Formation and ultrastructure of microsclerotia of Pyrenochaeta lycopersici

The formation of microsclerotia of Pyrenochaeta lycopersici Schneider & Gerlach was observed in culture, in isolated tomato root segments and in roots of intact tomato plants. In culture microsclerotia formed in the aerial mycelium, and submerged in the agar; in tomato roots they formed in single cells of the outer cortex. The microsclerotium of P. lycopersici is structurally unspecialized and has cell walls of similar thickness throughout. Microsclerotia are thought to have powers of long survival in soil. In isolation the microsclerotia of P. lycopersici maintained viability over 2 years with consistently high germinability. Transverse hyphae were never seen in sections of microsclerotia, suggesting that germ-tubes arise from the outermost layers of cells.