Development of macro‐ and microconidia of several species of Sclerotinia causing brown rot of rosaceous fruits was studied by light and electron microscopy. Macroconidia are produced by budding and develop in acropetal succession. Septa form centripetally across the constricted regions between the spore initials of a chain. A single pore in each septum permits movement of cytoplasm and organelles along the chains of spores and these pores are probably not plugged until just before the spores are liberated. The ultrastructure of the mature macroconidia is similar to that of the vegetative hyphae, indicating the unspecialized nature of these spores. Microconidia are formed in succession by extrusion from the tips of terminal phialides. The developing spore is protected by a thin wall and is finally cut off by formation of a basal septum. An uneven splitting of this septum occurs, the thicker part becoming the basal wall of the spore. The latter remains loosely attached to the phialide until another spore is produced. Extrusion of the spores leaves a collar or rim just behind the apex of the phialide. The mature spore contains a large lipid body, a single large nucleus, a small amount of cjrtoplasm and one or two small mitochondria. It is thus of simpler organization than the macroconidium. The development and structure of the two types of spore are compared and the differences correlated with function. Copyright © 1969, Wiley Blackwell. All rights reserved
