Diseases of melon (
Monosporascus vine decline
Monosporascus vine decline is caused by the fungus Monosporascus cannonballus. Plants may be infected when they are young, but symptoms usually do not appear until vines are mature and fruit are developing (photo 1). Foliage may have necrotic areas in the leaves and a progressive decline in the entire crown (photo 2). Tan lesions are often observed on roots, and, in the later stages of disease only, small black specks appear (photo 3). These specks are the sexual reproductive stage of the fungus that contain large black spores - thus the species name cannonballus (photo 4). 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 melons, but some varieties are much more susceptible than others. Caravelle and Desert Mark are among the most susceptible. 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. Monosporascus vine decline becomes more problematic when melon crops follow one another without rotation. Disease may become so severe that planting melons that will mature in late summer and early fall is not economical.
Monosporascus vine decline can be reduced in drip-irrigated systems by more frequent applications of water when plants have a heavy fruit load. Soil fumigation with 1.3 dichloropropene (Telone II) and chloropicrin applied prior to planting and application of fludioxonil (Cannonball) or thiophanate methyl (Topsin-M) through buried drip irrigation beginning at plant emergence (see label instructions), may suppress disease. Lifting roots out of the ground immediately after the last harvest may reduce build up of inoculum (spores) in the soil.
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March 4, 2009