Two Forecasting Models Show Similar Risks

Stewart's bacterial wilt, caused by Pantoea stewartii (formerly known as Erwinia stewartii), has been a problem in parts of Nebraska since the mid 1990s. In 1999 and 2000, Stewart's wilt was identified in at least 27 Nebraska counties scattered across the eastern two-thirds of the state.

This disease can cause severe yield losses in susceptible inbreds and popcorn and sweet corn hybrids. The bacterium also can be seed-borne and exportation to at least 50 countries is restricted when the disease is confirmed in seed.

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 the flea beetle.

Disease Life Cycle

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 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 from flea beetle feeding injury alone; however, more damage is probably caused by the bacterium they vector that causes Stewart's wilt.

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 on the likelihood for flea beetle survival during the previous winter. Both models base their predictions on the average temperatures during December, January, and February.

Stevens-Boewe Model. 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 (Table 1).

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 Model is based on the average temperature during each month. According to this model, the risk for Stewart's wilt development increases according to the number of months (December through February) with an average temperature that exceeds 24°F (Table 2).
 

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 are not an indication of flea beetle or disease distribution, but, based on these models, if flea beetles were present in 2011 and carrying the pathogen, then most of the state is at elevated risk for flea beetle damage and disease losses due to Stewart's wilt this season.

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 are found per plant. 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. Pay particular attention to corn not treated with a neonicotinoid seed treatment such as clothianidin or thiamethoxam, as flea beetle injury is more likely to be seen in these fields.

Insecticides

A variety of foliar insecticides are effective in controlling flea beetles. They include:

• Lorsban 4E, 2-3 pints per acre;
• Sevin XLR Plus, 1-2 quarts per acre;
• Asana XL, 5.8-9.6 fl oz per 1000 row-feet;
• Lannate LV, 0.75-1.5 pints per acre;
• Pounce 3.2 EC, 4-8 fl oz per acre;
• Warrior, 2.56-3.84 fl oz per acre;
• Mustang Max EC, 2.72-4.0 oz per acre; and
• Baythroid, 2 1.6-2.8 oz per acre.

Additional information is available on the UNL Department of Entomology website.

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, nutrients which have been shown to increase disease severity.

For more information, see the feature article from the American Phytopathological Society, Stewart's Wilt of Corn.

Tamra Jackson
Extension Plant Pathologist
Bob Wright
Extension Entomologist