AVOCADO TRUNK CANKER DISEASE CAUSED BY PHYTOPHTHORA-CITRICOLA - INVESTIGATION OF FACTORS AFFECTING INFECTION AND DISEASE DEVELOPMENT

Phytophthora citricola was able to cause stem canker of avocado only when inoculated into trunk or crown wounds. Neither intact bark nor lenticels on the crown or upper stem could be penetrated by P. citricola. While P. citricola infected and survived in the adventitious roots of avocado plants, it did not advance beyond the root base into the stem or woody root. Inoculum placed over cut wounds or injected under the bark on the crown or stem resulted in cankers in all cases. Avocado plants stressed by root pruning developed cankers more easily and at a greater rate than did nonstressed plants. The rate of disease development increased as the degree of stress increased. Younger plants were more easily predisposed to canker development by P. citricola than were older plants. Root pruning resulted in lower levels of soluble carbohydrates and higher levels of free amino compounds in the bark of avocado plants. Irrigation water sprayed on inoculation sites on avocado bark increased the rate of canker development more than sevenfold.