Postemergent Herbicides and the Biology of Drechslera sorokiniana: Influence on Severity of Leaf Spot on Poa pratensis

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Abstract
The influence of 4 chlorophenoxy (2,4-D, 2,4,4-T, 2,4,5-TP, MCPP and 1 benzoic acid (dicamba) postemergent herbicides were evaluated for their influence on the development of leaf spot caused by D. sarokiniana on P. 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 10-3M (2,4,5-TP at 10-4M.) 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 10-12, 10-9 and 10-6 M. In contrast, MCPP and dicamba at concentrations of 1012, 10-9 and 10-6 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 innoculated with conidia in droplets of distilled water; plants sprayed with 2,4-D and 2,4,5-TP showed increased disease at the 10-12 M concentration, but at a 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 10-6 and 10-4 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. 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-N metabolism of P. pratensis, and the severe chlortic to straw-colored streaking of infected leaves is suggestive of ethylene evolution in diseased tissue and (or) toxin production.