Managing Thousand Cankers Disease in High-value Plantings of Black Walnut (Fagales: Juglandaceae) in Washington State

Abstract

Abstract The health and productivity of black walnut (Juglans nigra L.) is currently threatened by Thousand Cankers Disease (TCD), a pest complex comprised of an insect vector, the walnut twig beetle (WTB) (Pityophthorus juglandis Blackman) (Coleoptera: Curculionidae), and associated pathogenic fungi (Geosmithia morbida Kolařík, Freeland, Utley, and Tisserat) (Hypocreales: Bionectriaceae). There is an urgent need for tools to manage TCD and avoid catastrophic losses in high-value plantations of black walnut. In this case study, we evaluated the efficacy of IPM strategies in three TCD-infested black walnut plantations in Walla Walla, WA. At the beginning of our study, average weekly captures of WTB per trap varied across sites; low (~32), moderate (~111), and high (>1,300). Each site received a separate management tactic: 1) girdled trap trees alone; 2) the insecticide emamectin benzoate alone or in combination with the fungicide propiconazole to protect healthy crop trees with superior form; and 3) the combination of girdled trap trees and chemically-protected crop trees. Walnut twig beetle populations remained at consistently low levels at both plantations after girdled trap trees were deployed. In plantations where crop trees were treated, the crown condition of those that received emamectin benzoate alone matched or exceeded that of trees treated with the combination of emamectin benzoate and propiconazole and the untreated control trees. Our findings suggest that using both trap trees and emamectin benzoate in an integrated approach has the most potential to reduce WTB populations and protect valuable crop trees in black walnut stands threatened by TCD.