Vacuum Steam Treatment Effectiveness for Eradication of the Thousand Cankers Disease Vector and Pathogen in Logs From Diseased Walnut Trees

Logs of high-value eastern black walnut (Juglans nigra L.) are commonly exported from the United States for production of veneer and lumber. Veneer logs are not debarked to minimize degradation of wood quality and reduce moisture loss. Thousand cankers disease (TCD) is caused by the walnut twig beetle (Pityophthorus juglandis Blackman) and the fungal pathogen, Geosmithia morbida M. Kolarik, E. Freeland, C. Utley and N. Tisserat sp. nov., which colonize the inner bark of Juglans species. Effective eradication of these organisms by heat or chemical fumigation treatment is required for walnut logs prior to export. Because vacuum steam is an effective and efficient means of heating round wood, its use in eliminating the TCD causal agents was evaluated using Juglans logs (12- to 44-cm small end diameter and 1.7- to 1.9-m length) from TCD-symptomatic trees in Oregon and Washington State. Five replicate trials with three logs per load were conducted in a portable vacuum chamber to test two treatment schedules: 60 degrees C for 60 min and 56 degrees C for 30 min. Complete elimination of P. juglandis and G. morbida was achieved when using a minimum of 56 degrees C at 5-cm targeted depth from bottom of bark furrow into the sapwood and held for 30 min. Treatment cycle time ranged from 298 to 576 min depending on log diameter and initial log temperature. Artificial inoculation of J. nigra trees with G. morbida within the TCD range in Pennsylvania was minimally successful in producing adequately colonized logs for experimental trials.