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Influence of the Pathogen on Disease Severity in Stemphylium Leafspot of Alfalfa in California. W. A. Cowling, Graduate research assistant, Department of Plant Pathology, University of California, Davis 95616; D. G. Gilchrist, assistant professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 70:1148-1153. Accepted for publication 19 May 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-1148.

Disease severity in Stemphylium leafspot of alfalfa (Medicago sativa) in California was assessed on a cloned alfalfa genotype at different levels of pathogen relative virulence and inoculum concentration. Measurements of disease severity, defined as percent leaf area necrotic (LAN), were based on the analysis of its two components: average lesion area and number of lesions per leaf. Isolates of Stemphylium botryosum from throughout California varied greatly in relative virulence. Further variation in relative virulence arose through monoconidial transfers in culture. However, average lesion area, previously used to distinguish races of the pathogen, was not affected by inoculum concentration or relative virulence of S. botryosum isolates, despite the large differences in percent LAN due to these factors. Similar mean values and frequency distribution patterns of lesion area were recorded from naturally infected plants collected in the field in California and from those inoculated in growth chamber studies. Therefore, disease severity was assessed validly by counting lesion numbers per leaf or visually estimating percent LAN. It is concluded that lesion size and restriction of lesion expansion by the host are independent of pathogen-related factors such as relative virulence and inoculum concentration. These conclusions are restricted to the form of the disease found in California.

Additional keywords: Pleospora herbarum, epidemiology, lucerne, crop loss assessment.