Stewart's disease is caused by the bacterium Pantoea stewartii. Corn flea beetles are important in the epidemiology of this disease because the bacterium can only survive the winter within the bodies of corn flea beetles. The seedling wilt phase of Stewart's disease occurs when overwintering adult corn flea beetles infested with the bacterium feed on young corn plants, transmitting the bacterium to these plants.
Although the corn flea beetle populations were quite low throughout most of Iowa during the 2001 growing season, the 2001-2002 winter was one of the warmest on record and we are predicting a high risk for the occurrence of Stewart's disease in the 2002 growing season. Based on 30 years of historical disease and weather data, we are predicting that 10-15 percent of the seed corn fields in Iowa will have Stewart's disease in 2002.
Stewart's Disease Risk Factors
- The predicted risk for Stewart's disease based on our mean monthly temperature model is "high" throughout the state.
- The prevalence of Stewart's disease was low in 2001, but was spotty and there is some carry-over of inoculum from 2001.
- Corn flea beetle populations in the fall of 2001 in the lower third of the state rebounded after being low for much of the year.
- Some corn flea beetles in the fall tested positive for the bacterium.
Our prediction is based on the fact that mean monthly temperatures for December, January, and February were each above 24°F and the fact that we are already detecting adult corn flea beetles in southern Iowa. Based on our prediction model, a high prevalence of Stewart's disease is predicted for seed corn and sweet corn when all three winter months are above 24°F. On young seedlings, look for linear, water-soaked lesions on leaves, followed by stunting and wilting. The seedling wilt phase can kill young corn seedlings.
Using the traditional Stevens-Boewe model to predict Stewart's disease in 2002, the risk is moderate-to-severe or severe in the southern two-thirds of the state, but is low in northern Iowa. In this system, the risk is calculated by summing the mean monthly temperatures (F) for December, January, and February. A sum below 80° is considered low risk; up to 85°, low to moderate; up to 90°, moderate to severe; and above 90°, severe. The Stevens-Boewe system was developed in the 1950s to predict the severity of Stewart's disease in the late summer leaf blight stage of the disease. Mid-to-late in the season, look for leaf symptoms originating at the site of corn flea beetle feeding scars. The symptoms initially are yellow or water-soaked lesions or streaks that elongate along the leaf veins. Later these streaks become necrotic and coalesce, eventually blighting entire leaves.Looking back to the 2001 growing season, the prevalence of Stewart's disease was quite spotty and low, as was predicted by both prediction methods (Figure 2). Look for Stewart's disease to be most severe in 2002 in areas where the disease was also found in 2001.
Seed treatments with imidacloprid (Gaucho Extra for inbred seed or Prescribe for hybrid seed) can provide protection from corn flea beetles (and therefore from Stewart's disease) for the first four weeks or so, depending on crop growth rate. During the season, the best control for Stewart's disease is to carefully watch corn flea beetle populations in individual fields and apply a foliar insecticide to reduce their populations. Thresholds for flea beetle control are based on the insect damage alone, not considering the damage due to Stewart's disease. But if corn flea beetle populations are high early in the season, they can damage corn plants even in the absence of P. stewartii. The following thresholds are used for flea beetle control: 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. Several foliar insecticides are registered for corn flea beetles (Table1).
Table 1. Foliar insecticides labeled for corn flea beetle.
|Asana XL||5.8-9.6 oz/acre|
|Capture 2EC||1.47-2.2 oz/acre|
|Furadan 4F||1 qt/acre|
|Lorsban 4E||2-3 pt/acre|
|Pounce 3.2EC||4-8 oz/acre|
|Sevin XLR Plus||1-2 qt/acre|
This article originally appeared on pages 50-51 of the IC-488 (6) -- April 29, 2002 issue.