Graduate Thesis Or Dissertation
 

Epidemiology of disease caused by Pseudomonas syringae as an epiphyte and its frequency, distribution, and characteristics as an endophyte of pear

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  • Conditions were established for inducing pear blossom blast caused by Pseudomonas syringae (Ps) on both attached and detached shoots. The incidence of disease was proportional to the log of Ps population under optimal temperature, moisture and bloom developmental stage. Highest incidence of infection followed occurrence of a major exotherm in the presence of Ps suspended in water drops on blossoms. An exotherm was detected inside ovary tissue at temperatures ranging from -1.8 to -3.5 C. Wetness duration following exotherm occurrence was less important than wetness during the freeze event. Blossoms inoculated, then either air -dried or removed from low temperature treatment prior to a freeze, had a low incidence of infection. The full bloom stage of blossom development was more susceptible to blossom blast than either the open cluster or tight cluster stages of development. Eight pear cultivars were evaluated for susceptibility to blossom blast with the red cultivars less susceptible than the green ones. Internal tissues of pear trees were sampled for Ps at seven orchard sites in Oregon. Endophytic Ps were found in 76% (52/68) of the trees with 81% (191/235) of the fluorescent (F+) isolates negative for oxidase reaction (Ox -). A total of 159 and 32 F+Ox- isolates were found in root and stem tissues, respectively. A heterogeneous endophytic Ps population exists based on variable results following phenotypic characterization tests including ice nucleation ability, hypersensitive response, pathogenicity, and sensitivity to bactericides. Visual evaluation of DNA restrictionfragment profile analysis resulted in similar profiles between isolates taken from the same tree as well as dissimilar profiles between isolates taken from the same tissue segments. Sensitivity to antibiotics was significantly different (P < 0.05) between concentrations tested, between streptomycin and oxytetracycline, and between epiphytic and endophytic isolates. Sensitivity to copper sulfate was similar between epiphytic and endophytic isolates but significantly different (P < 0.05) between concentrations tested. Ps inoculations into root and stem tissues of potted trees resulted in limited, detectable inoculum movement up to 3.0 cm in stems and no detectable movement above the crown from root inoculations.
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