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Inactivation of Verticillium dahliae in Peppermint Stems by Propane Gas Flaming. John L. McIntyre, Graduate Research Assistant, Department of Botany and Plant Pathology, Oregon State University, Oregon 97331, Present address: Department of Botany and Plant Pathology, Purdue University, Lafayette, Indiana 47907; Chester E. Horner, Plant Pathologist, Plant Science Research Division, ARS, USDA, Corvallis, Oregon 97331. Phytopathology 63:172-175. Accepted for publication 20 August 1972. DOI: 10.1094/Phyto-63-172.

In laboratory tests, internal stem temperatures of 60 C or higher killed 95 to 100% of the population of Verticillium dahliae in heavily infected stems of peppermint, Mentha piperita. In field tests using a boom-type field flamer at tractor speeds of 2.0 to 2.7 mph, internal stem temperatures usually reached or exceeded 60 C, and the average kill of the fungus was 99.5 and 99.2%, respectively. Speeds up to 2.7 mph also resulted in adequate incineration of infected plant debris on the ground, which could serve as an additional source of inoculum. Results of assays to determine the fate of the fungal propagules which persisted after flaming showed that few were viable after 1 week in the field; only three of 38 stems assayed over a 14-week period contained Verticillium, and only one showed an increase in propagule numbers. Flaming had little effect on soil temperature. At 2.0 mph, soil temperature increased 11.5 C at a depth of 1 mm, and only 1.0 C at a depth of 10 mm. At faster speeds, temperature changes were even less.