The use of one source of cytoplasmic male steriity, leading to widespread uniformity of maize, coupled with the appearance of a new virulent race of Helminthosporium maydis made the Southern Corn Leaf Blight epidemic possible. The weather during 1970 was very suitable for the spread of the disease and thus the epidemic occurred. The possible role of cytoplasmic DNA, carried most probably by the mitochondria, in the susceptibility or resistance of various maize lines in discussed. The relationship of cytoplasmic male sterility and disease resistance or susceptibility is considered. The effects of the pathotoxin, produced by H. maydis, its uses as an experimental tool in research, and its potential uses in further investigations are described in detail.