have come to the market or are being considered for registration. It was the objective of these studies to investigate a select number of these for potential use in Florida for lettuce downy mildew control.

Of those investigated, mandipropamid and fenamidone consistently provided for high levels of control. Fluopicolide, dimethomorph, dimethomorph plus ametoctradin, cyazofamid, and propamocarb also provided significant control. With the majority of these fungicides already being labeled or close to being labeled on lettuce, it would appear that lettuce growers now have a wide array of efficacious downy mildew fungicides with differing modes of action from which to choose. This is a far cry from the situation that existed during 1989, when the EBDC fungicides were being threatened with cancellation and metalaxyl insensitivity was becoming widespread.”

Authors: Raid, R. N., and D. D. Sui.
Affiliation: University of Florida, IFAS, Everglades Research and Education Center.
Title: Management of lettuce downy mildew with fungicides.
Source: Proc. Fla. State Hort. Soc. 2012. 125:218-221.

Consumer Expectations for High Quality Lettuce Require Insecticide Use

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.