The mild winter this year means that there is a high risk for Stewart's disease in the southern half of Iowa. We can expect high flea beetle populations in the spring, which will threaten susceptible dent corn inbreds and sweet corn hybrids. We also can expect considerable leaf damage during grain fill in some fields, resulting in yield loss. Seed producers in the southern part of the state should pay particular attention to early-season flea beetle populations.
||Stewart's disease symptoms.
Stewart's disease of corn is caused by the bacterium Pantoea stewartii (formerly 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 are almost always associated with flea beetle feeding. Stewart's disease symptoms on leaves are long, wavy streaks that are initially water soaked, and then turn yellow and die. When the disease is severe, whole leaves are killed. The pathogen can be seedborne, although seed transmission is extremely rare. Nevertheless, many countries prohibit the import of seed from affected fields.
The bacterium overwinters in the gut of the corn flea beetle, which in turn overwinters in the soil in ditches and other grassy areas.
||Corn flea beetle, carrier of Stewart's disease in corn.
||Severe late-season Stewart's disease in corn.
||Total collapse of susceptible sweet corn lines due to Stewart's disease.
The system to predict the survival of the insect and the risk of disease is based on winter temperatures. The 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, the environmental conditions favor survival of flea beetles and the risk of Stewart's disease is high. Table 1 shows the disease risk associated with different levels of the index.
In an average year, the index ranges from about 45 in Osceola County to about 79 around Keokuk. This year, the index is much higher, as indicated in Table 2. Values are near or above 90 for most of the southern half of the state, with the highest numbers occurring in the southeast.
If corn flea beetle populations are high early in the season, they can damage corn plants even in the absence of P. 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 plants with severe feeding injury and 5 or more beetles per plant; in seed corn on susceptible inbreds, 10 percent of the plants with severe feeding injury and 2 or more beetles per plant. There are a number of insecticides registered for flea beetles. These insecticides and the manufacturer's recommended labeled rates are as follows:
- Ambush (6.4-12.8 oz/acre)
- Asana XL (5.8-9.6 oz/acre)
- Furadan 4F (1 qt/acre)
- Lannate LV (12-24 oz/acre)
- Lorsban 4E (1-2 pt/acre)
- Penncap M (2-3 pt/acre)
- Pounce 3.2 EC (4-8 oz/acre)
- Sevin XLR Plus (1-2 qt/acre)
- Warrior (2.56-3.84 oz/acre)
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 it is expected to receive a label some time this year. There is a small possibility that it will be available in time for planting, but that possibility is shrinking each day.
Table 1. Stevens-Boewe Index for prediction of Stewart's disease.
||Leaf blight severity
||Moderate to severe
||Light to moderate
Table 2. Stevens-Boewe Indices for Iowa in 1995.
||Stevens-Boewe Index (selected stations)
This article originally appeared on pages 39-40 of the IC-480 (5) -- April 13, 1998 issue.