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Growth, Sporulation, and Pathogenicity of Stagonospora meliloti and Selection for Resistance to Crown Rot and Leaf Spot in Alfalfa. D. C. Erwin, Professor, Department of Plant Pathology, University of California, Riverside 92521. R. A. Khan, O. K. Ribeiro, and W. F. Lehman. Staff Research Associate, Department of Plant Pathology, University of California, Riverside 92521; Plant Pathologist, Microbiotica International Inc., 10744 Manitou Beach Dr., Bainbridge Island, WA 98110; and Agronomist, Department of Agronomy and Range Science, University of California, Davis 92516, and Imperial Valley Research Center, El Centro 92243. Plant Dis. 71:181-185. Accepted for publication 22 July 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0181.

The optimal temperatures for growth on V-8 juice agar (V8 A) of Stagonospora meliloti, the causal agent of a leaf spot and root rot of alfalfa, were between 18 and 24 C; for pycnidial production, 2124 C; and for pycnidiospore germination, 1827 C. More pycnidia formed under blue and green light than in darkness, red light, or near-ultraviolet light. In darkness, numbers of pycnidia were reduced but size was markedly increased. In almost all light regimes, immature pycnidiospores were predominantly nonseptate, but when mature (about 30 days), most were one-septate. The number of septa varied from one to six under different light regimes, except under green and red light, at which pycnidiospores were predominantly one-septate. Some single-ascospore isolates, which were obtained from perithecia (Leptosphaeria pratensis) on infected stem tissue, were pathogenic to leaves, but others were not. Single-ascospore isolates produced pycnidia and were similar morphologically to those isolated from diseased tissue. Inoculation of wounded roots and crowns of alfalfa plants with pycnidiospores was more efficient than inoculation of unwounded plants. None of several cultivars was highly resistant, but seed progeny from resistant plant selections that were intercrossed (UC129A and UC129B) were more resistant to root rot and leaf spot than the parent cultivars.