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Evaluation of a Strain of Myrothecium roridum as a Potential Biocontrol Agent Against Phytophthora cinnamomi. R. Gees, Postdoctoral researcher, Department of Plant Pathology, University of California, Riverside 92521-0122; M. D. Coffey, professor, Department of Plant Pathology, University of California, Riverside 92521-0122. Phytopathology 79:1079-1084. Accepted for publication 24 May 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1079.

Potential antagonists of Phytophthora cinnamomi were evaluated from among 36 fungi and 110 bacteria isolated from the rhizosphere of avocado roots growing in a soil suppressive to Phytophthora where P. cinnamomi had been present for 4050 yr. Strain TW of Myrothecium roridum proved to be the most active antagonist in controlling P. cinnamomi in repeated greenhouse pot tests with highly susceptible seedlings of Persea indica inoculated with P. cinnamomi. M. roridum was grown on a wheat-bran medium and introduced into a peat-perlite mixture at 2.5% (w/v) 2 wk before inoculation with P. cinnamomi. In a UC-mixture with P. indica inoculated with zoospores of P. cinnamomi, M. roridum suppressed root infection by 5094% compared with uninoculated controls. In the same experiments there was no significant difference in the level of control achieved by either M. roridum or the fungicide potassium phosphonate (2.5 mg/pot). In three naturally infested field soils, root infection ranged from 12 to 54% in the presence of M. roridum, compared with 58 to 93% for controls over the same 4-wk period. On a selective medium containing carbendazim, a fungicide-resistant mutant of strain TW, TWm14, was isolated consistently from the root tips of P. indica growing in infested soil 4 wk after transfer, demonstrating the apparent rhizosphere competence of this strain in all three soils.