WHEAT ROOT ROT PATHOGEN VARIABILITY AND SOIL MOISTURE STRESS EFFECTS ON HOST-PATHOGEN INTERACTIONS

Scope and Method of Study. This research was carried out to determine, (1) degree of pathogencity of different isolates of wheat root rot fungi: Helminthosporium sativum and Fusarium spp. collected from diseased wheat plants from various areas in Oklahoma; (2) the invasion patterns and penetration of wheat seedlings, first seminal root regions and subcrown internodes by Helminthosporium sativum, Fusarium sp. and Rhizoctonia solani in the laboratory; (3) effects of wheat root rot fungi and soil moisture stress conditions on growth and forage yield of transplanted and non-transplanted Danne wheat seedlings, and (4) effects of wheat root-rot fungi on six winter wheat cultivars: ('Danne,' 'Triumph 64,' 'TAM 101,' 'Payne,' 'Newton,' and 'Vona') grown under soil-moisture-stress conditions.;Findings and Conclusion. All five isolates of H. sativum and all six isolates of Fusarium spp. studied were pathogenic. There were no significant differences in the ability of H. sativum isolates to attack subcrown internodes of wheat cultivar, 'Danne;' but Fusarium spp. isolates differed significantly in their ability to attack the subcrown internodes. Isolates of the pathogens collected from diseased plants from the same wheat field differed appreciably in their degrees of virulence. Fusarium spp. isolates exhibited the least degree of aggressiveness compared to H. sativum isolates. Isolate 47 of H. sativum from Custer City and isolate 34 of Fusarium from Geary were found to be more aggressive than the other isolates of each pathogen. Helminthosporium sativum alone damaged the subcrown internodes more than Fusarium alone or both pathogens combined. Inoculum-density-level study showed that H. sativum recovery from subcrown internodes ranged from 48-97% as the inoculum levels increased from 1-1000 conidia/g soil, respectively. The percentage recovery plateaued between 250-1000 conidia/g soil, respectively. Disease severity ratings of the subcrown internodes followed a similar trend. Percentage recovery of Fusarium sp. from subcrown internodes ranged from 41-57% as inoculum density level increased from 1-100 macroconidia/g soil plateaued between 100 and 500 macroconidia/g soil and finally declined to 45% at 100 macroconidia/g soil. Disease severity ratings of the subcrown internodes followed a similar trend. Invasion patterns and penetration studies showed that H. sativum and Fusarium sp. penetrated wheat seedlings within 24 hours following inoculation but R. solani never did penetrate. The subcrown internodes and the region of differentiation of the first seminal root were found to be the most susceptible part to the pathogens and the root cap region the least susceptible. Effect of pathogens on transplanted and non-transplanted seedlings under soil moisture stress conditions showed greater percentage yield reductions in non-transplanted plants than with transplanted plants in all treatments. The study of six wheat cultivars showed that under water-stress conditions the pathogens significantly reduced forage and root yields in all the cultivars. Evapotranspiration rate did not differ significantly in any treatments or cultivars as compared to their controls. In general, the pathogens had no significant effect on stomatal resistance but Fusarium alone seemed to cause an increase in stomatal resistance while H. sativum alone and the combination of pathogens caused a decrease in stomatal resistance. Overall results showed that the cultivar, 'Payne,' consistently out-yielded the other cultivars in forage and root yields and also gave the lowest stomatal resistance while the cultivar, 'Newton,' consistently gave the lowest yields and the highest stomatal resistance in all treatments. Information obtained from the studies will be helpful in developing resistance-screening trials which will provide effective basis for integrated control of wheat root rot disease in Oklahoma.