Urediniospores of M. larici-populina were uniformly echinulate except for a smooth area on or near the rounded apex (Fig. I).

The abnormally-shaped urediniospores which are commonly observed in M. larici-populina are more likely to be formed as a result of the poorly structured uredium typical of this species.

Urediniospores of the 'parents', M. medusae and M.larici-populina are morphologically distinct and readily identified by microscopic examination of urediniospores. The most reliable character being the position of the smooth patch on the urediniospore, viz. the bald equatorial patch of M. medusae and the bald apex of M. Iarici-populina (Van Kraayenoord et al.. 1974).

With closer examination the two species are found to differ with regard to the size and shape of urediniospores, the apical thickness of paraphyses, and the location of the telia. (Walker et al.. 1974).