BIOCHEMICAL RESISTANCE OF OENOTHERA BIENNIS TO POWDERY MILDEW: THE ROLE OF PHENOLICS
Abstract
Oenothera biennis (evening primrose) consists of numerous isogenic races. Each race is characteristically either susceptible or resistant to a local isolate of the powdery mildew fungus Erysiphe polygoni. The time course of fungal development on susceptible and resistant Oe. biennis was studied by artificially inoculating hosts with conidia of the single-spore isolate of E. polygoni. Under conditions of 95% relative humidity, 20(DEGREES)C and 12 hour photoperiod, conidia germinated within 2-5 hours, appressoria were formed within 8-12 hours, penetration had been effected and haustoria initiated by 20 hours. On resistant plants, there was no further growth of the fungus. Secondary hyphae were initiated but developed poorly. On susceptible plants, secondary penetration occurred and secondary hyphae appeared 26h after inoculation, and sporulating colonies were visible in 4-5 days. Phenolic patterns of Oe. biennis leaves after inoculation with this isolate of E. polygoni were studied to determine whether any phenolic compounds could be correlated with resistance. Studies by Maranon (1924) indicated that phenolics are associated with resistance. Quantitative assays showed that absolute titer of phenolics does not determine resistance, but that quantitative and gross qualitative changed in phenolics are correlated with resistance. Paper, thin-layer, and high-performance liquid chromatography showed that a phenolic compound, tentatively characterized as a gallotannin, accumulates during the time frame of resistance in resistant races B-17 and B-37, but not in other resistant races. Apparently there is more than one biochemical mechanism of resistance to E. polygoni in Oe. biennis. In addition, field susceptibility of Oenothera biennis races was assayed. Several races which were resistant to the lab isolate were susceptible to E. polygoni at one or more field locations, showing that there is geographic variability in pathogenicity of E. polygoni strains. The pathogen variability, together with the non-generalizability of phenolic defense, imply that a host race may possess more than one resistance mechanism, and that resistance mechanisms differ among host races.
Degree
Ph.D.
Subject Area
Biology
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.