Following a hot and dry summer with lots of corn flea beetles, cold temperatures descended on Illinois this past winter. As a result, flea beetle survival and the threat of early season Stewart's wilt is expected to be low. Erwinia stewartii, the bacterium that causes Stewart's wilt, survives the winter in the gut of the corn flea beetle. Survival of the corn flea beetle is dependent on winter temperatures. Using the average temperature of December, January, and February, the potential of Stewart's wilt can be predicted (Table 2). The average temperature of December, January, and February fluctuates around the temperature for freezing and is easier to understand than the sum of average temperatures for December, January, and February. The average temperature for these 3 months predicts the potential for flea beetle injury and Stewart's wilt.
Stewart's wilt is spread in the spring as corn flea beetles feed on and infect seedling corn plants. The bacterium can spread systemically throughout the plant. Stewart's wilt has two phases: the seedling wilt phase and the leaf blight phase. The seedling wilt stage occurs when seedlings become infected at or before the V5 stage. The growing point is easily infected; eventually the seedling wilts and dies. Infections of older plants usually result in the development of the leaf blight phase of Stewart's wilt. This phase is characterized by long, yellow to chlorotic streaks, with wavy margins along the leaves. Although most commercial field corn hybrids are resistant to Stewart's wilt, the disease is still a concern for susceptible seed corn inbreds and sweet corn hybrids.
Based on the recent winter temperatures from the Midwestern Climate Center, estimates of early season Stewart's wilt are shown in Table 3. Little flea beetle activity is expected throughout much of Illinois. As is typical, a mild winter in southern Illinois has set the stage for the threat of Stewart's wilt. Remember, however, that these are only predictions; numbers of surviving corn flea beetles are not known. More information on the corn flea beetle and Stewart's wilt will follow as the growing season progresses.--Kelly Cook