The pathology and epidemiology of Monosporascus cannonballus, a root rot/vine decline pathogen of muskmelon new to Texas

Seven fungi were repeatedly isolated from diseased roots of muskmelon (C. melo) in four south Texas muskmelon fields in 1990. Macrophomina phaseolina and Monosporascus cannonballus caused conspicuous root rot and significant reductions in root mass when screened for pathogenicity to 'Magnum 45' muskmelon seedlings in the greenhouse. The latter fungus was reported from the United States only once previously (Arizona, 1970). Most, but not all, isolates of M. cannonballus (MC) caused significant reductions in root mass and vine length, and were readily isolated from lesioned or necrotic roots of Magnum 45 at the end of these tests.;The pathogenicity of MC (isolate TX(Cm)90-25) to nine cucurbit and eight non-cucurbit species was evaluated in the greenhouse by direct seeding into infested soil. MC was isolated from 70-100% of the cucumber, muskmelon, and watermelon plants, and from 33-77% of the bean, sorghum, and sugarbeet plants 8-9 wk after planting. Perithecia of MC were observed on diseased roots of all cucurbits tested, but only rarely on the non-cucurbits. Consistent reductions in top dry weight were observed in wheat and corn, in addition to all cucurbits tested. Watermelon and muskmelon were the most susceptible; typical vine decline symptoms were induced in these species in microplot tests. Greenhouse screening of 21 muskmelon cultigens indicated tolerance to MC in several commercial cultivars and two breeding lines.;MC ascospore numbers in the soils of three south Texas muskmelon fields in 1992 ranged from 8.8-14.8 spores/g dry wt soil in field A, 3.1-4.2 in field B, and 3.8-7.8 in field C. Samples were taken from within the rows at the 10-20 cm depth, where relatively high ascospore concentrations were found in 1991. A relationship between successive annual cropping of muskmelon in field A and high inoculum densities was indicated. However, significant positive correlations between ascospore numbers at planting and root disease severity at harvest were rare along individual sampling transects or across fields. Ascospore concentrations did not vary greatly within or between transects in a given field. This corresponds with an apparent lack of aggregation of plants with identical root disease severities, and implies a relatively uniform distribution of inoculum.