Stewart's disease risk for 1996

Stewarts disease of corn is a bacterial disease caused by Erwinia stewartii that primarily affects sweet corn and certain susceptible field corn inbreds. It is common in states to the south and east of Iowa. Although it occurs sporadically in Iowa, parts of the state had outbreaks in 1992 and 1995. Southeast Iowa has the highest risk for this disease each year.

The bacterium is spread by the corn flea beetle, and disease symptoms are almost always associated with flea beetle feeding. Stewarts disease symptoms on leaves are long, wavy streaks that are water-soaked, then turn yellow and die. The pathogen can be seedborne, although seed transmission is rare. Nevertheless, many nations prohibit the import of seed from affected fields.

The bacterium also overwinters in the corn flea beetle, which in turn, overwinters in grassy areas. We can predict the survival of the insect and, therefore, the risk of disease based on winter temperatures. Cold temperatures reduce the survival of corn flea beetles.

The Stevens-Boewe Index indicates the risk of Stewarts disease on susceptible corn varieties. The index is calculated by simply adding the mean monthly temperatures for December, January, and February. When the average temperature for these three months added together exceeds 90 F, environ-mental conditions favor the survival of the flea beetles and the risk of Stewarts disease is increased.

Stevens-Boewe Index for Prediction of Stewart's disease

Index Risk of leaf blight severity
90 or more Severe
85-90 Moderate to severe
80-85 Light to moderate
Below 80 Trace

Table 1 shows the disease risk associated with different levels of the index. In a normal winter in Iowa, the index would range from about 45 in Osceola County to about 79 around Keokuk. This would represent a low risk throughout the state. In 1992, the index was as high as 105 in southeast Iowa, and a severe outbreak occurred.

This year, southeast Iowa has a low to moderate risk for Stewarts disease on susceptible inbreds. We calculated indices for the warmest weather stations in each crop reporting district in Iowa, and they did not exceed 80 anywhere except in southeast Iowa. The highest index was about 82 for Keokuk.

If corn flea beetle populations are high early in the season, they can damage corn plants even in the absence of Erwinia stewartii. You can control Stewarts disease on susceptible corn by controlling the corn flea beetle with a foliar insecticide. Use the following thresholds: in field corn prior to stage V5, 50 percent of plants with severe feeding injury and 5 or more beetles per plant; in seed corn on susceptible inbreds, 10 percent of the plants with severe feeding injury and 2 or more beetles per plant.

This article originally appeared on pages 37-38 of the IC-476 (6) -- April 22, 1996 issue.

Updated 04/21/1996 - 1:00pm