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Assessment of Virulence of Acremonium cucurbitacearum and Monosporascus cannonballus on Cucumis melo

Assessment of plant response to inoculation with soilborne pathogens is essential for evaluation of fungal virulence. Combined plant responses (damage to hypocotyl, root-stem junction, primary root, secondary roots, and reduction in leaf area) used to derive a disease severity index (DSI) provided a functional assessment of both plant damage and pathogen virulence. Spanish isolates of Acremonium cucurbitacearum ranged from virulent to highly virulent on muskmelon cv. Magnum 45, whereas isolates from the Lower Rio Grande Valley (LRGV) of Texas ranged from weakly virulent to moderately virulent. Spanish isolates of Monosporascus cannonballus ranged from nonpathogenic to weakly virulent, whereas LRGV isolates ranged from weakly virulent to moderately virulent. Regression analysis of multiple inoculum densities established that CFU per gram of soil of 5, 10, 20, and 40 for M. cannonballus and 0.1 × 10⁴, 1 × 10⁴, 2 ×10⁴, and 3 × 10⁴ for A. cucurbitacearum, respectively, were adequate for virulence assessment of isolates of either fungus in greenhouse tests on seedlings. Seed planting depth had a significant effect on DSI. The 4-cm depth generally had smaller standard errors of the mean disease rating, indicating greater precision in determining isolate virulence.