Viability and maturation of Aphanomyces cochlioides oospores

Plasmolysis, tetrazolium bromide staining and microscopic appearance were tested for their usefulness in determining viability of oospores of Aphanomyces cochlioides. For comparison, three lethal treatments were employed to contrast the reaction of dead oospores and untreated, presumably viable oospores. Few oospores stained with tetrazolium bromide, even though plasmolysis and microscopic appearance indicated that 85% were viable. Cytoplasm of viable oospores was densely organized and uniformly granular (DOUG), whereas cytoplasm of oospores exposed to lethal treatments was loosely organized and non-uniformly granular (LONG). Dose-response bioassay experiments were conducted with untreated oospores of varying inoculum densities or with mixtures of untreated DOUG and heat-treated LONG oospores in varying proportions. The number of DOUG oospores was correlated (R(2) = 0.62, P < 0.001) with severity of damping-off of sugar beet seedlings caused by A. cochlioides. Thus, the granular appearance of cytoplasm offered a fast, easy and reliable indicator of viability of A. cochlioides oospores. Tests with newly formed oospores/oogonia showed that >80% harvested at 3-4 d after inoculation of hypocotyls stained with tetrazolium, but by 8-9 d <10% stained, apparently because of declining permeability of the spore wall to tetrazolium as oospores matured.