SUMMARY

The life history and cytology of Rhytisma punctatum on bigleaf maple was studied using inocula collected from infested areas in western Washington.

Results indicate that mature stromata rupture in late spring, and that ascospores are ejected for a short distance only, suggesting that air currents are necessary to distribute the fungus.

Soon after ascospore discharge, mycelia from germinating ascospores invade leaf cells both inter- and intra-cellularly.

Although spermagonia and spermatia occur in R. punctatum on Acer pseudoplatanus, these structures were not observed on A. macrophyllum.

About 10 weeks after invasion of host tissue or about mid-August, the fungus develops black dot-like stromata, each containing one apothecium. These appear in groups within lesions on surfaces of leaves. Individual apothecia develop slowly during winter months in fallen leaves. In late spring and early summer ascospores are formed and released.

Chromosomes studies indicated that N = 5.

Artificial inoculations were successful only when leaves were inoculated on upper surfaces.