Symptoms of eyespot are usually observed first in early spring, shortly after plants break dormancy and begin to grow. On young plants, symptoms consist of elliptical or eye-shaped lesions on leaf sheaths in the crown near the soil surface. Lesions frequently occur below the soil surface. One or several lesions can occur on a stem. When several lesions are present, they can coalesce, leading to a general discoloration of stems in the crown (Figs. 1, 2). Individual lesions are yellow-brown in color and may have dark-colored “scurf” in the center of the lesion (Figs. 3, 4). As plants develop, the fungus colonizes successive leaf sheaths and eventually the stem, which can be weakened and fall over, resulting in lodging (Fig. 5), or die standing, forming “whiteheads” (Fig. 6). Lodging of stems caused by eyespot typically occurs in multiple directions as opposed to one direction, such as occurs after wind or rain. The multi-direction lodging is known as “strawbreaker.” Lodged stems often break off at the soil line when pulled. Examination of the broken stem reveals a dark brown, charred appearance, also with the presence of “scurf.” Eyespot only affects stems—roots are not colonized.
Eyespot is caused by two closely related fungi; Tapesia yallundae and Tapesia acuformis, which were formerly known as the wheat- and rye-types, respectively, of Pseudocercosporella herpotrichoides. These fungi persist between crops only in the stubble of previously infected plants. Infection is favored by cool and moist conditions; therefore this disease is often most serious in the lower areas of a field, such as toeslopes and flats. When conditions are favorable, these fungi produce spores on infested plant debris in soil. Splashing raindrops spread spores short distances where they contact a plant and germinate. Upon germination, the fungus infects the outer leaf sheaths and eventually grows into the stem.
During colonization, the fungus forms masses of mycelium (scurf) on the leaf surface and between leaf sheaths, giving the lesions a charred appearance. If infection occurs early in the autumn, extensive colonization of the plant occurs and it is unable to obtain the water required to fill the kernels, resulting in shriveled kernels, whiteheads, and lodging. Less damage occurs when infection takes place later in spring because the pathogen does not have enough time to colonize the entire stem.