Metalaxyl and phosphonate as prophylactic and curative agents against stem infection of Leucadendron caused by Phytophthora cinnamomi

Abstract

Neither metalaxyl (100 mL of a solution of 2.0 g a.i./L, applied as a root drench to 10-L pots) nor phosphonate (foliar spray at 1.0 g a.i./L, applied to runoff) controlled Phytophthora cinnamomi stem infection of Leucadendron laureolum x L. salignum hybrid when applied 10 days after inoculation. Both fungicides were most effective when applied prior to inoculation. Neither fungicide was able to kill P. cinnamomi within established infections. The fungus survived within the vascular tissues of treated plants. Infection was confined to cortical tissue by wound periderm when the rate of tissue invasion by P. cinnamomi was slowed by the fungicides. The results confirm observations made in protea growers' fields that with highly susceptible species, systemic fungicides can have poor curative properties and should be used as a prophylactic, in conjunction with other disease control methods.