Date of Award


Document Type


Degree Name

Master of Science (MS)

Legacy Department

Plant and Environmental Science

Committee Chair/Advisor

Jeffers, Steve N

Committee Member

Kerrigan, Julia L

Committee Member

DeWalt, Saara J


Phytophthora cinnamomi is a devastating pathogen that can attack over 900 hosts. It is the most common species of Phytophthora isolated from woody ornamental crops in South Carolina but little is known about variability among isolates of P. cinnamomi that attack these plants. Therefore, 142 isolates of P. cinnamomi recovered from diseased plant samples submitted to the Clemson University Plant Problem Clinic between 1995 and 2011 were characterized for mycelium growth habit, growth rate, mefenoxam sensitivity, mating type, and sporangium morphology. Mycelium growth habit on PARPH-V8 selective medium was classified as aerial, appressed, or dwarf; 87% of isolates had the aerial mycelium growth habit. Isolates with different growth habits had significantly different growth rates. All isolates were uniformly sensitive to the fungicide mefenoxam at 100 ppm. The population was composed of 129 isolates that were mating type A2 and 13 isolates that were mating type A1, with six of these A1 isolates recovered from Camellia spp. All isolates with the aerial and appressed mycelium growth habits, which were most of the isolates in the study, produced few sporangia from clarified V8 juice agar plugs immersed in non-sterile soil extract for 1 to 4 days under continuous fluorescent light; however, the two isolates with the dwarf mycelium growth habit and a single isolate of P. cinnamomi var. parvispora produced abundant sporangia under these same incubation conditions. Isolates in this population originated from plant samples sent from 27 counties in South Carolina, two counties in Virginia, and one county in Georgia. The 142 isolates were found associated with 56 known species and 16 unspecified species in 46 genera and 31 families. Thirty-three previously unreported host plant associations were discovered.
The genetic diversity of this population was measured by sequencing DNA for several loci and building phylogenies based on these data. The ITS region was sequenced for all isolates in the population. There was a high degree of genetic uniformity in the ITS region for this population. Four other loci--β-tub, cox1, cox2, and rps10--were sequenced for a set of 59 isolates representative of the population. Two isolates, Pc40 (mating type A2) and Pc138 (mating type A1) from avocado trees in California, were incorporated into this study group as standards for comparison. Mating types were distinguishable at the β-tub locus, and both mating types and mycelium growth habit types were genetically distinct at the rps10 locus. Cluster analysis of the genotypes at each locus revealed seven distinct groups. Strong correlations were observed between cluster analysis groups and phenotypes of the isolates. This is the first study to find significant correlations between genotype and phenotypic characters other than mating type in P. cinnamomi.



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