By Forrest W. Nutter, Jr., Alison Robertson, and Sharon Eggenberger, Department of Plant Pathology; Erin Hodgson, Department of Entomology
Although December, January and February temperatures were somewhat warmer across Iowa, compared to the 2009/2010 winter, the risk for Stewart's disease of corn in 2011, based on two predictive models, is negligible to very low throughout most of Iowa.
Stewart's disease, also known as Stewart's wilt, is caused by the bacterium Pantoea stewartii. An insect vector, the corn flea beetle (Figure 1), plays a critical role in the overwintering survival and plant-to-plant spread of this microorganism. The bacterium survives the winter months within the gut of hibernating corn flea beetles. If winter temperatures are mild enough for corn flea beetle populations to survive locally, the bacterium will also survive. In the spring, surviving corn flea beetles infested with P. stewartii will emerge from grassy areas near corn fields and, as they feed, transmit the pathogen to corn seedlings.
Fig. 1. Corn flea beetle on a corn seedling leaf. Photo courtesy of Paul Esker, Univ. of Wisconsin.
Stewart's disease can occur at any stage of corn development, but symptoms are almost always associated with flea beetle feeding. Corn seedlings can wilt rapidly from systemic infection, and seedling death is common, especially in seed corn and sweet corn fields. Plants that survive the seedling wilt phase will be stunted and will serve as a source of the pathogen, for future generations of corn flea beetles to acquire and transmit throughout the crop.
Plant-to-plant spread by overwintering corn flea beetles will continue until late May, when corn flea beetles lay their eggs at the base of corn plants. The overwintering generation of adult corn flea beetles then dies. In early to late June, there is a "beetle-free" period, lasting two to three weeks, which ends as the next generation of adult corn flea beetles (known as the first summer generation) begin to emerge and feed. During this "beetle-free" period, foliar insecticide sprays are not effective.
The first summer generation of adult corn flea beetles emerges in late June, and feeds on infected corn plants. The beetles can acquire the bacterium and facilitate the further spread of the pathogen to healthy corn plants. Later in the growing season, usually after pollination, the leaf blight phase of Stewart's disease may occur. Diseased plants at this phase first exhibit long, wavy, water-soaked streaks (lesions); diseased leaf tissue then turns yellow and dies (Figure 2). Corn flea beetle feeding scars are usually visible within the lesions. If the disease is severe, whole leaves may wilt and die.
Fig.2. Early symptoms of Stewart's disease on a corn leaf. Photo by Alison Robertson.
A second generation of corn flea beetles will emerge about mid-August. Insects from this generation overwinter. If the overwintering adults harbor the bacterium, the pathogen can also survive until the next growing season.
Disease prediction models
Mild winters during the late 1900s and early 2000s resulted in the occurrence of severe epidemics of Stewart's disease in Iowa. However, severe winters and the widespread adoption of planting insecticide-treated seed have greatly reduced corn flea beetle populations throughout Iowa in recent years. Two disease prediction models are available to predict the seasonal and county-level risk of Stewart's disease: the Stevens-Boewe Index Model and the Iowa State Mean Monthly Temperature Model. Both models use the monthly mean winter temperatures for December, January and February to predict the degree to which corn flea beetle populations survived the winter.
The Stevens-Boewe Index predicts the severity (how much of the corn leaf tissue is infected) for the mid-to-late season leaf-blight phase of Stewart's disease based on the sum of the mean temperatures for December, January and February. A sum below 80 indicates a negligible risk; 80 to 85 is considered a low risk; 85 to 90 indicates moderate risk; and greater than 90 is considered a severe risk. The summed monthly mean temperatures for the nine Iowa agricultural climate districts are presented in the map below:
Fig. 3. The Stevens-Boewe risk level for the 2011 growing season is "negligible" for all nine agricultural climate districts in Iowa. Temperature data (summed mean temperatures for the months of December, January, and February, shown above) was obtained from the Bureau of Climatology, Iowa Department of Agriculture and Land Stewardship.
The Iowa State University Stewart's disease model predicts the prevalence of Stewart's disease, with prevalence being defined as the percentage of fields predicted to be infected by the Stewart's disease bacterium. A high prevalence of Stewart's disease is predicted if the mean monthly air temperatures for December, January and February are each above 24 degrees F. Eight of the nine agricultural climate districts had either zero months or just one month above 24 degrees F. This indicates that survival of large corn flea beetle populations is highly unlikely this winter, in all but the southwest climate district, which has a moderate risk. The Iowa State Model prediction for 2011 is shown below (Figure 4).
Continuous snow cover in parts of Iowa from late December through February could have functioned as an insulation blanket to protect corn flea beetles from subfreezing temperatures. Even so, we predict that corn flea beetle populations in 2011 will be extremely low and spotty this spring, so the anticipated risk of damage due to Stewart's wilt based upon both models is negligible-to-low statewide.
Fig. 4. Predicted risk of Stewart's disease in Iowa agricultural climate districts in 2011, calculated using the Iowa State Model.
0 = negligible risk, 1 = low risk, 2 = moderate risk, 3 = high risk.
Corn flea beetle, the vector of Stewart's disease, can be suppressed with IPM tools such as hybrid selection, scouting and insecticides. Areas with potential risk should incorporate resistant hybrids to minimize adult attraction and subsequent egg laying. Susceptible hybrids planted in historically infected areas should be planted later to discourage adult colonization. Regardless of hybrid selection, all corn fields should be scouted for adult corn flea beetles several times a week during emergence and seedling stages. Look for the shiny, black adults feeding on leaves. Try to walk quietly, as they are easily disturbed and will jump off the plants. Also look for long, light feeding scars on the leaves.
Seed treatments may also provide early season management for the beetle and Stewart's wilt. A 2000 study at the University of Illinois demonstrated that two insecticides, imidacloprid (Gaucho®) and thiamethoxam (Cruiser®), applied to sweet corn seed, reduced the incidence of Stewart's wilt by 50 to 85 percent under field conditions with naturally occurring populations of corn flea beetles. According to the researchers, these seed-treatment insecticides controlled Stewart's disease during the very early growth of corn seedlings when applications of conventional, foliar insecticides were ineffective. The full article, Control of Stewart's Wilt in Sweet Corn with Seed Treatment Insecticides, is available online.
Consider using treatment threshold guidelines to protect yield. Corn flea beetle can be controlled with timely applications. Currently labeled products include pyrethroids (Asana XL, Mustang Max, Warrior II), organophosphates (Lorsban 4E, Nufos 4E), and carbamates (Lannate LV, Sevin 4F). See manufacturer's labels for use rates and restrictions. Use the following thresholds for rescue treatments in corn:
- Field corn--prior to stage V5, 50 percent of plants with severe feeding injury and five or more beetles per plant.
- Seed corn--on susceptible inbreds, 10 percent of the plants with severe feeding injury and two or more beetles per plant.
Forrest W. Nutter, Jr. is a professor in the Department of Plant Pathology working on disease risk models for improved disease management. Alison Robertson is an assistant professor in the Department of Plant Pathology with extension and research responsibilities. Sharon Eggenberger is a Research Assistant in the Department of Plant Pathology. Erin Hodgson is an assistant professor in the Department of Entomology with extension and research responsibilities.