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Histological Responses of Peanut Germplasm Resistant and Susceptible to Cylindrocladium crotalariae in Relationship to Inoculum Density. N. E. Harris, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27650; M. K. Beute, professor, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Phytopathology 72:1250-1256. Accepted for publication 22 February 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1250.

Taproots of nine peanut entries including susceptible (Florigiant and NC 2) and resistant (NC 3033, NC 3033 NC 2, NC 3033 Florigiant, Florigiant NC 3033, Argentine, Argentine Florigiant, and Florigiant Argentine) germplasm inoculated in greenhouse tests with one, two or four microsclerotia (ms) of Cylindrocladium crotalariae showed no qualitative histological differences in the basic formation of original or additional taproot periderms or in the suberization of periderm. Florigiant sustained more breachments of its original periderm and protected fewer of its periderm breachments with additional periderm than did the other entries. Additional phellogens were initiated in all parts of the peanut taproot where viable parenchymatic cells existed. In infested field plots with an inoculum density <2.9 ms/g of soil, the nine entries gave responses similar to those obtained in greenhouse tests. Primary branch roots in all entries showed potential for limited secondary growth including additional periderm formation when infected by C. crotalariae. The sites of emergence of branch roots from the taproot cortex were favorable entry points for the fungus. All entries initiated phellogens in infected nodules. The Poisson distribution was used to describe the theoretical probability of the occurrence of microsclerotia on the taproot surface, assuming mean densities of ms commonly occurring in North Carolina peanut fields. Resistant NC 3033 remained highly resistant with four ms per square millimeter of taproot, whereas susceptible Florigiant was moderately to severely diseased with one ms per square millimeter of taproot.