Resistance to Polymyxa betae in wild Beta species

Resistance to Polymyxa betae, the fungal vector of beet necrotic yellow vein virus, was studied in two wild beet species, Beta patellaris and B. procumbens. Plants grown in naturally infested soil or exposed to zoospore suspensions were examined in order to determine the stage in the life cycle of the fungus at which resistance was operating. Resting spores were never observed in the resistant species. Microscopic examination of stained transverse sections of fibrous roots taken at intervals after inoculation showed no evidence of even the earliest infection structures, the plasmodia; these were detected frequently in the epidermal cells of the susceptible Beta vulgaris. Use of the fluorescent stain DiOC(6)(3) to label zoospores showed that these were attracted to and attached themselves to the roots and root hairs of resistant species in the same way as to susceptible species. Maximum zoospore attachment was observed 1-6 h after roots were exposed to zoospore suspensions. There was no obvious difference in the numbers attracted to resistant and susceptible hosts. Apparent infection of root hairs by encysted zoospores was observed in all three species. The resistance mechanism in the wild species must operate soon after this initial infection, possibly involving a hypersensitive response that prevents the subsequent development of plasmodia in epidermal cells.