Survival of the Oak Wilt Fungus in Logs Fumigated with Sulfuryl Fluoride and Methyl Bromide

Abstract

Abstract Methyl bromide (MB) fumigation of oak (Quercus sp.) logs destined for export is required to mitigate risks associated with movement of the oak wilt fungus, Bretziella fagacearum. Alternative fumigants with efficacy against B. fagacearum are needed because of MB's ozone-depleting properties. Fumigation with sulfuryl fluoride (SF) is considered a promising substitute. Logs (1.8 m long) were obtained from Quercus trees (18.3 to 29.2 cm diameter at breast height) that were naturally infested (NI) or artificially inoculated (AI) with B. fagacearum to compare pathogen colonization and survival following fumigation with SF and MB. The logs were fumigated with SF for 72 hours with 240, 280, and 320 g/m3 or 96 hours with 128 and 240 g/m3. MB fumigations were conducted using the current treatment schedule for oak logs destined for export (240 g/m3 for 72 h). Frequencies of successful pathogen isolation before treatment were higher for AI logs than for NI logs based on isolation rates from sapwood chips. Treatments greatly reduced frequencies of viable pathogen presence, but no treatment was successful in eradicating the pathogen. Experiments were conducted on blocks (10.2 by 10.2 by 11.4 cm3) obtained from Quercus trees to investigate simulated penetration and diffusion of SF and MB into oak logs. Slow, variable fumigant diffusion never reached concentration–time products lethal to B. fagacearum. Based on these results, reliance on SF alone as a quarantine measure may require higher concentration × time products to achieve quarantine level control of the oak wilt fungus in logs.