Process of Ceratocystis fimbriata infection in Aspen

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