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Histopathology of Sclerotinia sclerotiorum Infection of Bean. R. D. Lumsden, Research Plant Pathologist, Plant Protection Institute, ARS, USDA, Beltsville, Maryland 20705; R. L. Dow, Research Assistant Plant Pathologist, Plant Protection Institute, ARS, USDA, Beltsville, Maryland 20705. Phytopathology 63:708-715. Accepted for publication 19 December 1972. DOI: 10.1094/Phyto-63-708.

Infection cushions were formed on bean hypocotyls within a few hours after inoculation with Sclerotinia sclerotiorum-infested oat kernels. The dome-shaped cushions were composed of three safranin-stained hyphal types: (i) dark red hyphae on the upper surface; (ii) light red, granular, inflated hyphae in the center; and (iii) light red, granular, dichotomously-branched, penetration hyphae next to the host surface. Multiple penetration pegs from the latter hyphae forcibly entered the host through the cuticle and formed inflated, lightly stained vesicles between the cuticle and epidermal cells. The vesicles were focal points for production of lightly stained subcuticular and cortical “infection” hyphae (modal diam 17.0 µm). The subcuticular hyphae were oriented parallel to one another and formed organized infection fronts. These hyphae eventually became oriented parallel to the long axis of the hypocotyl and grew up the stem more rapidly than the cortical hyphae and generally on the same side as the point of penetration. Cortical infection hyphae also were oriented parallel to the hypocotyl, and girdled the stem. In contrast to the infection hyphae, “ramifying” hyphae (modal diam. 8.5 µm) were stained dark red with safranin and grew throughout the invaded tissue both inter- and intracellularly. These hyphae arose as branches, ca. 55 µm from the apex of the infection hyphae. A primary role in pathogenesis is suggested for the intercellular infection hyphae, and a secondary and perhaps nutritional role is suggested for the ramifying hyphae. Upon maturation of lesions (48 to 72 hr), ramifying hyphae moved to the surface layer of hypocotyl cells and, being unable to penetrate the still intact cuticle, the hyphae emerged through stomata. On the surface, tufts of mycelia developed into dense cottony mats of mycelia or formed sclerotium initials. The vascular and pith regions of the stem were not colonized until the cortex was thoroughly invaded.