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Responses to Septoria glycines of Soybeans Nearly Isogenic Except for Seed Color. S. M. Lim, Research plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, and Department of Plant Pathology, University of Illinois, Urbana 61801; Phytopathology 73:719-722. Accepted for publication 6 December 1982. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1983. DOI: 10.1094/Phyto-73-719.

Two distinct types of chlorotic and nonchlorotic lesions on the leaves of soybean plants infected with Septoria glycines are associated with plants grown from yellow and green seeds, respectively. This research was undertaken to determine the effects of two lesion types on the development of brown spot and subsequent effect on yield of three derivatives of Clark soybeans nearly isogenic except for seed color (Clark-L-1 [yellow], L64-2545 G d1d2, and L62-1027 cyt-G [green]) in the field in 1978 and 1979. Treatments included inoculation and control (protected with benomyl application) in both years and a check (natural infection) was added in the 1979 experiments. Brown spot severity was rated three to five times during the reproductive growth stages. Defoliation was also determined by counting the number of defoliated nodes at each disease rating. In both years, there were no significant differences in brown spot severity, apparent infection rate, or number of defoliated nodes between chlorotic and nonchlorotic lesion types of Clark lines inoculated with S. glycines. Yield reductions in these isogenic lines ranged from 8 to 11% at Urbana in 1978, 12 to 14% at Urbana in 1979, and 9 to 10% at Brownstown in 1979 compared to yields of the same lines protected with benomyl. However, differences in yield reductions among lines producing two lesion types were not significant for either years. Therefore, resistance and tolerance to brown spot cannot be characterized by these two lesion types associated with seed color.

Additional keywords: brown spot, disease reaction, Glycine max, soybean disease, soybean yield loss.