Influence of assessment methods on selection of fungal antagonists of the sclerotium-forming fungus Phymatotrichum omnivorum

Abstract

Four attributes required by microorganisms for biocontrol of plant diseases caused by sclerotium-forming fungi, where the strategy is to reduce the number of sclerotia, are (1) an ability to adversely affect propagule germination and (or) viability, (2) growth potential through soil, (3) activity over a range of environmental conditions, and (4) survival and reproductive potential. Three methods of assessment (soil plate, SPM; soil immersion, SIM; inert carrier, ICM) were evaluated to assess biocontrol agents with regard to attributes 1 and 3. Based on the method of assessment, the fungal antagonists (Gliocladium roseum, G. catenulatum, Trichoderma harzianum, Tr. pseudokoningii, Aspergillus ustus, A. versicolor, Penicillium crustosum, Fusarium solani, Chaetomium globosum, and Stilbum sp.) were grouped by their ability to colonize sclerotia of Phymatotrichum omnivorum in nonsterile soil. Group 1 strains colonized greater than 50% of the sclerotia with any method. Group 2 strains failed to colonize greater than 18% of the sclerotia with any method. Group 3 strains effectively colonized sclerotia when assessed by the SPM but not the ICM; opposite results were obtained with group 4 isolates. With the three methods of assessment, Gliocladium roseum (strain 1620) colonized sclerotia at 19, 23, 27, and 31 °C. Colonization with the SPM or SIM was not temperature dependent, but colonization significantly decreased with increased temperature (27 and 31 °C) for the ICM. The method of assessment altered conclusions about the effectiveness of these potential biocontrol agents.