EPIDEMIOLOGY OF NORTHERN LEAF-BLIGHT ON SWEET CORN

The epidemiology of northern leaf blight of corn, caused by Exserohilum turcicum (Pass.) Leonard and Suggs, is reviewed. The minimal dew period required for infection is temperature-dependent. At 25-degrees-C, 1 h of dew is sufficient to cause infection and at this temperature the minimal dew period for sporulation is 14 h. Under natural conditions when one dew night is not long enough for conidia to develop, the dew period on the following night enables the completion of conidial formation. The amount of conidia formed is dependent on temperature, light, plant age, leaf position and plant susceptibility. Both qualitative and quantitative types of resistance were identified in several hybrids. Subsequently, there developed additional biotypes of E. turcicum which are aggressive to plants containing qualitative monogenic resistance. Within the same physiological race, a significant variation in aggressiveness between isolates from various locations is observed. The pathogen overwinters as mycelia and conidia in infected leaves, husks and other plant parts, or on Sorghum halepense L. Reduction in yield due to northern leaf blight is associated with the level of resistance of the host plant, with disease severity, plant age during infection, and position of infected leaves.