A survey of soils associated with oak species was conducted in 2003 and 2004 in Indiana, Illinois, Maryland, Michigan, Minnesota, Pennsylvania, Ohio, West Virginia, and Wisconsin to investigate the occurrence of Phytophthora spp. Soils taken from around the base of healthy and declining oak trees were flooded with H2O and Quercus robur leaflets were used as bait for Phytophthora spp. From 829 soil samples collected near trees, 21% were positive for Phytophthora spp., with 55% of the 125 sites surveyed yielding a Phytophthora sp. Phytophthora cinnamomi was the most frequently isolated species, representing 69.4% of the Phytophthora-infested sites surveyed. Other species, in decreasing order of isolation frequency were Phytophthora sp. 2, P. citricola, P. europaea, P. cambivora, P. quercina-like isolates, and Phytophthora sp. 1. No significant association was found between the presence of Phytophthora organisms and site characteristics such as latitude, elevation, soil pH, or the crown condition of the trees. However, in P. cinnamomi-infested sites, a significant association was found with the deteriorating crown status of Q. alba and the presence of P. cinnamomi. The absence of P. cinnamomi above the 40°N latitude range also was noteworthy.
Chestnut blight is a devastating disease of Castanea spp. Mycoviruses that reduce virulence (hypovirulence) of the causative agent, Cryphonectria parasitica, can be used to manage chestnut blight. However, vegetative incompatibility (vic) barriers that restrict anastomosis-mediated virus transmission hamper hypovirulence efficacy. In order to effectively determine the vegetative incompatibility genetic structure of C. parasitica field populations, we have designed PCR primer sets that selectively amplify and distinguish alleles for each of the six known diallelic C. parasitica vic genetic loci. PCR assay results were validated using a panel of 64 European tester strains with genetically determined vic genotypes. Analysis of 116 C. parasitica isolates collected from five locations in the eastern United States revealed 39 unique vic genotypes and generally good agreement between PCR and tester strain coculturing assays in terms of vic diversity and genotyping. However, incongruences were observed for isolates from multiple locations and suggested that the coculturing assay can overestimate diversity at the six known vic loci. The availability of molecular tools for rapid and precise vic genotyping significantly improves the ability to predict and evaluate the efficacy of hypovirulence and related management strategies.
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