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Infection Court and Factors Affecting the Expansion of Stem Canker of Avocado Caused by Phytophthora citricola . Z. A. EL-HAMALAWI, Department of Plant Pathology, University of California, Riverside 92521. J. A. MENGE, and F. B. GUILLEMET, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. 79:384-388. Accepted for publication 5 December 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0384.

Stem wounds are the main infection court for Phytophthora citricola on avocado. P. citricola in cankers formed after inoculation of stem wounds or wounds caused by removal of suckers could move upward in the inner bark and cause infection of stem wounds above these inoculation sites. Inoculation of the root systems of either nurse seed plants or clonal rootstock plants with P. citricola caused infection to both root systems, but the pathogen did not move upward to the stems during the first 4 wk after inoculation. There was no direct transmission of P. citricola through the phloem from the infected stems of nurse seed plants to wounds made on the stems of clonal rootstock plants. P. citricola infected and survived in wound sites caused by removal of seed cotyledons without further advancement. P. citricola was isolated from healed-over cankers, but only when these cankers were scraped to reveal healthy stem tissue did P. citricola move into the phloem and infect distant wounds. Healed wounds and healed-over cankers were not susceptible to infection unless they were reinjured. P. citricola was isolated from the sugary exudate that emanated from cankers caused by P. citricola