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Seasonal Dynamics of Alfalfa Foliar Pathogens in Iowa. S. Shabbir A. Rizvi, Postdoctoral Research Associate, Department of Plant Pathology, 351 Bessey Hall, Iowa State University, Ames 50011. Forrest W. Nutter, Jr., Associate Professor, Department of Plant Pathology, 351 Bessey Hall, Iowa State University, Ames 50011. Plant Dis. 77:1126-1135. Accepted for publication 23 July 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-1126.

The frequency of occurrence of 11 foliar pathogens of alfalfa was monitored at four locations in Iowa over a 2-yr period. At each location, disease severity and percent defoliation were assessed weekly beginning in early May and continuing until late September or early October. For each sampling period and for all four locations, 100 lesions or blotches were incubated in moisture chambers (12 hr of light per day) for 48 hr at 23 C to facilitate pathogen sporulation and identification. Percentage of the total lesions or blotches caused by each pathogen was plotted vs. time to identify seasonal patterns. Correlation analyses were performed between the percentages of lesions or blotches caused by each pathogen sampled and the date of sampling (day of year). Correlations were also determined between frequency of pathogen occurrence and mean rainfall and mean maximum and minimum temperatures occurring 1, 2, and 3 wk before each sampling date. Ten fungal pathogens (Cercospora medicaginis, Colletotrichum dematium, Colletotrichum trifolii, Leptosphaerulina trifolii, Leptotrochila medicaginis, Phoma medicaginis, Pseudopeziza medicaginis, Stagonospora meliloti, Stemphylium botryosum, and Uromyces striatus) and one bacterial pathogen (Xanthomonas campestris) were identified. Of these, Phoma medicaginis, Leptosphaerulina trifolii, Pseudopeziza medicaginis, and Cercospora medicaginis occurred in the highest frequencies. Occurrences of Phoma medicaginis, Colletotrichum dematium, and Xanthomonas campestris were negatively correlated with date of sampling (early-season pathogens), whereas occurrences of Cercospora medicaginis, Leptotrochila medicaginis, Stagonospora meliloti, Pseudopeziza medicaginis (in most location years), and Uromyces striatus were positively correlated with date of sampling (late-season pathogens). Disease severity and defoliation at harvests varied among locations and years. Defoliation ranged from as little as 3% at Knoxville in 1992 to as high as 71% at Ames in 1991, and disease severity at the time of harvests ranged from 3% at Knoxville in 1992 to 73% at Ames in 1991.