Diseases of watermelon (
Monosporascus vine decline
Monosporascus vine decline is caused by the fungus Monosporascus cannonballus. Plants may be infected when they are young, but symptoms often do not appear until vines are mature and fruit are developing. Foliage may have necrotic areas in the leaves and a progressive decline in the entire crown. Tan lesions are often observed on roots, and, in the later stages of disease only, small black specks appear (photo 1). These specks are the sexual reproductive stage of the fungus that contain large black spores - thus the species name cannonballus (photo 2). These spores are commonly found in many different soils, and the fungus seems to be indigenous to certain semiarid soils.
There is no resistance in watermelon. The presence of spores in soils can be determined in laboratory assays, but to date there has been no direct correlation between the numbers of spores in soil samples and the incidence or prevalence of disease. Disease can be reduced in drip irrigated systems by more frequent applications of water when plants have a heavy fruit load. No fungicides have been shown to be effective, but soil fumigants such as chloropicrin may be effective.
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August 1, 2003