While the temperatures in January did little to affect average soil temperatures (https://blogs.illinois.edu/view/7447/618008), the same can not be said for average air temperature. Average winter temperatures in Illinois for 2017-2018 were much colder than 2016-2017 (Figures 1 and 2). Cool temperatures during the months of December, January and February favor increased mortality of the corn flea beetle and the bacterium it vectors.
Corn flea beetles are the primary vector of Stewart’s wilt. Erwinia stewartii, the bacterium that caused Stewart’s wilt, survives the winter in the gut of the corn flea beetle and the survival of the corn flea beetle is dependent on winter temperatures. Warmer winters result in greater survivorship of corn flea beetles, thus increasing the potential for Stewart’s wilt. Using the average temperatures of December, January, and February, the potential for Stewart’s wilt can be predicted (Table 1).
Table 1. Projected risk of Stewart’s wilt based on the average temperatures of December, January, and February.
|Average temperature of December, January, & February||Probability of early season wilt||Probability of late season blight|
|<27° F||Absent||Trace, at most|
|27-30° F||Light||Light to Moderate|
|30-33° F||Moderate||Moderate to Severe|
Corn flea beetles become active in the spring when temperatures rise above 65°F, and they feed on and transmit Stewart’s wilt bacteria to seedling corn plants. The bacterium can spread systemically throughout the plant. Although most commercial field corn hybrids are resistant to Stewart’s wilt, the disease is still a concern for susceptible seed corn inbreds and many sweet corn hybrids.
There are two phases of Stewart’s wilt: 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 vascular system becomes plugged with bacteria, causing the seedling to wilt, become stunted, and die. Infections of older corn 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. When the late infection phase or “leaf blight phase” of Stewart’s wilt occurs after tasseling, it is generally not a concern in sweet corn because ears are harvested before damage occurs.
Based on the recent winter temperatures from the Midwest Regional Climate Center, early season Stewart’s wilt are estimated to be absent to light in the northern half of the state, while the risk of in the southern portion is much greater. Remember, however, that these are only predictions; numbers of surviving corn flea beetles are not known.