Field and laboratory studies of gray leaf spot of corn caused by Cercospora sp.

With the acceptance and increased adoption of minimum-tillage cultivation of corn in Tennessee, the incidence and severity of Cercospora gray leaf spot of com has increased. A resistance evaluation of 25 cultivars of corn was conducted in the summer of 1977. Varying degrees of resistance were observed and the experimental inbred T222 was the most highly resistant cultivar tested. Comparisons of five cultivars for symptom development between those treated with a foliar fungicide and those left untreated resulted in significant differences. The study showed that the severity of gray leaf spot symptoms could be diminished with periodic applications of Manzate. Harvest data comparing the same treated and untreated plants failed to show that Cercospora gray leaf spot caused significant reduc-tions in grain yield or quality. Artificial inoculation of corn with Cercospora zeae-maydis resulted in infection when virulent conidia were atomized onto leaf surfaces and when fresh, ground diseased leaf material was dusted into seedling whorls under conditions of high humidity. Potato dextrose agar proved best of five media tested for growth and sporulation of C. zeae-maydis cultivars. Optimum sporulation in total darkness occurred at 28°C, and conidial lengths varied greatly with temperature. Conidia produced in culture were longest when produced at 16°C. Conidia from naturally infected leaf tissue were generally longer than those produced in culture, reaching lengths of 155μ. Conidio-phores were observed having as many as six geniscars per conidiophore. A study of the effect of temperature on conidial germination was inconclusive.