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 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.
Breeding for Resistance
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.