Being early March, it is time to put out the annual corn flea beetle and Stewart’s leaf blight prediction based on the average temperatures the past three months (Dec, Jan, and Feb.). The occurrence of Stewart's bacterial disease is totally dependent on the level of bacteria-carrying flea beetle survival over the winter. Because higher populations of the flea beetle survive during mild winters than during cold winters, the winter temperatures have been used to predict the risk of Stewart's disease.
The 'flea beetle index' is calculated as the sum of the average temperatures (Fahrenheit) of December, January and February. We find that all areas of the state except for the north have indexes over 95 suggesting that risk is moderate to severe, while those in the north are still over 90 suggesting more low to moderate risk this coming spring. The locations and the corresponding indexes are: Wooster (OARDC) 94.4, Ashtabula 92.2, Hoytville (Northwest Research Station) 91.6, South Charleston (Western Research Station) 96.3, Jackson 105.2, and Piketon 103.7. While not as high as last summer (2012), they are much higher than what we saw in 2010 and 2011.
The flea beetle index is:
- Index values less than 90 indicate negligible disease threat,
- 90-95 indicate low to moderate levels,
- 95-100 indicate moderate to severe and
- values over 100 predict severe disease threat.
A question is how prevalent Stewart’s bacterial blight is in our state. While the warmer temperatures this winter might allow for an increase in corn flea beetle numbers, it doesn’t automatically result in higher incidence of Stewart’s. The surviving flea beetles need to be carrying the bacterium in order to infect plant in the spring, and in order for them to acquire the bacterium, they needed to feed on diseased plant last season. So, with the level of Stewart’s disease being extremely low during 2012, it is quite possible that beetles, even if they survived due to the mild winter, may not be carrying the bacterium.
We would recommend that growers scout for flea beetles, especially if they have planted a hybrid that is susceptible to Stewart's disease. Normally we would recommend that growers wanting to take preventive action against flea beetles apply a commercially applied insecticide seed treatments labeled for flea beetles. However, the realization is that most field corn planted these days, especially all transgenic hybrids, already comes with an insecticide seed treatment applied. Thus, it is mostly non-transgenic corn that might need to be treated specifically for this concern. Also, most field corn hybrids are more resistant to wilt than sweet corn. Dent corn hybrids vary greatly in their resistance to the leaf blight stage phase of the disease. All sweet corn varieties are susceptible to wilt in the first leaf stage. A few are resistant by the second leaf stage and many are resistant in the third and fourth leaf stage. Consult your seed supplier for information on resistant varieties and hybrids.