Stewart’s wilt could be back in sweet corn in 2012

Stewart’s wilt of sweet corn can cause significant damage. The extent is directly correlated to winter temperatures. What can we expect in 2012?

Stewart’s wilt is a bacterium that attacks primarily sweet corn and survives in close association with corn flea beetles. First symptoms are usually flea beetle feeding lesions on the leaves (Photo 1). These lesions will be followed shortly by a yellowish-white leaf streaking parallel to the leaf midrib (Photo 2). Theses streaks eventually dry and turn brown. The bacterium can penetrate the stalk, killing the entire plant.

Corn flea beetle
Photo 1. Corn flea beetle and feeding damage.
Photo credit: Univ. of Illinois at Urbana-Champaign, Bugwood.org

Corn plants
Photo 2. Corn plants showing Stewart’s wilt symptoms.
Photo credit: J.K. Pataky, Univ. of Illinois at Urbana-Champaign, Bugwood.org

Symptoms are often worse in early planted corn, but it can occur throughout the season and is more severe if the plant is exposed in the early stages of growth. Severity varies from year to year and it has been a while since we have had a severe problem.

The Stewart’s wilt bacterium overwinters in the gut of the corn flea beetle and is transmitted as the beetle feeds. Due to this close association, survival and transmission of the bacteria is directly related to survival of the beetle and severe Stewart’s wilt years are those when greater numbers of beetles survive mid-winter temperatures. Beetle survival and Stewart’s wilt severity can be estimated by calculating the average temperature of December, January and February. Years when the temperature averages below 27°F during this three-month period, Stewart’s wilt is generally not a problem. But when the temperature averages above 33°F, Stewart’s wilt can be severe (Table 1).

Table 1. Relationship of December, January
and February average temperature and
Stewart’s wilt severity.

Average temperature
Dec., Jan. and Feb.

Stewart’s wilt
Severity

Below 27°F

Nearly Absent

27°F to 30°F

Light

30°F to 33°F

Moderate

Above 33°F

Severe

The last couple winters have been severe enough that there has been low beetle survival. However, the winter of 2011-12 was quite mild across much of Michigan. Table 2 data indicates growers across much of the Lower Peninsula can expect to experience slight to possibly severe Stewart’s wilt symptoms. Average temperatures for other Michigan sites can be calculated by going to Enviro-weather and retrieving the daily data records from the site nearest your location.

Table 2. Average monthly temperature during
December 2011, January and February 2012
at six Michigan locations.

Michigan Location

Average Temperature
Dec., Jan. and Feb.

Benton Harbor

32.5°F

Escanaba

25°F

Fremont

30.5°F

Hudsonville

31.5°F

Petersburg

31.5°F

Traverse City

29°F

The first step in controlling Stewart’s wilt is to choose cultivars having some genetic tolerance to the bacterium. The next step is to control the beetle. This can be accomplished by purchasing seed already treated with the proper insecticide. Treated seed will protect the early, most susceptible growth stages of the plant. Growers can get untreated seed and then wait to see what kind of beetle activity occurs. Control measures need to be taken when beetle numbers reach five beetles per 20 plants from five locations in the field. Controlling after you see beetles requires scouting fields several times a week, so many growers use the treated seed.

Be aware that Stewart’s wilt has the potential of making a comeback this year. So don’t let it catch you off guard.

For more information on commercial vegetable production, contact Ron Goldy at 269-944-1477 ext. 207.

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