The cambium of balsam poplar appeared to be stimulated by Rhytidiella moriformis and Keissleriella emergens infections of bark, colonized to the outer periphery of the phloem but not beyond. Instead of producing normal xylem in groups of four cells, it produced hyperplastic and hypoplastic deformed xylem, each group consisting of variable combinations of tracheids, vessel elements, and wood parenchyma. Deformities were accompanied by composite cell structures and rapid aging of thick-walled ray cells. Tracheids and vessel elements were branched, curvate, attenuate, and short. Vessel elements without perforations had more bordered pits arranged on all sides. Perforations, if present, were often lateral and not oriented for vertical transport of solutes. Morphogenetically, host tissues are believed to be chimeral, as evidenced by stunting, incomplete cell-plate formation, and hyperplasia and hypoplasia. Changes in structure of cells and tissues of the host were caused by both pathogens, but they occur over a larger surface area of the bark and sapwood on trees infected by R. moriformis in nature.

Rhytidiella baranyayi n.sp. is described from corky proliferations of aspen (Populus tremuloides Michx.) in British Columbia, Canada, and from its growth in culture. A bitunicate ascomycete, it is suspected of being the causal agent of the disease with which it is consistently associated.

Ceratocystis fimbriata Ell. and Halst. is reported for the first time to be pathogenic to aspen in Manitoba and Saskatchewan. Infection studies revealed that the fungus causes two types of symptoms, one a leaf spot and twig dieback, and the other a target canker of the stem. Twig and branch dieback resulted from a rapid infection of the young petiole, leaf trace, cortex, phloem, and cambium of the stem. Target canker was caused by slow infection of older petioles and bark. A continuous cicatricial zone walled off the healthy bark from the infected area and formed a callus fold at the margin of the lesion.Histological studies of the penetration phase of infection revealed that hyphae of C. fimbriata penetrate directly through the epidermis of leaves, petioles, and young stems. Colonization of the tissue is intracellular, although intercellular hyphae may be found in the mesophyll and pith. Hyphae colonize mainly the phloem and cambium of the stem.

Rhytidiella moriformis n. gen., n. sp. is a parasite which causes perennial rough-bark of Populus balsamifera L. The fungus produces perfect and imperfect states in nature. Cultural studies show that the macroconidia belong to the form-genus Phaeoseptoria Speg.

Dolabra nepheliae Booth and Ting is redescribed from the type and its imperfect state is described as Rhabdospora nepheliae sp. nov. New information on host and geographic distribution is also reported.