|LI, YONGGANG - University Of Georgia|
|JI, PINGSHENG - University Of Georgia|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2017
Publication Date: 4/21/2017
Citation: Li, Y., Harris-Shultz, K.R., Wang, H., Wadl, P.A., Ji, P. 2017. Population structure and genetic diversity of Phytophthora nicotianae from tobacco in Georgia. Plant Disease. 101:1113-1118.
Interpretive Summary: Phytophthora nicotianae, a soilborne pathogen, causes black shank of tobacco. Black shank causes yellowing and wilting of tobacco leaves as the pathogen causes root and crown rot. Field losses of susceptible cultivars can reach 100% in years favorable for disease development (high rainfall). The 2014 tobacco growing season was one of the worst in Georgia for black shank disease. Thus knowledge is needed about the P. nicotianae present in GA and what races are present. Knowledge of the races present in an area is important as this effects what tobacco cultivar is grown as some cultivars are resistant only to certain races of the pathogen. Furthermore information is needed if the P. nicotianae present in GA is resistant to mefenoxam, the most widely used fungicide for disease control. In this study, P. nicotianae was collected in GA from five tobacco growing counties. Three races (race 0, 1, and 3) were isolated from infected plants, with race 3 being identified in Georgia for the first time. All isolates were found to be the same mating type suggesting an absence of sexual spores. This lack of sexual reproduction was further confirmed by DNA fingerprinting as the isolates obtained could be grouped into two groups, with one group containing isolates primarily from Berrien County. Additionally, all isolates were sensitive or intermediately sensitive to mefenoxam which suggest the fungicide can be used by growers to control the disease. Thus, this study provides valuable information that can be readily applied by extension agents for tobacco cultivar and fungicide recommendations to growers in Georgia.
Technical 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 repeats (SSR) markers were used. Three races of P. nicotianae (race 0, 1, and 3) were isolated from infected tobacco plants, with race 3 identified in Georgia for the first time. All isolates of P. nicotianae were identified as A2 mating type and were sensitive or intermediately sensitive to mefenoxam at 1 or 10 µg/ml, with EC50 values ranging from 0.0005 to 0.12 µg/ml. Bayesian and unweighted pair group method with arithmetic mean (UPGMA) analysis 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 apparently associated with geographical location of collection and isolates in group I were primarily (75%) race 1, whereas those isolates in group II were primarily (69%) race 0. The presence of a single pathogen mating type implies the absence 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. This study provides new information about population structure and diversity of the pathogen which facilitates development and implementation of effective disease management programs.