Figure 1. Stewart’s wilt lesions from the leaf blight phase.

Nebraska corn at higher risk for Stewart’s wilt and flea beetle damage in 2006

Stewart’s bacterial wilt, caused by Pantoea stewartii (formerly known as Erwinia stewartii) has been a problem in parts of Nebraska since approximately the mid-1990s. In 1999 and 2000, Stewart’s wilt was identified in at least 27 counties in Nebraska that were scattered across the eastern two-third's of the state. This disease can cause severe yield losses in susceptible inbreds and popcorn and sweet corn hybrids. But, the bacterium can also be seed-borne and exportation of seed to some countries is restricted when the disease is present. The pathogen is most commonly spread by the corn flea beetle (Chaetocnema pulicaria) and incidence of this disease has been related to the overwintering survival of its flea beetle vector. Stewart’s wilt appears in two phases: systemically infected seedlings and leaf blight. The seedling phase is less common than the leaf blight phase, except in sweet corn and some inbreds that tend to be more susceptible. Severely infected plants may develop decaying cavities inside the stalk near the soil line. The leaf blight phase typically develops after tasseling and lesions are usually long, wavy and may be associated with flea beetle feeding injury.

Corn flea beetles overwinter as adults in protected areas near corn fields. They have been reported from throughout Nebraska, but typically are most common in the southern half of the state. They become active in April and feed on a variety of grasses before corn emerges. Corn flea beetles can directly injure corn by feeding on seedling plants; in rare cases economic damage may occur just from flea beetle feeding injury alone. However, more damage is probably caused by the bacterium that they vector, which causes Stewart’s wilt.

Figure 2. Nebraska map illustrating the sum of the average temperatures for December 2005-February 2006 for the Steven-Boewe Index. Based on this model, the areas shaded in red and yellow are at high and moderate risk, respectively, for flea beetle survival and Stewart’s wilt development in 2006. (All maps developed by Al Dutcher, Nebraska State Meteorologist, Department of Agricultural Meteorology.)

Table 1. Stevens-Boewe Index to predict the flea beetle survival and Stewart's wilt development.
Temperature Index	Risk of Stewart’s Wilt
= 90°F	High
80-90°F	Moderate
= 80°F	Low
Table 2. Iowa State University Model for predicting the risk for Stewart's wilt development. It looks at the average temperature for each month from December through February.
Number of months with 24°F	Predicted Risk of Stewart’s Wilt
0	Negligible
1	Low to moderate
2	Moderate to high
3	High
Iowa State Model (Nutter et al., 2002. Comput. Electron. Agric. 37:7-14).

Disease forecasting

Disease forecasting systems for Stewart’s wilt have been under development for more than 50 years. Currently, there are at least two predictive models for the leaf blight phase of Stewart’s wilt that are based upon the likelihood for flea beetle survival during the previous winter. Both models base their predictions on the average temperatures during the months of December, January, and February.

In the 1940s, G.H. Boewe of Illinois modified an earlier model for predicting Stewart’s wilt. The predictions for flea beetle survival and Stewart’s wilt development in this model are based upon the sum of the average temperatures of each of the three winter months. When that temperature index exceeds 90°F, then the risk of developing Stewart’s wilt is high.

Iowa State University Model

Within the last ten years, the Stevens-Boewe Index was modified by scientists in Iowa to improve disease predictions. The new Iowa State Model is based on the average temperature during each individual month. According to this model, the risk for Stewart’s wilt development increases for an area according to the number of months with an average temperature that exceeded 24°F from December through February.

Both of these models make disease predictions based on the potential for flea beetle survival. It is important to keep in mind that, historically, flea beetles have not been evenly distributed across Nebraska, and that not all flea beetles will be carrying the bacterium that causes Stewart’s wilt. These maps are not an indication of flea beetle or disease distribution, but, based on these models, if flea beetles were present in 2005 and carrying the pathogen, then most of the state is at higher risk for flea beetle damage and disease losses due to Stewart’s wilt this season.

December 2005 January 2006 February 2006 Figure 3. Nebraska maps showing in red those areas with average temperatures higher than 24°F for the months given. Using the model developed by Iowa State, most of Nebraska is, at least, at moderate risk for Stewart’s wilt development. And, all of the southern counties had average temperatures above 24°F during all three months and are at high risk. (All maps developed by Al Dutcher, Nebraska State Meteorologist, Department of Agricultural Meteorology.)

Controlling potential damage

To minimize damage from flea beetles:

• Avoid hybrids or inbreds known to be more susceptible to Stewart’s wilt (see seed catalogue or local seed company representative).
• Avoid early planting dates if susceptible inbreds or hybrids are planted.
• Seed treatments containing clothianidin (Poncho) or thiamethoxam (Cruiser) are systemic and provide protection from feeding by flea beetles and other early season soil insects.
• Scout for corn flea beetles on seedling corn. Postemergence treatment may be warranted on dent corn if 50% of plants show severe flea beetle injury (plants look silvery or whitish, or leaves begin to die), and five or more flea beetles per plant are found. If susceptible inbreds or hybrids are grown, an insecticide may be needed when two to three flea beetles per plant are present and 10% of the plants show severe flea beetle injury.