Risk of Stewart’s Disease to be low

WOOSTER, Ohio — The risk of Stewart’s bacterial wilt and leaf blight is predicted to be low throughout much of Ohio’s corn crop this year.

The disease is caused by a bacterium carried and spread by adult flea beetles.

Negligible threat

Based on the flea beetle index, conducted by Ohio State University Extension and Ohio Agricultural Research and Development Center specialists this winter, Stewart’s Disease should a negligible threat in 2009.

“The numbers indicate that the risk of Stewart’s bacterial leaf blight should be low in much of Ohio, with only southern Ohio considered to have a low to moderate threat,” said Ron Hammond, an OSU Extension entomologist with OARDC.

The flea beetle index is calculated as the sum of the average temperatures of December, January and February.

Index values

Index values less than 90 indicate a negligible disease threat, 90-95 indicates low to moderate levels, 95-100 indicates moderate to severe and values over 100 indicate a severe disease threat.

The results covered six locations: Hoytville with a value of 80; Wooster with a value of 79.6; Ashtabula with a value 79.8; South Charleston with a value of 90.4; Jackson with a value of 93.6 and Piketon with a value of 95.1.

The numbers were lower than what was calculated last 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. Because of a relatively cold January, much colder than normal, the index values are lower than we often see,” said Hammond.

Scout for the insect

Despite the low disease predictions, specialists still recommend growers scout for the insect, especially if they have planted a hybrid that is susceptible to Stewart’s disease.

“Beetles survive in the soil and emerge when soil temperatures warm to about 65 degrees Fahrenheit. The flea beetle is a small, black, shiny insect,” said Pierce Paul, an OARDC plant pathologist.

Preventive action

For those growers wanting to take preventive action against the flea beetle, commercially applied insecticide seed treatments Cruiser and Poncho are labeled for flea beetle control.

Stewart’s wilt is characterized by two major disease phases. One phase is seedling blight. Young plants develop pale green to yellowish streaks on the leaves.

These young plants usually wilt and die and those that survive are stunted and usually produce no ears.

The other phase is the leaf blight stage. Leaf blight is recognized as long, pale green streaks on leaves. As the streaks enlarge, portions turn pale yellow and eventually become brown.

Streaks may run the entire length of the leaf. A few characteristic lesions may be seen early in the season, but numerous lesions are usually not detected until after tasseling.