VIEW ARTICLE

Ecology and Epidemiology

The Effect of Chlorophenoxy Acid Herbicides on Growth and Rhizomorph Production of Armillariella mellea. J. Pronos, Former research assistant, Department of Plant Pathology, University of Wisconsin, Madison, 53706, Present address of senior author: U. S. Forest Service, Forest Insect and Disease Management, 630 Sansome Street, San Francisco, CA 94111; R. F. Patton, professor, Department of Plant Pathology, University of Wisconsin, Madison, 53706. Phytopathology 69:136-141. Accepted for publication 30 August 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-136.

Rhizomorph production by Armillariella mellea from dead oak roots in the field was assessed on trees that had been killed 2 to 14 yr before sampling. More rhizomorphs were produced from roots of trees killed by herbicides than from roots of trees killed by hand-girdling. The maximum quantity of rhizomorphs was recovered from root systems 10 yr after trees were treated with herbicides. In culture, 2,4-dichlorophenoxyacetic acid (2,4-D) stimulated the growth rate or the production of rhizomorphs of four A. mellea isolates. The range of growth-regulating activity for 2,4-D was between 10 and 250 μg/ml. Picloram, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), and 2(2,4,5-trichlorophenoxy)propionic acid (2,4,5-TP) either had no effect on the fungus at any concentration or were inhibitory at and above 10 μg/ml. The addition of 2,4,5-T or 2,4,5-TP to an equal quantity of 2,4-D negated the stimulatory effect of 2,4-D. Gas-liquid chromatography was used to detect 2,4-D and 2,4,5-TP in root-collar phloem of oaks treated with aerial applications of the herbicides. Traces of both compounds were detected as early as 2 days after treatment and as late as 39 days after treatment. In general, less than 2.5 μg/ml of 2,4-D and 0.5 μg/ml 2,4,5-TP were present in analyzed tissue. We concluded that, under field conditions, these herbicides were not present in root phloem at levels high enough to have a direct effect on A. mellea.