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ABSTRACT
Eight different vital stains were tested on mycelium and oospores of Phytophthora megasperma f. sp. glycinea race 1. Acridine orange, fluorescein diacetate, neutral red, and tetrazolium bromide differentiated living from dead mycelium in vitro and on membrane filters in soil. Fluorescein diacetate and tetrazolium bromide also differentiated living from dead mycelium added directly to soil. Mycelium buried in soil or on the surface of soil with a moisture content adjusted to -0.1 bars matric potential at 24 C for 8 da failed to stain. Fluorescein diacetate and tetrazolium bromide selectively stained living oospores in vitro, on membrane filters incubated in soil, or directly added to soil. Oospore viability was measured best by tetrazolium bromide. Oospores on membrane filters were buried in soil adjusted to 0 bars at 24 C. After 12 wk, tetrazolium bromide stained at least 90% of the oospores.