12 years (1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) and still only controlling a fraction of infections (Wilson, 2001). Although oak wilt occurs more sporadically and acts differently in the Appalachian region and northeast (Juzwik et al., 2011), land managers continue to be concerned about the disease as its range expands northwards. In areas where the disease has not been found, such as Canada, oak wilt is a pathogen of regulatory concern, and modelled estimates of removal and replacement costs were hundreds of millions of Canadian dollars (Pedlar et al., 2020).Early detection is crucial to management success, as finding and removing infected trees before the fungus can reproduce allows managers to stop local spread (Juzwik et al., 2011).Our native oak trees contribute numerous timber, wildlife and aesthetic benefits, as well as improving the quality of our environment.
The 2015–2018 outbreak of spongy moth (Lymantria dispar) in southern New England initiated a severe oak decline and mortality event. While defoliation was a primary driver, increased secondary pest and pathogen activity contributed to decline and death. Following this large defoliation event, Diplodia was frequently recovered from northern red oaks (Quercus rubra) with serious outbreaks of twig cankering. Given the many recent reports of Diplodia corticola on oak in eastern North America, it was presumed to be the causal agent. To confirm, a limited survey was conducted from five states in the region (Maine, Massachusetts, New Hampshire, New York, and Vermont). Based on ITS, tef1 and tub2 sequences generated from 28 isolates collected primarily from northern red oak, Diplodia gallae, two potentially novel Diplodia species and Diplodia sapinea were identified. Surprisingly, D. corticola was not found in this study. ITS sequences alone failed to discriminate among isolates of D. gallae and D. corticola, creating uncertainty over previous reports of D. corticola in eastern North America. Only a combined ITS + tef1 dataset successfully distinguished D. gallae and D. corticola along with two other closely related species that also occur on oak (Diplodia quercicola and Diplodia quercivora). Additional cankering and endophytic fungi (Coryneum, Dendrostoma, Gnomoniopsis, Pestalotiopsis and Tubakia) were also found on symptomatic oaks in the region. Identification of Diplodia isolates from non‐Quercus hosts also detected Diplodia neojuniperi on Juniperus chinensis and Microbiota decussata, which has not been reported previously in North America.
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