Apricot pruning wounds made at all seasons of the year were susceptible to infection and canker formation resulting from mycelial inoculation with Eutypa armeniacae. Cambial inoculations resulted in comparatively little infection, and inoculations of the inner bark (phloem) were negative. Evidence indicated that the pathogen is inactivated in some infections, and thus sometimes is prevented from forming perennial, girdling cankers. Death of the pathogen did not appear to result from the action of other microorganisms. Invasion of the xylem by E. armeniacae resulted in the formation of gum plugs in some of the tracheary elements, and the development of hyphae in the lumens of vessels, fiber-tracheids, and ray parenchyma cells. The hyphae penetrated cell walls directly or through pits. In advanced stages of infection, the hyphae grew longitudinally within the secondary wall, especially of fiber-tracheids, where, presumably through enzymatic action, they caused the development of soft rot. This soft-rot condition appears to be responsible for the commonly encountered brittle nature of apricot branches killed by this pathogen. The ability of E. armeniacae to cause soft rot was demonstrated histologically, and by its capacity to cause weight loss in inoculated birch wood. The soft-rot phenomenon appears to develop late in the disease syndrone. In vitro studies indicated the ability of the pathogen to produce enzymes that degraded cellulose and xylan. Almond and peach trees were slightly susceptible when wounded tissues were inoculated with mycelia of the fungus.