Corn flea beetle and Stewart's leaf blight
The adult corn flea beetle is not only a pest itself, but it is the vector and means for spread of the bacterium that causes Stewart's bacterial wilt and leaf blight on both field and sweet corn. Beetle adults that overwinter become active in the spring when the soil temperatures reach 65 F, and are most active on sunny, warm, windless days. Those adults that fed on corn plants with Stewart’s disease in the previous late summer or fall may acquire and carry the bacterium from one growing season to another. By feeding on young plants in the spring, they may spread the bacterium which in turn causes seedling wilt and leaf blight. The occurrence of Stewart's bacterial disease is totally dependent on the level of bacteria-carrying flea beetle survival over the winter.
For many years the winter temperatures have been used to predict the risk of Stewart's disease because higher populations of the flea beetle survive during mild winters than during cold winters. An index is developed that helps to predict the likelihood of the disease threat. This 'flea beetle index' is calculated as the sum of the average temperatures (Fahrenheit) of December, January and February. We checked the indices for the past 8-10 years, and the last time the numbers were high suggesting an elevated concern was in 2006. Having checked average temperatures for various locations in Ohio the past three month, we find that all areas of the state have indexes over 100 suggesting that risk is severe in 2012. The locations and the corresponding indexes are: Wooster (OARDC) 102.5, Ashtabula 103.1, Hoytville (Northwest Research Station) 100.2, South Charleston (Western Research Station) 105.7, and Piketon 103.1.
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.
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 treatment 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.
The question becomes, "how prevalent is Stewart’s bacterial blight in our state?" While the warmer temperatures this winter might allow for an increase in corn flea beetle numbers, in 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 plants last season. So, with the level of Stewart’s disease being low during 2011, it is quite possible that beetles, even if they survived due to the mild winter, may not be carrying the bacterium.
You can see pictures of flea beetle injury and Stewart’s bacterial blight, and get additional information on Stewart's disease of corn, on the Ohio Field Crop Diseases web site at http://oardc.osu.edu/ohiofieldcropdisease/t01_pageview2/Home.htm. Additional information on the flea beetle can be obtained from OSU Extension Fact Sheet AC-37-2001 http://ohioline.osu.edu/ac-fact/pdf/0037.pdf .