Stewart's disease of corn, which was severe in Iowa last year, is not expected to be a major problem this season. This disease, also known as Stewart's wilt or bacterial wilt, is caused by the bacterium Erwinia stewartii. It primarily affects susceptible sweet corn varieties and inbred lines of field corn. Most hybrids are fairly resistant to Stewart's disease, but a few are susceptible.
Symptoms can be divided into an early (wilt) phase and a late (leaf blight) phase. The early phase, which occurs almost exclusively on highly susceptible sweet corn, appears as wilting and stunting, with pale green or yellow streaks on the leaves. The streaked portions of the leaves quickly die and turn brown. The bacterium spreads systemically in the plants through the vascular system. Severely affected plants wither and die.
The disease on field corn usually consists of the late season leaf blight. These infections result in leaf streaks (described above) and death of leaves. The bacterium usually does not become systemic in these plants, but is restricted to the leaf lesions. Yield is reduced by the loss of leaves. Stalk piths may become deteriorated in severe cases. Plants with Stewart's disease are more susceptible to stalk rots caused by other pathogens.
A system has been used to predict the severity of Stewart's disease. The system was based on the influence of winter temperature on the survival of the corn flea beetle, Chaetocnema pulicaria. Erwinia stewartii overwinters in the gut of this insect. The beetles are also responsible for spread of the disease during the growing season.
Researchers in Illinois first devised the predictive system during the 1930s, and it has been modified since then. Basically, the beetles survive in larger numbers after a mild winter, so the disease is more severe. An index is calculated by adding the mean monthly temperatures for December, January, and February. The severity of Stewart's disease is related to this index. Table 1 summarizes the predictions for Stewart's disease severity on field corn based on this system, known as the Stevens-Boewe system. Last year, Stewart's disease was severe in some parts of the state, following the unusually mild winter. The map shows the ranges of Stevens-Boewe indices within each of the nine crop reporting districts of Iowa for 1992 and 1993. The high values for 1992 would have predicted a bad year for Stewart's disease.
You can determine the prediction for your area by comparing the numbers on the map to the numbers in Table 1. You should also notice that there can be a wide range of values even within a district. Higher temperatures tend to occur in the southern portion of each district or along the major rivers. Clearly, the 1993 values are much lower than 1992, so the risk of Stewart's disease is also lower this year.
Remember, this system is an oversimplification. It does not take into account other factors that influence the insect and the pathogen, such as snow cover, seed transmission, possible movement of corn flea beetles, and the role of other insect vectors. Therefore, predictions should be interpreted with caution. In particular, disease levels could be higher than predicted this year because the extensive snow cover insulated the overwintering beetles from the cold air temperatures.
If corn flea beetle populations are high early in the season, they can damage corn plants even in the absence of Erwinia stewartii. Control of Stewart's disease on susceptible corn can be accomplished by controlling the corn flea beetle. Recommendations for flea beetle control were given by Marlin Rice in the December 1992 issue of Integrated Crop Management.
Table 1. Stevens-Boewe indices
Index Leaf blight severity
90 or more Severe
85 to 90 Moderate to severe
80 to 85 Light to moderate
Below 80 Trace