SCLB: Biology and Disease Cycle
Southern corn leaf blight (SCLB) is favoured by warm temperatures (68-90°F or 20-32°C) and high humidities. Thus, it tends to be a problem in the Southern half of Illinois, although it can be found farther north if weather conditions are favourable. Frequent rainy periods enhance disease development.
The symptoms of SCLB are leaf lesions ranging from minute specks to spots of one-half inch wide and one and one-half inches in length. They are oblong, parallel-sided, and tan to greyish in colour. A purplish to brown border may appear on the lesions depending on the genetic background of the plant. Early and severe infections in susceptible plants predisposes them to stalk rots.
The fungus overwinters in corn debris as spores or mycelium. Spores are spread by wind or splashing water to growing plants. After infection and colonization, sporulation from these primary lesions serves as the source for secondary spread and infections as long as weather conditions are favourable for disease development and living tissues are present. The disease cycle may repeat every few days under ideal conditions. Germination of spores and penetration into the plant can occur within six hours when free water is present on the leaf surface and temperatures are between 15 and 28°C. Control of SCLB is easily accomplished with resistant hybrids. Although some slight flecking may be found in some hybrids, this is simply a part of the resistant reaction and does not lead to any economic loss.
Burial of crop residue is helpful where erosion is not a problem. Crop rotation is especially suggested where no-till is used or where heavy crop residues are found. Since this fungus overwinters on debris, the planting of corn into such residues may result in earlier infection and poor seedling performance. Foliar fungicides are useful in seed production fields. For optimal control, it is important to control foliar disease during the period from 14 days before tasselling to 21 days after tasselling. Research has shown that this four-week period is the most critical for leaf blight damage and that yields and quality are most affected if susceptible inbred are not protected at this time.