One of the potential impacts of the warm winter of 2001-2002 is an increased chance of Stewart's bacterial wilt of corn this summer. This disease is primarily a problem on susceptible inbreds and sweet corn; however, some hybrids with low levels of resistance or other hybrids may also be affected when corn flea beetle populations are very high. In the March 22, 2002, issue of this newsletter, Dr. Kevin Steffey provided a report on the predicted survival of corn flea beetles in Illinois. This article will add to the Stewart's wilt story this season in Illinois.|
A key to whether Stewart's wilt will be a problem is based in part on winter survival of corn flea beetles. Corn flea beetles are the primary vectors and overwintering location of the bacterial pathogen (Pantoea [= Erwinia] stewartii) that causes Stewart's wilt. The bacterium survives in the gut of hibernating adults, and the bacteria are transmitted when flea beetles feed on corn. The bacterium can also be transmitted at low levels via infested seed, but generally transmission via the corn flea beetle is considered to be much more important and common.
Winter survival of corn flea beetles, and the incidence of Stewart's wilt, is strongly influenced by winter temperatures. As reported by Rick Weinzierl and Kelly Cook in the Illinois Fruit and Vegetable News newsletter, the winter was warm enough for survival of corn flea beetles in much of Illinois. Survival of the corn flea beetle, and the potential incidence and severity of early-season Stewart's wilt, can be predicted on the basis of average December, January, and February temperatures. Nearly no disease is expected if the average is below 27; the incidence will be low if the average is 27-30; the incidence will be moderate if the average is 30-33; and above 33 the incidence is predicted to be high. Kelly Cook, a pest management graduate student in the Department of Crop Sciences, summarized temperature data from the Midwestern Climate Center to determine the average temperatures for December through February (see Table 3).
They also reported that because the winter in 2000-2001 was fairly cold, corn flea beetles and Stewart's wilt were somewhat rare north of I-80 in the 2001 season. Thus, although a high percentage of beetles probably survived the past winter, their numbers and the portion carrying the Stewart's wilt bacterium are likely to be low this spring in northern Illinois. Through much of the state, however, Stewart's wilt will be a concern this year.
There are two phases of Stewart's wilt. In the first (seedling blight) phase, young plants become systemically infected and often quickly wilt. Leaves develop yellow or light green streaks with wavy margins that parallel veins, and lesions turn brown and dry. Cavities may form in the stalk pith near the soil line, and plants may be killed. The second (leaf blight) phase of Stewart's wilt is more common and usually affects plants after tasseling, but is usually not as damaging as the seedling phase. Pale green streaks develop along the veins, and these often die and become tan in color. In severe cases whole leaves may die.
Stewart's wilt seedling blight.
Stewart's wilt leaf blight and different levels of severity.
Based on the predictions, scouting efforts should be intensified to identify problems with Stewart's wilt this spring and summer. Preventative management with resistance and seed treatment insecticides are the keys for reducing problems with Stewart's wilt.
For more information on the value of seed treatment insecticides for Stewart's wilt and hybrid resistance for sweet corn, see the Stewart's wilt link on Dr. Pataky's Web site at http://www.sweetcorn.uiuc.edu. Foliar insecticides for control of corn flea beetles and Stewart's wilt may be warranted in some cases if leaves on seedling plants are severely damaged. See the 2002 Illinois Agricultural Pest Management Handbook for more information on the use of insecticides to control corn flea beetles.--Dean Malvick