Based on a standard system for predicting Stewart's disease in corn, the disease should be light in Iowa this year. However, we have noticed that the disease sometimes is more prevalent than the model predicts, possibly because factors other than temperature influence beetle survival, and the beetles reproduce and disperse during the season. Therefore, we expect Stewart's disease levels to be similar to last year. Some fields in southern Iowa may have some yield reduction and seed producers in the southern part of the state also may want to pay attention to early-season flea beetle populations.
Stewart's disease symptoms.
Stewart's disease of corn is caused by the bacterium Erwinia stewartii. It causes a fatal wilt disease in young plants of sweet corn and certain susceptible field corn inbreds, but usually we see the leaf blight phase, which can affect most dent corn inbreds and hybrids after pollination. The bacterium is spread by the corn flea beetle, and disease symptoms almost always are associated with flea beetle feeding. Stewart's disease symptoms on leaves are long, wavy streaks that are water-soaked, then turn yellow and die. The pathogen can be seedborne, although seed transmission is extremely rare. Nevertheless, many nations prohibit the import of seed from affected fields.
The bacterium overwinters in the gut of the corn flea beetle, which, in turn, overwinters in soil in ditches and other grassy areas. The system used to predict survival of the insect and risk of the disease is based on winter temperatures. An index is calculated by simply adding the mean monthly temperatures for December, January, and February. When the average temperature for these three months added together exceeds 90° F, environmental conditions favor survival of flea beetles and the risk of Stewart's disease is high.
The table shows Stewart's disease risk associated with different levels of the index. Our data indicate that the numbers in the table should be lower for Iowa. Last year, we saw some fields with economic damage caused by Stewart's disease in areas where the index was 80-83. This year the index ranges from about 73-81 in southern Iowa, and considerably lower throughout the rest of the state.
Stevens-Boewe Index for prediction of Stewart's disease.
||Leaf blight severity
|90 or more
||Moderate to severe
||Light to moderate
If corn flea beetle populations are high early in the season, they can damage corn plants even in the absence of Erwinia stewartii. You can control Stewart's disease on susceptible corn by controlling the corn flea beetle with a foliar insecticide. Use the following thresholds:
- in field corn prior to stage V5--50 percent of the plants with severe feeding injury and 5 or more beetles per plant, and
- in seed corn on susceptible inbreds--10 percent of the plants with severe feeding injury and 2 or more beetles per plant.
A new systemic insecticide, formulated as the seed treatment Gaucho, also has been shown to reduce flea beetle feeding and Stewart's disease. This compound is not yet registered for corn, but is expected to receive a label some time this year.
This article originally appeared on page 57 of the IC-478 (8) -- May 12, 1997 issue.