Diseases of lettuce (
Bottom rot
Bottom rot is caused by the fungus Rhizoctonia solani. Infected plants have sunken, reddish-brown lesions of varying sizes on leaf petioles and midribs that touch the soil (photo 1). When plants are cut, the lesions are visible on the bottom of the head. White to brown colored mycelium may grow in the lesions, and the fungus can move into other leaves of the plant. When only the outer leaves are affected (photo 2), they are removed in the field and the heads can be harvested, but if the fungus has invaded the entire head, lesions are too deep and plants are discarded.
Rhizoctonia solani is one of the most common soil borne pathogens in Arizona. It survives indefinitely in soils because of its ability to colonize soil organic matter in absence of a living host. It is carried wherever infested plant parts or soils are moved. Disease can occur over a fairly wide range of temperatures. Other hosts include cotton and alfalfa, and rotation with these crops should be avoided, especially if R. solani has been a problem in the field previously.
Fungicides may be effective if applied early. Applications of iprodione should be made just after thinning.
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February 10, 2003