Outlook for Stewart’s disease for 2007

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Growers of sweet corn and seed corn are well acquainted with Stewart’s disease (Stewart’s wilt), a bacterial disease transmitted to corn in spring by corn flea beetles carrying the disease feeding on emerging corn seedlings. Corn flea beetles survive over winter in grassy areas, in soil and plant debris, and become active in the spring once temperatures warm to around 65-70ºF There are several generations of corn flea beetles per year. Some sweet corn and seed corn hybrids are susceptible to Stewart’s wilt, and management includes (in order of preference) the use of resistant hybrids, insecticidal seed treatments (Cruiser or Poncho), soil insecticides and foliar insecticide to control the beetle. There are no treatments aimed at controlling the bacterium, Pantoea stewartii, that causes the disease; you control the beetles that carry the bacterium. The disease is not transmitted plant-to-plant. It spreads with the assistance of corn flea beetles carrying the bacterium and depositing it with their feces into wounds created as they feed on the corn foliage.

For field corn, the primary means of managing this disease is through the use of resistant hybrids. In looking through seed catalogs, there seem to be fewer earlier maturing field corn hybrids that list resistance to Stewart’s disease among their characteristics than later maturing hybrids. Resistance limits the movement of the bacterium in the vascular system and keeps the plants from becoming systemically infected.

An outbreak of Stewart’s disease was reported on field corn hybrids in a number of locations last August and September of 2006. But we do not know if Stewart’s wilt infection affects the yield of field corn: We are not aware of any studies that show yield is affected. We do not know whether or not the disease will show up in field corn this spring. Plant pathologists from other states with a history of Stewart’s disease on field corn report that it generally appears as a foliar blight, later in the season, rather than in the wilt phase on seedlings. Most field corn is thought to have some resistance to the seedling wilt phase of the disease, and if the disease shows up, it occurs later in the season (after flowering), although that isn’t always so.

Although a number of fields showed symptoms of the disease, we do not have comparative research data that tells us whether the late season Stewart’s disease resulted in yield losses. Similarly, we do not know how many of the flea beetles carrying the disease survived over the winter. Corn fields bordering woodlots or protected areas that would shelter beetles from extreme weather may have higher survival of flea beetles. There are no “rescue treatments” or applications recommended for the leaf blight phase that shows up in late summer.

However, we do know that mild winter temperatures were favorable for beetle survival in much of the state. If Stewart’s disease was a problem in your area last year, the winter probably didn’t have much effect on killing the beetles. The average monthly air and soil temperatures for December, January and February were calculated for the Lower Peninsula MAWN (Michigan Agricultural Weather Network) weather station locations. Predictions for Stewart’s disease risk in Michigan for spring, 2007 is based on two models as shown in the accompanying table.

A paper published in Plant Disease, (October 2006, pp. 1353-1357) compared the accuracy of the models used to predict the prevalence of Stewart’s disease based on winter air temperatures during December - February. The Stevens-Boewe model predicts the risk of the late leaf blight phase of Stewart’s disease. At most of the locations in the table, the Stevens-Boewe model indicates a low (trace) risk of the foliar blight phase of the disease. This model tends to under predict the risk of Stewart’s disease.

Although it works well in some cases, with heavy snow cover, hedgerows or woods bordering a field, beetles may survive in higher numbers than the model would predict. The Iowa State model, which was developed for seed corn, hasn’t been tested in Michigan. It uses the number of months (0-3) during December - February that the average monthly air temperature exceeded 24ºF. Most of the locations in the table reflect a moderate level of risk for Stewart’s disease this spring. The coldest month was February, but even with air temperatures mostly in the teens, we had good snow cover to act as insulation in many locations.

We may have higher than normal survival of corn flea beetle and more Stewart’s disease, but we still don’t know if it will affect yield in field corn. Any management method will have an associated cost. Growers will need to assess the cost/risk benefit to determine their course of action. Seed corn and sweet corn growers growing susceptible hybrids should be prepared to scout their fields when plants begin to emerge in spring and take appropriate measures. To recap, the four methods of managing Stewart’s disease are: planting resistant hybrids (best), using Poncho or Cruiser seed treatments (effective for 35 to 40 days), soil insecticides and foliar treatments. Keep in mind that rapid plant growth, weathering, and an influx of new beetles limit the effectiveness of foliar insecticides. The economic treatment thresholds, prior to stage V5 in seed corn are 10 percent of the plants with severe feeding injury and two or more beetles per plant, and in commercial hybrid corn, 50 percent of plants with severe feeding injury and five or more beetles per plant. Comparison of forecasting models for Stewart’s Disease (Wilt), and survival potential of corn flea beetles. View Table

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