Date of Award

5-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Plant and Environmental Science

Committee Member

Dr. Steven N. Jeffers, Committee Chair

Committee Member

Dr. Stacy L. Clark

Committee Member

Dr. William C. Bridges

Committee Member

Dr. Julia L. Kerrigan

Abstract

Phytophthora root rot was first described on American chestnut (Castanea dentata) in a preliminary report in 1932 and the causal agent was tentatively and erroneously identified as P. cambivora. Soon after, the causal agent was correctly identified as P. cinnamomi and, since that time, P. cinnamomi has been the only species reported to cause Phytophthora root rot on the American chestnut. In the early 1980s, The American Chestnut Foundation initiated a backcross breeding program to develop chestnut trees that had resistance to Cryphonectria parasitica which causes chestnut blight. In the early 2000s, backcross hybrid chestnut seedlings ([American × Chinese] × American) were planted in field plots in the eastern United States to evaluate field performance of these seedlings. Between 2010 and 2014, 271 root and 353 soil samples associated with diseased American, Chinese, and backcross hybrid chestnut seedlings were collected in eight field sites in four states in the southeastern United States, and these samples were assayed for presence of species of Phytophthora. Phytophthora cinnamomi, P. cambivora, P. heveae, P. quercetorum, and P. cryptogea were recovered by isolation from roots on PARPH-V8 selective medium and baiting soil with rhododendron and camellia leaf pieces. A total of 248 isolates were recovered and tentatively identified based on morphology; species identifications were confirmed by sequencing the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA). Phytophthora cinnamomi was recovered most frequently"”from 17% of root and 34% of soil samples. Phytophthora cambivora and P. heveae also were recovered"”from 10% and 1% of root samples and 9% and 5% of soil samples, respectively. Nine of the isolates of P. cambivora were from roots of and soils associated with backcross hybrid chestnut seedlings growing in in the field at a tree nursery in western VA. Phytophthora quercetorum was recovered from one soil sample, and P. cryptogea was isolated from roots of five backcross hybrid chestnut seedlings at a separate field site in northwestern South Carolina. Isolates were characterized for mycelium growth habit, mating type, mefenoxam sensitivity, and sporangium production. Three mycelium growth habits were identified when isolates of P. cinnamomi were grown on PARPH-V8 selective medium. All 165 isolates of P. cinnamomi were mating type A2; but both A1 and A2 mating types were present in the population of isolates of P. cambivora. Isolates of all species recovered from forest sites and the site in South Carolina were sensitive to the fungicide mefenoxam at 100 ppm, but two isolates of P. cambivora from the nursery site were insensitive to this fungicide. The cytochrome c oxidase subunit 1 (cox1) locus was sequenced for 52 isolates of P. cambivora, and all sequences were similar. A single nucleotide polymorphism was observed in sequences of the rps10 gene for a small subset of P. cinnamomi isolates with a sparse mycelium growth habit. Four species of Phytophthora were tested for pathogenicity to American and Chinese chestnuts"”P. cinnamomi, P. cambivora, P. heveae, and P. cryptogea. To fulfill Koch's postulates, 3-month-old, open-pollinated American and Chinese chestnut seedlings were artificially inoculated by soil infestation using individual isolates of each species and periodic flooding. There was a significant amount of root rot caused by P. cinnamomi, P. cambivora, and P. cryptogea, and all species caused necrotic lesions on the main tap root of American chestnut seedlings. P. cinnamomi was the only species that caused significant root rot on Chinese chestnut seedlings, but P. cambivora and P. cryptogea occasionally caused necrotic lesions on the main tap root of Chinese chestnut. Virulence of P. cinnamomi and P. cambivora was compared on 2- and 3-year-old American chestnut seedlings using inoculum treatments composed of isolates from different geographical locations and substrates; each treatment was a mixture of two, three, or four isolates of one species. P. cinnamomi was more virulent than P. cambivora based on lesion height on the main stem and amount of root rot. No differences were observed among isolate treatments within each species. Three treatments of P. cinnamomi produced significant lesions on the main stems of seedlings. P. cambivora did not consistently produce symptoms under the experimental conditions used in the virulence experiment. This is the first study to report multiple species of Phytophthora as pathogenic to American and backcross hybrid chestnuts.

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