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Developmental Morphology of the Black Knot Pathogen on Plum. S. H. Wainwright, Graduate Assistant, Department of Plant Pathology, The Pennsylvania State University Fruit Research Laboratory, Arendtsville, Pennsylvania 17303; F. H. Lewis, Scientist-in-Charge, Department of Plant Pathology, The Pennsylvania State University Fruit Research Laboratory, Arendtsville, Pennsylvania 17303. Phytopathology 60:1238-1244. Accepted for publication 20 March 1970. DOI: 10.1094/Phyto-60-1238.

Plum trees (Prunus domestica ‘Stanley’) were inoculated with ascospores of Dibotryon morbosum, the incitant of black knot disease, and the development of the pathogen within and on the host was studied. The fungus established a well organized tissue within the branch, from the xylem to the epidermis, and on the surface of the hyperplastic host tissues. Within the host the fungus formed a pseudoparenchymatous tissue which was devoid of any type of host cell penetrations. On the host surface the fungus developed a darkly-pigmented, thick-walled ascostroma on which conidiophores formed and within which many erumpent locules arose. Each mature centrum consisted of a basal layer of asci that had grown upward toward the apex among previously formed pseudoparaphyses. Ascospores were two-celled, and were two-ranked within bitunicate asci.