Evaluation of phosphonate treatments for control of phytophthora crown rot of walnut

Authors: Browne, Greg; Schmidt, Leigh; PRICHARD, TERRY - University Of California; KRUEGER, WILLIAM - University Of California - Cooperative Extension Service

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2011
Publication Date: 6/1/2011
Citation: Browne, G.T., Schmidt, L.S., Prichard, T.J., Krueger, W.H. 2011. Evaluation of phosphonate treatments for control of phytophthora crown rot of walnut. Plant Health Progress. doi:10.1094/PHP-2011-0601-01-RS.

Interpretive Summary: Foliar spray and irrigation water applications phosphonate were evaluated for control of trunk cankers caused by Phytophthora citricola in an experimental Persian walnut orchard. The walnut variety was ‘Chandler’. In each of two experiments, the foliar treatment was applied once in the second week of September, whereas the soil treatment included three chemigations with phosphonate applied at weekly intervals from the last week of August to the second week of September. Following tree trunk wound inoculations with P. citricola, the trees that had received a foliar spray with phosphonate consistently developed less necrotic bark area than the non-sprayed control trees. Canker suppression from the foliar treatment was evident up to 11 months after its application. Irrigation water applications of phosphonate (i.e., phosphonate chemigation) contributed to canker suppression during an initial 2-month assessment period in one of the two experiments, but otherwise the chemigation did not significantly affect canker development. The results indicate that late-summer foliar spray treatments with phosphonate can suppress trunk cankers caused by P. citricola on walnut, whereas chemigation treatments with phosphonate may not be effective.

Technical Abstract: Foliar and soil applications of phosphonate were evaluated in a factorial manner for control of trunk cankers caused by Phytophthora citricola in a Persian walnut orchard, cultivar ‘Chandler’. In each of two experiments, the foliar treatment was applied once in the second week of September, whereas the soil treatment included three chemigations with phosphonate applied at weekly intervals from the last week of August to the second week of September. Following tree trunk wound inoculations with P. citricola, the trees that had received a foliar spray with phosphonate consistently developed less necrotic bark area than control trees. Canker suppression from the foliar treatment was evident up to 11 months after its application. Chemigation with phosphonate contributed to canker suppression during an initial 2-month assessment period in one of the two experiments, but otherwise the chemigation did not significantly affect canker development. The results indicate that late-summer foliar spray treatments with phosphonate can suppress trunk cankers caused by P. citricola on walnut, whereas chemigation treatments with phosphonate may not be effective.