Characterization and Management of the Race Structure of Phytophthora parasitica var. nicotianae

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Title: Characterization and Management of the Race Structure of Phytophthora parasitica var. nicotianae
Author: Sullivan, Melinda Jo
Advisors: Thomas Melton, Committee Co-Chair
H. David Shew, Committee Chair
Shuijin Hu, Committee Member
Earl Wernsman, Committee Member
Marc Cubeta, Committee Member
Abstract: Deployment of tobacco cultivars with single-gene (Ph), complete resistance to race 0 of the tobacco black shank pathogen has resulted in a rapid increase in the occurrence of race 1 in N.C. A four-year cultivar rotation study was conducted in three fields to assess how different levels and types of resistance affected the race structure and population dynamics of the pathogen. In a mixed race field, the high level of partial resistance in 'K 346' was most effective in reducing disease and race 1 populations decreased. The deployment of complete resistance in 'NC 71' resulted in intermediate levels of disease, and race 1 increased. 'K 326', with a low level of partial resistance, had the highest levels of disease, and race 0 was dominant. In a field where no race 1 was detected initially, disease incidence was high with the use of partial resistance. Complete resistance was very effective in suppressing disease, but race 1 was recovered after only one growing season. By the end of the third growing season, race 1 was recovered from most 'NC 71' treatments. In a field where race 1 was predominant, a high level of partial resistance was most effective in controlling disease and race 0 increased rapidly. A rotation of single-gene resistance and a high level of partial resistance was the most effective rotation for disease management and it minimized race shifts in the pathogen. This may serve to prolong the usefulness of the Ph gene. Populations of race 1 decreased relative to race 0 when cultivars with partial resistance were rotated with complete resistance, suggesting that race 1 isolates are not as fit as race 0 isolates. Experiments were conducted to compare their pathogenic and ecological fitness. Forty isolates of race 0 and 20 isolates of race 1 were used to inoculate tobacco cultivars with low, moderate, and high levels of partial resistance. Race 0 isolates were more aggressive than the race 1 isolates; incubation period was shorter and root rot severity greater with race 0 isolates than with race 1 isolates. Isolates of race 1 caused greater stunting of plants than race 0 isolates. Field microplots were infested with either a single race or an equal mixture of each race. Soil samples were collected and populations determined at the end of each growing season and again the following spring. There were no statistical differences in survival between races, but over both years of the study there was a trend for race 0 to survive better than race 1. One-hundred ninety five isolates of P. parasitica var. nicotianae were subjected to amplified fragment length polymorphism (AFLP) analysis to characterize the genetic diversity among isolates and within pathogen races 0 and 1. Isolates included 20 diverse isolates and an additional 175 isolates obtained over years from a field in Duplin Co., N.C. From all isolates evaluated, 256 of 304 markers (85%) were polymorphic and provided 106 AFLP profiles. The AFLP phenotypes initially detected within each plot were maintained throughout the study but additional phenotypes were recovered over years. At least 6 race 0 and race 1 isolates collected from a single test plot were similar and clustered together in the unweighted pair-group mean analysis phenogram. Examination of the AFLP profiles showed race 0 and race 1 isolates differed by only 2 to 4 markers. Results indicated that P. parasitica var. nicotianae is diverse and that the multiple occurrences of race 1 that were recovered throughout this field over years were independent events where race 1 was selected from within the pathogen population.
Date: 2005-01-30
Degree: PhD
Discipline: Plant Pathology

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