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Histology of Leaf Infection of Susceptible and Resistant Soybeans by Peronospora manshurica. J. H. Riggle, Research Assistant, Department of Botany and Plant Pathology, Iowa State University, Ames 50010; J. M. Dunleavy, Research Plant Pathologist, Agricultural Research Service, United States Department of Agriculture, and Professor of Plant Pathology Iowa State University, Ames 50010. Phytopathology 64:522-526. Accepted for publication 29 October 1973. DOI: 10.1094/Phyto-64-522.

Detached leaves of a susceptible soybean cultivar ('Wayne') and a resistant isoline ('SL9') were inoculated with conidia of Peronospora manshurica. After 12, 24, 36, 48, and 51 h, the leaves were cleared, stained, and mounted on microscope slides for observation. There were no discernible differences in the formation of germ tubes, appressoria, or penetration pegs on the two cultivars. Penetration and early hyphal growth was as rapid on resistant tissue as on susceptible tissue, but by 24 h the hyphae had ramified farther in the susceptible leaf than in the resistant leaf. At 36 h the fungus in the susceptible tissue had penetrated 40% farther than in the resistant tissue, and susceptible leaf tissue had a mean of 3.1 haustoria/penetration; whereas the resistant tissue had a mean of only 0.2 haustoria/penetration. After 36 h there were 14 haustoria/mm of hyphae in the susceptible tissue, but only 2.7/mm in the resistant tissue.

Additional keywords: downy mildew.