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Adaptation to host resistance genes by Phytophthora nicotianae
K. L. MCCORKLE (1), R. S. Lewis (1), H. D. Shew (1). (1) North Carolina State University, Raleigh, NC, U.S.A.

Black shank, caused by <i>Phytophthora nicotianae,</i> is an important disease of tobacco worldwide. Host resistance is the easiest and most cost effective tool for disease control; however, the widespread deployment of complete resistance failed due to a rapid race shift in the pathogen from wild type race 0 to race 1. Partial resistance also is available, and while offering some protection to both races, severe losses may still occur. All partial resistance in commercial varieties is from variety Florida 301. To increase the level of partial resistance, variety Beinhart 1000 was investigated. Since all resistance genes place selection pressure on the pathogen, the objectives of our study were to determine how isolates adapt to varying sources and levels of partial resistance and if adaptations are specific to the source of resistance. Ten tobacco lines with resistance from Florida 301, Beinhart 1000, or single genes <i>Wz </i>or <i>Php </i>were grown in the greenhouse. Plants were inoculated with race 0 and race 1 isolates for six continuous generations. Adapted isolates were compared across generations for each variety and pathogen race combination. An increase in pathogen aggressiveness was observed over generations for both races on multiple varieties. More aggressive isolates had decreased incubation periods and increased root rot. Understanding how <i>P. nicotianae </i>adapts to resistance will improve recommendations for variety rotation to manage the pathogen and the disease.

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