Semester

Fall

Date of Graduation

2009

Document Type

Thesis

Degree Type

MS

College

Davis College of Agriculture, Natural Resources and Design

Department

Horticulture

Committee Chair

William L. MacDonald.

Abstract

Phytophthora cinnamomi isolates, collected from eastern US forest soils, were studied to determine the extent of phenotypic, genotypic, and pathogenic variation present. Four microsatellite loci were targeted for genetic analysis. Phenotypic characteristics measured were sporangia length, width, and length:width ratio; chlamydospore, oogonia, and oospore diameter; antheridia width, colony morphology on PDA; and colony growth rate on clarified V8 agar at 20°C, 24°C, 28°C, and 32°C. Red oak logs were inoculated with select isolates to determine relative pathogenicity. Microsatellite analysis showed that the genetic variability of the eastern US oak forest P. cinnamomi isolates was low. Isolates were grouped into two microsatellite fingerprint groups (MFG). Isolates in microsatellite group one (MFG1) were characterized by DNA fragment lengths of 120 and 122 bp at locus d39, 169 and 170 bp at locus e16, and 254 and 255 bp at locus g13. Microsatellite Fingerprint Group 2 (MFG2) isolates were characterized by marker sizes of 122 and 124 bp at locus d39, 161 and 163 bp at locus e16, and 247 and 248 bp at locus g13. Similar microsatellite profiles have been found in P. cinnamomi isolate populations from Australia and other regions of the world. The low level of genetic viability and microsatellite marker similarities to P. cinnamomi isolates from other continents supports the theory that P. cinnamomi is not indigenous to the U.S. and may provide clues as to the origin of US P. cinnamomi. Asexual and sexual spore dimension varied greatly within the populations, but were similar to previous descriptions. Differences in phenotypic characteristics among isolates were most pronounced when data were grouped by MFG, the most significant being colony morphology and growth rate. Isolates expressed either rosaceous or stellate colony morphology. There was some correlation between colony morphology and MFG, but colony morphology alone could not be relied upon to predict the genotype of an isolate. Differences in growth rates of microsatellite fingerprint groups were observed with MFG1 being less tolerant of increased incubation temperatures. No variation in pathogenicity on red oak logs was observed.

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