The disease usually spreads from the lower leaves upward. When spores are blown in from neighboring fields, lesions can be random or concentrated in the upper leaves. Infections appear initially as small, water-soaked, circular lesions that are about 1/16 inch in diameter. The lesions can enlarge to about 1/8 inch in diameter, and become chlorotic, then necrotic, with a tan center and a darker brown or purple margin (Figures 1 and 2). The spot is usually surrounded by a larger yellow “halo” which is most visible when light passes through the leaf (Figures 3 and 4). You can observe the halo by holding a leaf up toward a blue sky background.
Spots can vary in size and color depending on the hybrid. Spots can coalesce into larger necrotic areas (Figure 5). It is common to observe bands of lesions across a leaf, indicating that infection took place in the moist environment of the whorl after a period of spore dispersal. Lesions often are concentrated along the leaf edges and leaf tips. Severely infected leaves can be entirely blighted. The dark margins of the lesions remain visible on dead leaves.
Eyespot symptoms can be confused with physiologic or genetic leaf spots, which are noninfectious, or with insect feeding wounds. Eyespot also can be confused with Curvularia leaf spots and the early symptoms of northern leaf spot or gray leaf spot. Noninfectious leaf spots sometimes do not develop a necrotic center, and if they do, it does not have the darker margin. Northern leaf spot lesions can enlarge to be substantially larger than eyespot lesions, and usually are less circular in shape. Fully developed gray leaf spot lesions are long, rectangular and easily distinguished from eyespot.