Conidial length, width, septation, curvature and pigmentation of 39 isolates of Bipolaris zeicola (Stout) Shoemaker from different geographical areas and representing distinct physiological races were examined. Wide variability in conidial morphology was found among isolates of each race. No association was found between races or groups of isolates producing the same shape of lesions and conidial morphology.

Morphology and expansion of lesions induced by three races of B. zeicola producing different shapes of lesion on susceptible maize (Zea mays L.) inbred lines were studied. Resistant type lesions induced by three races of B. zeicola were indistinguishable based on their shape or size. However, lesion size correlated with the lesion type induced by each race, indicating that it may be useful to evaluate resistance to different races of B. zeicola.

Inheritance of reaction to one isolate of the proposed pathotype of B. zeicola was studied. Resistance was controlled by a single dominant gene in the cross Pr x B37.

Forty-nine isolates of B. zeicola from different geographical areas including all the previously described races and pathotype were characterized by their disease reaction, severity and symptoms incited on 14 maize inbred lines. Eleven physiological races were distinguished based on their differential reaction on the 14 inbred lines; however, 6 inbred lines were adequate to differentiate the races. A system based on binary notation is proposed to designate races of B. zeicola on maize. Two races induced typical symptoms of Helminthosporium carbonum-toxin (HC-toxin) production on Pr. Seven races produced oval to irregular lesions on susceptible hosts and did not produce typical lesions of HC-toxin production on Pr. One race induced predominantly long, linear lesions, and another was avirulent on all 14 maize inbred lines.