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

Abstract

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°C for 60 min and 56°C for 30 min. Complete elimination of P. juglandis and G. morbida was achieved when using a minimum of 56°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.

Evaluation of Kiln Heating as a Phytosanitary Treatment for Ceratocystis-Infested ‘Ōhi‘a (Metrosideros polymorpha) Wood

“…Thus, the dehumidification kiln treatments were successful in eradicating viable Ceratocystis propagules from the colonized poles. Like our previous work with a vacuum-steam unit, a threshold temperature of 608C eradicated C. lukuohia, C. huliohia and the oak and walnut pathogens Bretziella fagacearum and Geosmithia morbida, respectively (Juzwik et al 2019b(Juzwik et al , 2021(Juzwik et al , 2022. The current standard of 608C for 60 minutes (heat treatment standard T314-a) was accepted for treating ash (Fraxinus spp.)…”

“…Thus, the vacuum kiln treatments were successful in eradicating viable pathogen propagules from the colonized logs. This threshold temperature eradicated Ceratocystis and several tree pathogens, including Phytophthora cinnamomi and Lasiodiplo-dia theobromae (Ramsfield et al 2010;Juzwik et al 2019bJuzwik et al , 2021Juzwik et al , 2022. The heating schedule of 568C/30 min is the accepted wood treatment standard to eradicate gypsy moth (Lymantria dispar) and Asian longhorn beetles (Anoplophora glabripennis) in the United States (Myers and Bailey Within this trial, log-core temperatures were monitored at a radial depth of 70 percent to the core since neither ROD Ceratocystis species colonizes the core heartwood (Hughes et al 2020, Juzwik et al 2019a.…”

“…With the dehumidification kiln, humid air is passed over a cooling coil where water condenses, and the cool water is drained outside the kiln while the heated air is recirculated. This recirculation of hot air can result in within-kiln air temperatures !728C, which exceeds the 568C and 608C temperatures known to kill fungal pathogens in wood (Ramsfield et al 2010;Mayfield et al 2014;Juzwik et al 2019bJuzwik et al , 2021Juzwik et al , 2022.…”

“…In a previous study with ' ohi'a, submersion in a 15 percent borate solution and a 10-week drying/diffusion period reduced C. lukuohia viability in 1-m-long bolts, but did not completely eliminate the pathogen (Hughes et al 2021). Vacuum steam treatment has been effective in eliminating tree pathogenic fungi from oak and walnut logs (Juzwik et al 2019b(Juzwik et al , 2021. The same portable vacuum steam unit that successfully treated oak and walnut logs was shipped to Hawai'i to test large-diameter ' ohi'a logs (24 to 43 cm in diameter), and heating to 608C for 60 minutes to a depth of 70 percent of the log radius completely eradicated ROD fungi (Juzwik et al 2022).…”

Thousand Cankers Disease in Walnut Trees in Europe: Current Status and Management

“…Therefore, attention has been focused on alternative treatments for infested logs that kill the two agents of TCD without negatively affecting wood quality. Among the protocols that have been developed in the US for the treatment of infested logs, vacuum steam treatment is one of the most promising, achieving complete elimination of both GM and the WTB by applying a minimum of 56 °C for at least 30 min [ 62 ]. Specific surveillance and monitoring techniques consist of visual examinations and pheromone traps in both the US [ 63 ] and Europe.…”