Evaluation of soybean resistance to Phialophora gregata culture filtrate in tissue culture.

Resistance to the fungal pathogen, Phialophora gregata (Allington and Chamberlain) W. Gams, the cause of brown stem rot (BSR) in soybean [Glycine max (L.) Merr.], is an important trait for cultivars grown in the northern USA. A novel tissue culture method was developed where ten soybean cultivars were differentiated on the ability of their excised cotyledons to remain green and initiate callus in a tissue culture medium containing P. gregata culture filtrate. Cultivar BSR classifications by the cotyledon method corresponded to greenhouse root-dip assay classifications in 80%, 100%, and 90% of the three P. gregata isolate treatments. Another method, employing pieces of somatic callus exposed to the culture filtrate, had a 70% average correspondence to the greenhouse results. Physiologic specialization was demonstrated in parallel in vivo/in vitro assays for the first time. These data suggest that the cotyledon method would accurately identify soybean lines resistant to certain aberrant or wild-type P. gregata isolates.