THE RELATION OF BARK MOISTURE TO THE DEVELOPMENT OF CANKER DISEASES CAUSED BY NATIVE, FACULTATIVE PARASITES: 2. FUSARIUM CANKER ON BLACK COTTONWOOD

Abstract

A close correlation was found to exist between the development of a bark canker in nursery-grown Populus trichocarpa Torrey and Gray, caused by Fusarium lateritium Nees., and the moisture content of the living bark. When bark moisture was expressed as a percentage of the amount of water required to saturate the sample under experimental conditions, relative turgidities of 80% or more inhibited canker development which, however, occurred normally at lower percentages. Studies of the epidemiology of Fusarium canker in the nursery afforded evidence in support of the limiting effect of bark moisture on canker development. Thus during the dormant season of 1957–58, the monthly average temperature was higher than the minimum temperature for the growth of F. lateritium on potato dextrose agar, and the relative turgidity did not reach the inhibiting value of 80%. Cankers continued to develop throughout this period. During the growing season while temperatures were still more favorable for fungus development, no extension of cankers occurred in the nursery trees, apparently because, during the growing period, the relative turgidity was in excess of 80%. Canker development was prevented during the winter by placing dormant cuttings in water, which likewise increased the moisture content of the bark above the 80% level.During the dormant season the bark of field-grown black cottonwoods maintained relative turgidity values of approximately 80% or higher and the disease could not be found on these trees.