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Title: ASSESSMENT OF VIRULENCE OF ACREMONIUM CUCURBITACEARUM AND MONOSPORASCUS CANNONBALLUS ON CUCUMIS MELO

Author
item Bruton, Benny
item GARCIA-JIMENEZ, J. - UNIVERSIDAD POLITECNICA
item ARMENGOL, J. - UNIVERSIDAD POLITECNICA
item Popham, Thomas

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Muskmelon response to soilborne disease is determined by the stage of plant development, level of plant resistance, pathogen density and virulence, and environmental conditions. Accurate assessment of virulence in soilborne pathogens is necessary for monitoring gene introduction and/or genetic drift within the pathogen population and prerequisite for effective plant breeding programs. The disease rating system used in the present study wa effective in evaluating virulence in both Acremonium cucurbitacearum and Monosporascus cannonballus. It provided good assessment of damage to the hypocotyl, stem-root junction, primary root, secondary roots, and reduction of the first two true leaves. Historically, pathogenicity of M. cannonballus and A. cucurbitacearum isolates has been determined without knowledge of inoculum density. The purpose of this study was to develop a method for assessing virulence of two important vine decline pathogens of cucurbits. For cucurbit root rot pathogens, reduction in leaf area of th first two true leaves may be preferable to taking root weight due to the difficulty in obtaining all the roots and the great amount of variability that is normally experienced. Adventitious roots produced along the hypocotyl of diseased plants often obscure actual reduction in root mass of primary and secondary roots. Alone, area of the first two true leaves is insufficient to estimate plant damage. Combining ratings of damage to the hypocotyl, stem-root junction, primary root, and secondary roots with a scaled leaf area reduction provided a useful method for assessment of isolate virulence or plant damage. Perhaps this study can provide a basis for the development of a "Universal Testing System" to standardize methods for evaluating soilborne pathogens of cucurbits.

Technical Abstract: Assessment of plant response to inoculation with soilborne pathogens is essential for evaluation of fungal virulence. Plant responses (damage to hypocotyl, root-stem junction, primary root, secondary roots, and reduction in leaf area), were used to derive a Disease Severity Index, and provided a asensitive estimation 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 to moderately virulent. Spanish isolates of Monosporascus cannonballus ranged from non- pathogenic to weakly virulent, whereas, LRGV isolates ranged from weakly to moderately virulent. Regression analysis of multiple inoculum densities showed that colony forming units (CFU)/g of soil of 5, 10, 20, and 40 for M. cannonballus and 0.1 x 10**4, 1 x 10**4, 2 x 10**4 and 3 x 10**4 for A. cucurbitacearum, respectively, could provide for virulence assessment of isolates of either fungus in greenhouse tests on seedlings. Seed depth was demonstrated to have a significant effect on disease severity with 4 cm being selected as optimal.