Adaptation of Phytophthora nicotianae to Multiple Sources of Partial Resistance in Tobacco.

Host resistance is an important tool in the management of black shank disease of tobacco. Race development leads to rapid loss of single-gene resistance, but the adaptation by Phytophthora nicotianae to sources of partial resistance from Beinhart 1000, Florida 301, and the Wz gene region introgressed from Nicotiana rustica is poorly characterized. In greenhouse environments, host genotypes with quantitative trait loci (QTLs) conferring resistance from multiple sources were initially inoculated with an aggressive isolate of race 0 or race 1 of P. nicotianae. The most aggressive isolate was selected after each of six host generations to inoculate the next generation of plants. The race 0 isolate demonstrated a continuous gradual increase in disease severity and percentage root rot on all sources of resistance except the genotype K 326 Wz/-, where a large increase in both was observed between generations 2 and 3. Adaptation by the race 0 isolate on Beinhart 1000 represents the first report of adaptation to this genotype by P. nicotianae. The race 1 isolate did not exhibit significant increases in aggressiveness over generations but exhibited a large increase in aggressiveness on K 326 Wz/- between generations 3 and 4. Molecular characterization of isolates recovered during selection was completed via double digest restriction-site associated DNA sequencing, but no polymorphisms were associated with the observed changes in aggressiveness. The rapid adaptation to Wz resistance and the gradual adaptation to other QTLs highlights the need to study the nature of Wz resistance and to conduct field studies on the efficacy of resistance gene rotation for disease management.