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The Effect of Light and Physiologic Races on Leptosphaerulina Leaf Spot of Alfalfa and Selection for Resistance. M. C. Pandey, Department of Plant Pathology, University of Minnesota, St. Paul, Minnesota 55101, Present address of senior author: Inter-Asian Corn Improvement Programme, U.P. Agricultural University, P.O. Pantnagar, U.P., India; Roy D. Wilcoxson, Department of Plant Pathology, University of Minnesota, St. Paul, Minnesota 55101. Phytopathology 60:1456-1462. Accepted for publication 24 April 1970. DOI: 10.1094/Phyto-60-1456.

Symptoms on alfalfa (Medicago sativa) caused by Leptosphaerulina briosiana were (i) of the resistant type (black spots with no chlorosis) when light intensity was 450 ft-c or less and day length was 8 hr/day; (ii) of the moderately susceptible type (black or brown spots with a trace of chlorosis) when light intensity was 1,100 ft-c and day length was 8 to 12 hr/day; and (iii) of the susceptible type (brown spots with tan centers, dark margins, and chlorotic halos) typical of those seen in the field, when light intensity was 2,000 ft-c or more and day length was 12 or more hr/day. Temperatures between 20 and 29 C and high atmospheric humidity during symptom development did not alter the effects of light. Microscopically, symptoms on susceptible and resistant plants were similar until about 120 hr after infection. Symptoms on susceptible plants could always be distinguished from those on resistant plants by the greater number of injured cells, by the collapse of cell walls of the upper epidermis and mesophyll cells, by the presence of abundant mycelium, and by sporulation in the lesions. Three physiologic races were distinguished on alfalfa clones using symptom type as the basis for judging resistance. In three different experiments of 14,000 alfalfa plants from 87 varieties and clones in greenhouse trials, 48 resistant plants were selected. When these were further evaluated in the field, four remained resistant, probably because the races present in the field were different from the one used in the greenhouse. In the greenhouse, the host range was extended to include Pisum sativum, Gomphrena globosa, Arachis hypogaea, and Glycine max. These hosts probably have little or no importance in the life cycle of the fungus or the disease it incites.

Additional keywords: Leptosphaerulina briosiana.