Apr 29, 2021

Here’s how to deal with gray mold in lettuce

By the Yuma Center of Excellence of Desert Agriculture at the University of Arizona

Gray mold of lettuce occurs most everywhere that lettuce is grown, but its prevalence varies depending on the particular environment of the lettuce production area. The disease affects all lettuce types but is more prevalent in leaf types.

Symptoms include wilting and collapse of young seedlings, followed by a wet decay and necrosis.  In mature plants, the disease may begin on the upper foliage or at the soil level. In a disease of the upper plant, irregular necrotic spots appear across the mature leaves and may coalesce into larger necrotic areas.

In a disease of the base of the plant, wilting and collapse follows necrosis of the lettuce stem. If the necrotic lettuce plant is turned over, a gray fuzzy mold may be seen across the necrotic area, often with irregularly shaped flattened sclerotia. At this stage, the disease can be mistaken for lettuce drop caused by Sclerotinia sclerotiorum; however, with lettuce drop, the mold will be white and the sclerotia will be irregular and more rounded in shape.

The disease is a common problem in greenhouse production, when ambient moisture may be higher and more constant than that in field production.  This occurs both in transplants and in hydroponic production systems.  In leaf lettuce production, the disease affects the foliage at all stages of development, resulting in irregular necrotic spots that can coalesce and consume entire leaves.  If left untreated, the disease can rapidly spread to all neighboring plants within the growing facility.

The disease is caused by the fungus Botrytis cinerea.  This fungus is very common in most agricultural systems and causes disease on a wide range of crops.  The fungus grows well on dead and decaying plant tissue and can live indefinitely in the absence of a living host.  Cool moist conditions favor the growth of the fungus and in such environments, it can produce abundant airborne spores on a weekly basis.  The fungus can also produce hard irregularly shaped sclerotia which are long-lived resting structures.  However, not all strains produce sclerotia.

Cultural control

Cultural control of the disease begins with a good field and greenhouse sanitation.  Since the fungus lives indefinitely on decayed organic debris, all residue from previous crops should be either tilled into the soil or removed from the production areas.  The fungus thrives in cool moist conditions.  As such, management should focus on reducing free water from surfaces as much as possible.  In the field, excessive leaf wetness will promote the disease and efforts should focus on reducing or avoiding sprinkle irrigation.  As the fungus can infect a wide range of hosts or live on non-living plant residue, crop rotation has limited effects.

Chemical control

There are a number of fungicides that are effective against gray mold on a variety of hosts. Much work has been done on controlling gray mold on grapes, strawberries, tomatoes, and floriculture products, and more limited work on control on lettuce, including a recent study by Matheron and Porchas.

Most studies show similar findings in sensitivity to different products, as well as the common occurrence of fungicide resistance development in local strains of Botrytis.  Therefore, all product use should be accompanied by a robust program of fungicide resistance management.   Before using any product to control gray mold on lettuce, check product labels and restrictions and consult with your local Extension Specialists and Pest Control Advisers.

Biological control

Considering the importance of gray mold on a number of high-value crops such as grapes and strawberries, considerable research has been done in evaluating different biological control agents for efficacy in disease management.  The most studied of these are strains of Trichoderma, a fungal parasite of other fungal species, and strains of Bacillus, which have been used widely as a biocontrol antagonist against a wide range of pathogens.  Specific research on gray mold control on lettuce revealed that Pseudomonas, Ulocladium, and Epicocum species reduced lesions on lettuce by over 78%.  Additional studies have explored the use of Coniothyrium minitans as a biocontrol agent, a fungus that parasitizes sclerotia and has been used successfully to control lettuce drop disease.

Several studies have been published on gray mold resistance in lettuce including recent work by Matheron and Porchas, which found variation among several commercial cultivars.  With more resources available from the recent genomic characterization of lettuce germplasm, it is expected that breeding efforts will expand.  Recent work exploring gray mold resistance in wild relatives of lettuce has revealed several accessions with complete resistance.  These accessions are strong candidates for inclusion into ongoing lettuce breeding programs.

– The Yuma Center of Excellence of Desert Agriculture at the University of Arizona