Expression of Disease Reaction Types in Soybean Callus from Resistant and Susceptible Plants

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Abstract
Conditions influencing expression of disease reaction types were examined in calluses derived from soybean [Glycine max] plants resistant (cultivar Cutler 71) or susceptible (cultivar Cutler) to race 1 of Phytophthora megasperma var. sojae (Pms). ''Cutler 71'' calluses were colonized less than those of ''Cutler'' when both were grown on medium containing 6 or 10 mg of 2,4-D/l and incubated at 16 or 20.degree. C prior to and following inoculation with Pms zoospores. Differences between colonization rates of ''Cutler'' and ''Cutler 71'' calluses were greater in callus secretions 5 mm thick than in thicker or thinner sections. Differences in colonization rates remained high with inoculum doses of 50-1000 zoospores/callus section. Sections of ''Cutler'' and ''Cutler 71'' calluses 5 mm thick were colonized equally by race 3 of Pms which is pathogenic to plants of both cultivars. No combinations of incubation temperatures, 2,4-D concentrations, sizes of calluses, or numbers of zoospores used for inoculum resulted in ''Cutler 71'' calluses with the nearly absolute resistance to race 1 of Pms found in whole plants of that cultivar.

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