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The Interaction of Armillaria mellea and Trichoderma spp. as Modified by Methyl Bromide. Howard D. Ohr, Former Graduate Student, Department of Plant Pathology, University of California, Riverside 92502, Senior author’s address: Southern Weed Science Research Laboratory, ARS, USDA, Stoneville, Mississippi 38776; Donald E. Munnecke(2), and James L. Bricker(3). (2)(3)Professor of Plant Pathology, and Research Associate, respectively, Department of Plant Pathology, University of California, Riverside 92502. Phytopathology 63:965-973. Accepted for publication 7 February 1973. DOI: 10.1094/Phyto-63-965.

Investigations were made of the early sequence of events occurring after methyl bromide fumigation of citrus roots infested with Armillaria mellea and their storage in nonfumigated, nonsterile soil. When infested roots were treated with sublethal concentrations of methyl bromide and stored in nonsterile soil, Armillaria died, but when stored in sterile soil the fungus survived. Isolations of Trichoderma from the roots reached a maximum after storage in nonsterile soil for 7-8 days, and then declined as Armillaria populations approached zero. The population changes of the two fungi were directly correlated; a regression of viable Armillaria isolations on viable Trichoderma isolations had a linear correlation coefficient of –0.888 and a significance level of 99.9%. Bacteria and Fusarium spp. also increased after fumigation but were not correlated with the decline of Armillaria. In vitro fumigation of mycelium growing in water agar using carefully controlled concentrations of methyl bromide in a moving air stream, demonstrated that Trichoderma was approximately two times more resistant to the chemical than Armillaria. The mean LD50 value for two Armillaria isolates was 128 CT (concentrations of methyl bromide times hours of treatment); whereas, the mean for four Trichoderma isolates was 261 CT. In this case concentration was 35 ml methyl bromide/liter air.

Additional keywords: integrated chemical-biological control, parasitism, dosage response.