Efficacy of Chlorothalonil for Control of Spring Black Stem and Common Leaf Spot of Alfalfa. F. A. Gray, Plant Science Department, University of Wyoming, Laramie 82071. J. A. Fernandez, and J. L. Horton. Associate Professors, and Research Assistant, Plant Science Department, University of Wyoming, Laramie 82071. Plant Dis. 71:752-755. Accepted for publication 23 January 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0752.

Biweekly applications of chlorothalonil were highly effective in controlling spring black stem (SBS), caused by Phoma medicaginis var. medicaginis, and common leaf spot (CLS), caused by Pseudopeziza medicaginis, of alfalfa. Flowable formulation of chlorothalonil provided better control of SBS measured as disease severity and percentage of canopy diseased than copper hydroxide applied at the same rate. Disease control increased in all cultivars (because cultivar Ramsey is resistant to CLS, there was no response to chlorothalonil), years, and locations with an increase in the rate of chlorothalonil applied. Severity of SBS significantly decreased and forage yield significantly increased in the cultivar Agate at the first harvest but not at the second harvest with the 1.12-kg a.i./ha rate of chlorothalonil. There was a trend toward increased seed production and decreased seed infection by P. medicaginis var. medicaginis with an increase in rate of chlorothalonil in the susceptible cultivar Ranger but not in the resistant Ramsey.