CHARACTERIZATION OF Exserohilum turcicum INFECTION SITES IN MAIZE GENOTYPES

ABSTRACT Northern leaf blight caused by Exserohilum turcicum is an important disease of maize (Zea mays L.), and its severity depends more closely on growth lesions than on spot number. Here, we characterized the infection sites of E. turcicum on resistant and susceptible maize genotypes by analyzing the histology of lesions as well as the structural and biochemical mechanisms of infection. Maize leaves were inoculated with the pathogen at specific points and incubated in a microhumidity chamber. Samples were obtained to follow fungal development and host tissue lignification using light and electron microscopy, and the activity and electrophoretic patterns of peroxidases were determined. The time course of spore germination and appressorium formation was essentially the same for both genotypes; however, a delay of 12 h in fungal penetration, accompanied by host tissue lignification, was noted in the resistant genotype, as opposed to that in the susceptible one. Scanning electron microscopy revealed fungal mycelium in the xylem vessels of both genotypes; however, in the resistant genotype, pathogen colonization was restricted to mesophyll cells around the penetration point, where chlorotic flecks were produced. Meanwhile, in the susceptible genotype, following penetration and chlorotic fleck formation, the pathogen continued to grow inside the xylem vessels and profusely colonized mesophyll tissue distant from the penetration point, resulting necrotic lesion development. Electrophoretic patterns of peroxidases were similar between the two genotypes, with three isoenzymes present in all tissues. In addition, two novel isoenzymes were detected in chlorotic flecks, necrotic lesions, and green tissue around the lesions.