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. 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.