The effects of the 2000-2001 winter on the survival of corn flea populations will be interesting to monitor regarding the occurrence of Stewart's disease in corn. All indications are that the disease will be much less prevalent this year compared with last year, but there may still be a risk of Stewart's disease in 2001.

Stewart's disease is caused by the bacterium Pantoea stewartii. Two phases of symptoms can occur with Stewart's disease: seedling wilt and late leaf blight. The seedling wilt phase occurs when overwintering adult corn flea beetles infested with the bacterium feed on young corn plants, thereby transmitting the bacterium to these plants. After transmission, linear, water-soaked lesions occur, followed by stunting and wilting. The seedling wilt phase can kill corn plants, as occurred in numerous fields throughout the state in spring 2000. The late leaf blight phase occurs later in the growing season with symptoms originating at the site of feeding scars caused by corn flea beetles. Once the bacterium has been successfully transmitted into the corn plant, it multiplies and yellowish water-soaked lesions or streaks soon appear. These lesions elongate along the leaf vein and coalesce. They eventually become necrotic and may blight or kill entire leaves.

Factors influencing the occurrence of Stewart's disease in Iowa in 2001 include the following:

1. Prevalence of Stewart's disease in 2000. Figure 1 shows Stewart's disease prevalence in seed production fields. Of 1,303 fields inspected in 2000, 58 percent (755 fields) were found to have Stewart's disease during phytosanitary inspections (Figure 2). This percentage tied the 30-year record for the prevalence of Stewart's disease and is an important risk factor.
2. The corn flea beetle population was high at the end of the 2000 growing season. Even if the 2000-2001 winter killed a high percentage of corn flea beetles, there may still be a sufficient number to transmit the bacterium to corn seedlings.
3. The proportion of corn flea beetle populations found to be infested with the bacterium going into the fall of 2000 ranged from 8 to 20 percent statewide. This range is a little less than that of infested beetles going into the spring 2000 growing season, but it is still sufficiently high to cause serious damage this spring if the beetles survived the winter.
4. The predicted risk for Stewart's disease using the Nutter model is low to moderate and spotty in the majority of the state (except the extreme northwestern and southwestern counties) and moderate to high in the bottom two tiers of counties. The expected prevalence of Stewart's disease for the moderate-to-high regions would be greater than 9 percent (i.e., more than 9 percent of the fields in a county would have Stewart's disease). The prevalence of Stewart's disease may be much more variable in the rest of the state.

In comparison, the traditional Stevens-Boewe system for predicting the severity of Stewart's disease would indicate that nothing is to be expected in 2001 (Figure 3). None of the climatic regions even approached the minimum threshold of 80ºF. This temperature threshold is the lowest level for Stewart's disease when using the Stevens-Boewe system.

The influence of snow cover in the other 80 percent state may help increase corn flea beetle survival. For example, in Des Moines, there was a record 99 consecutive days of snow cover of at least 1 inch. Thus, we expect that some localized corn flea beetle populations have probably survived the 2000-2001 winter, which is why risk for Stewart's disease may be low to moderate and spotty for much of Iowa and moderate to high for the lower two tiers of the state.