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Disease Control and Pest Management

Postemergent Herbicides and the Biology of Drechslera sorokiniana: Influence on Severity of Leaf Spot on Poa pratensis. Clinton F. Hodges, Professor of Horticulture and of Botany and Plant Pathology, Department of Horticulture, Iowa State University, Ames, IA 50011; Phytopathology 68:1359-1363. Accepted for publication 8 March 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1359.

The influence of four chlorophenoxy (2,4-D, 2,4,5-T, 2,4,5-TP, MCPP) and one benzoic acid (dicamba) postemergent herbicides were evaluated for their influence on the development of leaf spot caused by Drechslera sorokiniana on Poa pratensis. Leaf spot development was severely inhibited on plants not previously exposed to any of the herbicides and leaf-inoculated with conidia in droplets of each herbicide solution prepared at a concentration of 103 M (2,4,5-TP at 104 M). Leaf spot development was uneffected or inhibited on plants not previously exposed to the various herbicides and leaf-inoculated with conidia in droplets of 2,4-D, 2,4,5-T, and 2,4,5-TP solutions at concentrations of 1012, 109, and 106 M. In contrast, MCPP and dicamba at concentrations of 1012, 109, and 106 M in droplets containing conidia and applied to leaves of plants not previously exposed to the herbicides, resulted in increased leaf spot severity. There was a substantial increase in leaf spot severity on the foliage of plants spray-treated with 2,4,5-T, MCPP, and dicamba, at all concentrations and then inoculated with conidia in droplets of distilled water; plants sprayed with 2,4-D and 2,4,5-TP showed increased disease at the 1012 M concentration, but at higher concentrations, 2,4-D had no effect, and 2,4,5-TP remained inhibitory to disease development. All plants grown in soil treated with various concentrations of each herbicide and inoculated with conidia in droplets of distilled water showed an increase in disease, except for 2,4,5-TP at concentrations of 106 and 104 M, which continued to inhibit disease development. Four lesion types produced by D. sorokiniana were observed on the herbicide-treated plants. These ranged from small purple-brown necrotic areas with or without halos to enlarged necrotic areas with or without severe chlorotic to straw-colored streaking of the uninfected tissue of the leaf. The results suggest that herbicide-increased disease is, for the most part, independent of any stimulatory effect of the herbicides on in vitro growth of D. sorokiniana. Instead, herbicide-induced increases in disease may involve imbalances in the carbohydrate-nitrogen metabolism of P. pratensis, and the severe chlorotic to straw-colored streaking of infected leaves is suggestive of ethylene evolution in diseased tissue and (or) toxin production.

Additional keywords: Bipolaris, Helminthosporium, mecoprop, silvex.