The production of a host-specific pathotoxin by Cochliobolus heterostrophus

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Abstract
Certain isolates of Cochliobolus heterostrophus which induce blight reactions on corn produce a host-specific pathotoxin. Blighting symptoms are characterized initially by a diffuse, generalized watersoaking and chlorosis of leaf tissue apparent 2 to 3 days after inoculation, then rapid necrosis of the affected tissue. Filtrates were obtained from cultures grown on Fries' modified medium with 2% yeast extract. Toxin production was measured by dilution end point methods using a seedling bioassay measuring inhibition of root elongation and detached leaves floated on culture filtrates. Minimum pH of filtrates was correlated with maximum mycelial growth and occurred 9–11 days after inoculation. Maximum toxin production occurred 4–6 days later. Culture filtrates and extracts from diseased plants induced all disease symptoms and exhibited the same specificity as the pathogen to susceptible and resistant lines of corn. Sorghum vulgare and Sorghum Sudanese were susceptible to the isolates and their toxin, while eight other gramineous species were resistant to both sources of inocula. Toxin production was shown to be under genetic control in that ascospore progeny with greater or lesser toxin capacities than parental isolates were obtained from genetic crosses. Certain of the chemical characteristics of the pathotoxin were determined.

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