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First Report of Diplodia quercivora Causing Stem Cankers on Chestnut Oak (Quercus montana) in Maryland
- Haines, S. L., Stauder, C. M., Martin, D. K. H., Thompson, B. A., Kasson, M. T.
- Plant disease 2019 v.103 no.6 pp. 1423
- Castanea, Diplodia, Quercus alba, Quercus montana, Quercus rubra, antibiotics, blight, bolts, dieback, discoloration, forests, genes, glucose, leaves, mortality, mycelium, pathogenicity, ribosomal DNA, ribosomal RNA, saplings, stem cankers, stems, tissues, yeast extract, Maryland, Mid-Atlantic region, West Virginia
- In the Mid-Atlantic region, oak species (Quercus spp.) have been in decline for the past several decades (Oak et al. 2016). Recently, Diplodia corticola, an emerging pathogen, has been implicated in contributing to decline of red oaks in West Virginia (Martin et al. 2017). A second species, D. quercivora, has been associated with decline of white oak family members elsewhere in the U.S. (Dreaden et al. 2014). In 2018, outwardly symptomatic chestnut oaks (Quercus montana) exhibiting acute wilt, blighted leaves, and/or dieback were observed in Allegany County, MD. Radial cross sections were taken from symptomatic trees, surface disinfested, and plated on glucose yeast extract agar with antibiotics (GYEA). Within a week, grayish-white sterile mycelial colonies with irregular margins characteristic of Diplodia spp. (Dreaden et al. 2014) emerged from plated tissues. DNA was obtained from pure cultures, PCR amplified, and sequenced for a portion of the nuclear ribosomal RNA gene repeat (rDNA) comprising the internal transcribed spacer region (ITS). GenBank BLASTn search showed the isolate sequence (deposited as MK160499) was 98% identical to D. quercivora. To confirm pathogenicity, sapling inoculations and a cut stem assay were performed on Q. montana from West Virginia University Forest in Morgantown, WV, using D. quercivora (isolate Dq703) from Q. montana in MD and D. corticola (isolate Dc103) from Q. rubra in WV (Martin et al. 2017). For sapling inoculations, 0.5-cm diameter GYEA plugs colonized by either Dq703 or Dc103 were used to inoculate three scalpel-wounded saplings/treatment. Three scalpel-wounded saplings received sterile 0.5-cm diameter GYEA plugs. Inoculation sites were sealed with Parafilm. Saplings (mean diameter 0.95 cm) were potted and maintained in a growth room at 23°C for 4 weeks. At 4 weeks, all Dq703 and Dc103 saplings had cankers while the controls remained canker free and showed limited vascular discoloration from wounding. Both Dq703 and Dc103 mean canker linear growth ([length+width]/2) were significantly larger (5.80 cm, P < 0.01 and 3.93 cm, P = 0.01, respectively) than the control (0.60 cm). For the cut-stem assay, harvested stems were cut into approx. 10-inch bolts (mean diameter 1.39 cm), waxed, and surface-disinfested before being wounded with a 9 mm diameter punch. Ten 0.5-cm diameter GYEA plugs of Dq703 and Dc103 were used to inoculate five borer-wounded chestnut bolts/treatment. Five borer-wounded bolts received eight sterile 0.5-cm diameter GYEA plugs. Inoculation sites were sealed with tape and bolts were maintained in the dark at 23°C for 6 weeks. At 6 weeks, all Dq703 and Dc103 bolts had cankers on all stems while the controls showed limited vascular discoloration from wounding. Both Dq703 and Dc103 mean canker linear growth were significantly larger (9.15 cm, P < 0.01, and 3.94 cm, P < 0.01, respectively) than the control (1.23 cm). Reisolation of both Dc and Dq from 100% of the saplings and 80% of bolts confirmed pathogenicity on white oak family members (Lynch et al. 2010), although Dc was not recovered during this study. Additional field studies are needed to assess the ability of Dq to cause blight and mortality in chestnut oak. To our knowledge, this is the first documented report of D. quercivora causing disease on Q. montana in Maryland.