Evaluation of Phosphonate Treatments for Control of Phytophthora Crown Rot of Walnut

Abstract

Preventive foliar and soil (chemigation) applications of phosphonate were evaluated in a factorial manner for control of cankers caused by Phytophthora citricola in a Persian walnut orchard, cultivar Chandler. In each of two experiments, a foliar treatment was applied once in the second week of September, and a soil chemigation treatment was applied three times at weekly intervals from the last week of August to the second week of September; the treatments were applied separately and in combination to the walnut trees. At 1 and at 7 months after completion of the treatments, the trees were wound inoculated with P. citricola. Treatment efficacy was assessed by measuring the area of resulting trunk cankers. Trees that had received a foliar spray with phosphonate consistently developed less necrotic bark area than trees that had not received the treatment. This effect was evident following the inoculations made 1 and 7 months after phosphonate treatments. Chemigation with phosphonate contributed to canker suppression only following inoculations made 1 month after treatment in experiment 1; the suppression was not evident in experiment 2. The results suggest that a late-summer foliar spray treatment with phosphonate, but not necessarily short-term chemigation treatments, may help to reduce losses caused by P. citricola in Persian walnut. Accepted for publication 4 March 2011. Published 1 June 2011.