EFFECT OF WOUND AGE AND FUNGICIDE TREATMENT OF WOUNDS ON SUSCEPTIBILITY OF AVOCADO STEMS TO INFECTION BY PHYTOPHTHORA-CITRICOLA

Bark wounds of avocado (Persea americana) stems of the cultivar Topa Topa became resistant to infection by Phytophthora citricola within 12-14 days after injury. The size of cankers declined significantly as the time interval between wounding and inoculation increased. Wounds that were reinjured after 2 mo of aging became susceptible to infections Similar to those of wounds made on previously unwounded stems. These results suggest that chemical treatment applied to fresh bark wounds must protect against P. citricola for at least 14 days to be effective. Fungicide treatments for curing as well as protecting avocado stems against trunk canker disease caused by P. citricola were investigated. Wounds on intact and excised avocado stems were treated with fosetyl Al, fosetyl Al + Tree Seal, metalaxyl, Bordeaux mixture, Tree Paint, or Tree Seal after infection with P. citricola. The development of cankers was suppressed only by treatments including fosetyl Al. Wounds were protected by fosetyl Al through the period required for the wound to become resistant to infection. Brief washing of wounds treated with fosetyl Al 24 hr after treatment did not reduce protection of the wound site.