The roots of susceptible sorghum plants are badly rotted and seedlings are stunted. In addition, the leaves tend to curl as if affected by drought. The crowns of diseased plants, when split, show a dark red discoloration. Plants may die as seedlings, with the roots completely rotted, or they may grow on, though stunted, until the time of heading when they begin to show signs of firing. The heads do not develop fully and the plants die just when grain of resistant plants is at the hard-dough stage.
The fungus produces a virulent, relatively thermostable toxin that kills the plant cells in the absence of fungal mycelium. Pringle and Scheffer (1963) purified the selective toxin and showed that growth of susceptible sorghum roots was inhibited by 0.1 µg/ml toxin but resistant plants were unaffected. Disease symptoms are correlated with the enhanced synthesis of specific proteins (see Biology and Ecology).