Population dynamics of coexistence by plant pathogens of the rhizosphere of spring wheat

Abstract

The dryland root rot complex is a collection of root pathogens that significantly affect small grain production in the semiarid regions world-wide. The complex includes Cochliobolus sativus, Fusarium pseudograminearum, Rhizoctonia solani, Pythium ultimum, and Penicillium sp. The purpose of this thesis was to document the interactions among these pathogens that commonly coexist within the wheat rhizosphere. The thesis had two objectives: 1) examine variation in interactions amongst isolates of C. sativus and F. pseudograminearum within the wheat crown and 2) identify interactions among pathogens in the dryland root rot complex (C. sativus, F. pseudograminearum, R. solani, P. ultimum, and Penicillium sp.) in-field, as reflected in plant response variables. For objective 1, wheat in both field and greenhouse settings were inoculated singly and in all pathogen isolate combinations. Both C. sativus and F. pseudograminearum, alone and in combination, reduced yield (P<0.001, P=0.003, respectively), but C. sativus isolates had a greater effect on both yield and emergence (P<0.001). Inoculations with some isolates of C. sativus and F. pseudograminearum were suppressive on populations of each other. Significant variability in suppressiveness was observed among isolates for both species. For objective 2) plant health as measured by emergence, vigor, plant height and yield, was observed in response to inoculations with all single, pairwise, four-pathogen and five-pathogen combinations of C. sativus, F. pseudograminearum, R. solani, P. ultimum, and Penicillium sp. Antagonistic relationships that favored overall plant health were observed between several pathogens, including P. ultimum, Penicillium sp. and C. sativus. These antagonistic interactions affected seedling emergence and plant vigor. Conversely, F. pseudograminearum in combination with R. solani significantly reduced emergence beyond expected (P=0.002). Within the community interaction studies overall, F. pseudograminearum was the dominant pathogen, causing more disease and more consistent disease than any other pathogen, and generating increased damage to emergence when inoculated in the community (P=0.017). In conclusion, interactions among members of the dryland root rot complex is a dynamic process, one that varies by species and pathogen isolate. These interactions are more often antagonistic, with one pathogen suppressing another. Additional studies may improve how disease control measures are deployed in the future.