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Effect of Root-Knot Nematodes on Fusarium Wilt of Watermelon. Donald R. Sumner, Assistant Professor, Department of Plant Pathology, University of Georgia Coastal Plain Station, Tifton 31794; A. W. Johnson, Nematologist, Agricultural Research Service, USDA, Coastal Plain Station, Tifton 31794. Phytopathology 63:857-861. Accepted for publication 16 January 1973. DOI: 10.1094/Phyto-63-857.

Eight cultivars of watermelon with known reactions to Fusarium wilt were grown in soils collected from 21 fields with a history of watermelon production in eight growing areas of South Georgia. More plants wilted in soils when Meloidogyne incognita was present. Wilt symptoms were increased more in resistant than in susceptible cultivars in soils naturally infested with root-knot nematodes. Wilt severity was significantly correlated with initial populations of root-knot larvae, inoculum density of all Fusarium oxysporum in the soil, and wilt severity in the previous crop of watermelons, but not with soil pH or the number of years since the previous crop of watermelons was grown. In another study, Dothan loamy sand was artificially infested with 3% cornmeal-sand cultures of F. oxysporum f. sp. niveum 1:400 or 1:16,000 (v/v) or noninfested. Each treatment was then infested with M. arenaria, M. javanica, M. hapla, or noninfested. Root-knot nematodes did not significantly increase wilt at 17, 584, or 218 propagules of total F. oxysporum per gram of air-dry soil. However, at 650 propagules per gram of total F. oxysporum, M. arenaria reduced foliage weights 11-13% in susceptible ‘Jubilee’, moderately resistant ‘Charleston Gray’, and resistant ‘Crimson Sweet’, and significantly increased wilting and root necrosis in Charleston Gray. Total plant weight of Charleston Gray was significantly less than Crimson Sweet with M. hapla, but not with other root-knot nematodes or in the controls at the high level of F. oxysporum. Significantly more root galls were caused by M. arenaria and M. javanica than by M. hapla.