Population structure and genetic diversity of Phytophthora nicotianae from tobacco in Georgia

Abstract

Black shank, caused by Phytophthora nicotianae, occurs worldwide and is responsible for significant yield loss in tobacco production in Georgia. Management of the disease has primarily relied on utilization of tobacco cultivars with resistance to race 0 of the pathogen and application of the fungicide mefenoxam. Races of P. nicotianae currently prevalent in tobacco production in Georgia, their sensitivity to mefenoxam, and genetic diversity of the pathogen are largely unknown. To determine population structure and genetic diversity of the pathogen, simple sequence repeat (SSR) markers were used. Three races of P. nicotianae (races 0, 1, and 3) were isolated from infected tobacco plants, with race 3 identified in Georgia for the first time. The majority of isolates were identified as A2 mating type and all isolates were sensitive or intermediately sensitive to mefenoxam at 1 or 10 mg/ml, with effective concentration of mefenoxam for 50% mycelial growth reduction values ranging from <0.01 to 0.12 mg/ml. Bayesian and unweighted pair group method with arithmetic means analyses of 59 isolates using SSR markers grouped the isolates in two major groups. Group I contained 20 isolates, of which 19 isolates were collected from Berrien County. Group II contained 39 isolates collected from Bacon, Cook, Tift, and Toombs Counties as well as one sample from Berrien County. Genetic diversity of the isolates was associated with geographical location of collection, and isolates in group I were primarily (75%) race 1, whereas isolates in group II were primarily (69%) race 0. The presence of a single pathogen mating type at most of the locations implies low probability of sexual recombination that may have contributed to the low genetic diversity at a particular geographical location. Sensitivity of the isolates to mefenoxam indicates that the fungicide remains to be a potent tool for growers to combat the disease. Information generated in the study advances our knowledge about diversity and population structure of P. nicotianae, which facilitates development and implementation of effective disease management programs.