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

April 13, 2007

Compared to 2006

Nebraska corn at lower risk for Stewart’s wilt and flea beetle damage in 2007

The pathogen is most commonly spread by the corn flea beetle (Chaetocnema pulicaria) and disease incidence 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.

Figure 2. Nebraska map illustrating the sum of the average temperatures for December 2006-February 2007 for the Stevens-Boewe Index. Based on this model, the areas shaded in green and yellow are at low and moderate risk, respectively, for flea beetle survival and Stewart’s wilt development in 2007. (Links to larger version.) (All maps developed by Al Dutcher, Nebraska State Meteorologist, Department of Agricultural Meteorology.)

Corn flea beetles overwinter as adults in protected areas near corn fields. They have been reported 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 and in rare cases this has caused economic damage; however, usually more damage is caused by the bacterium they vector, which causes Stewart’s wilt.

Disease Forecasting

Disease forecasting systems for Stewart’s wilt have been under development for more than 50 years. Currently, at least two predictive models for the leaf blight phase of Stewart’s wilt are based upon the likelihood of flea beetle survival the previous winter. Both models base their predictions on the average temperatures during 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 on the sum of the average temperatures of each of the three winter months. When that temperature index exceeds 90°F, the risk of developing Stewart’s wilt is high.

Iowa State University Model

Within the last 10 years, the Stevens-Boewe Index was modified by Iowa scientists to improve disease predictions. The new Iowa State University Model is based on the average temperature during each 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.

Table 1. Stevens-Boewe Index to predict the flea beetle survival and Stewart's wilt development.		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.
Temperature Index	Risk of Stewart’s Wilt	Number of months with >24°F	Predicted Risk of Stewart’s Wilt
> 90°F	High	0	Negligible
80-90°F	Moderate	1	Low to moderate
< 80°F	Low	2	Moderate to high
		3	High
		Iowa State Model (Nutter et al., 2002. Comput. Electron. Agric. 37:7-14).

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 carry the bacterium that causes Stewart’s wilt. These maps do not indicate flea beetle or disease distribution, but rather areas where overwintering conditions would support low to moderate risk of flea beetle damage and disease losses due to Stewart’s Wilt if flea beetles were present in 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 University, most of Nebraska is at low to moderate risk for Stewart’s wilt development, with southern Nebraska having a moderate to high risk. (All maps developed by Al Dutcher, Nebraska State Meteorologist, Department of Agricultural Meteorology.) Each map links to a larger version.

December 2006

January 2007

February 2007

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.

A variety of foliar insecticides can control flea beetles. Products and rates include: Lorsban 4E, 2-3 pints per acre; Sevin XLR Plus, 1-2 quarts per acre, Asana XL, 5.8-9.6 fl. ounces per 1000 row-feet; Lannate LV, 0.75-1.5 pints per acre; Pounce, 3.2 EC 4-8 fl. ounces per acre; Warrior, 2.56-3.84 fl. ounces per acre, Mustang Max, 2.72-4.0 ounces per acre; Baythroid 2, 1.6-2.8 ounces per acre. Additional information is available on the UNL Department of Entomology Web site.

It’s probably too late to limit risk through hybrid selection since most seed selections have already been made for this season. Minimizing flea beetle damage and careful fertility management may decrease disease severity, especially by avoiding the use of excess nitrogen and phosphorus, which has been shown to increase disease severity.