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Beetles in the family Nitidulidae can transmit the oak wilt fungus, Ceratocystis fagacearum, to fresh wounds on healthy oak trees, leading to infection and disease development. Historically, nitidulid beetles have not been considered important vectors of the pathogen in Missouri. Studies were conducted in the spring of 2005 and 2006 to determine frequencies of nitidulid beetle species contaminated with C. fagacearum visiting fresh wounds on red oak trees in central Missouri. Colopterus truncatus, C. niger, and C. semitectus were the most abundant species collected from fresh wounds and the only species found to be contaminated with Ceratocystis fagacearum. Of 230 beetles assayed for C. fagacearum, 23 yielded the fungus. Contamination frequencies were higher for beetles collected in April than May; no beetles collected in June were contaminated. We hypothesize that Colopterus truncatus, C. niger, and C. semitectus are principal nitidulid beetle vector species in Missouri during spring. The risk for pathogen transmission by these beetles appears to be greatest in April and least in June.

Periodic episodes of oak decline have occurred in the Missouri Ozark forests since the early 1900s and the disease is currently severe (2). Several Phytophthora spp. contribute to oak decline in Europe (1), but the role of Phytophthora spp. in oak decline in the eastern United States is not known. Mineral soils collected around the bases of declining and nondeclining oaks in paired sites in central Missouri forests were assayed for the presence of these taxa by baiting flooded soil with Quercus robur leaves. Q. rubra and Q. velutina were the oak species on three sites and Q. alba was on the fourth. Isolates from symptomatic baits plated on PARPNH selective medium were identified tentatively on classical taxonomic characteristics. Five isolates of P. cambivora, two of P. quercina, and eight of P. cinnamomi were obtained from soils around one, one, and three trees, respectively, on decline sites. The internal transcribed spacer sequences for each isolate were compared to those in GenBank; BLAST searches for all isolates had nucleotide identities of 99% and E values of 0, which confirmed the identifications. Greenhouse pot trials were conducted to assess pathogenicity of isolates on stems of 2-year-old Q. alba and Q. rubra. A mycelial agar plug was inserted into a 1-cm long slit cut into the cambium 5 cm above the root collar and covered with sterile, moist cotton and wrapped with laboratory film. The treatments included two isolates of P. cambivora, one of P. quercina, three of P. cinnamomi, and a sterile agar plug. Each host × isolate combination was replicated four times, and the experiment was conducted twice in the greenhouse (natural lighting; temperature ≤32°C in summer and ≥7°C in winter). Stem lesions were produced commonly by P. cambivora (28 of 32 seedlings) and P. cinnamomi (46 of 48 seedlings) within 3 months; none was found on control seedlings or those inoculated with P. quercina. Mean lengths (cm) of lesions caused by P. cinnamomi were greater for Q. rubra (5.6) than for Q. alba (4.3) and lesion lengths for P. cambivora were greater for Q. alba (5.2) than for Q. rubra (4.4). Seven Q. alba seedlings inoculated with P. cambivora and one Q. alba inoculated with P. cinnamomi died before 3 months. All Phytophthora species were recovered from inoculated stems at 3 months except that P. quercina was not recovered in one trial. To our knowledge, this is the first report of Phytophthora species in soils of Missouri oak forests, of P. quercina in the United States, and of the ability of P. cambivora to cause stem lesions on Q. alba. P. cinnamomi and P. cambivora should be investigated in situ as possible contributing factors of oak decline in Missouri. References: (1) T. Jung et al. Plant Pathol. 49:706, 2000. (2) R. Lawrence et al. MO. Conserv. 63:11, 2002.