Corn flea beetle survival and potential for Stewart’s wilt

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Corn flea beetle is a sporadic corn pest in Indiana and has had little impact of recent years. Still, winter temperatures in regions where beetles were abundant last season will determine if there is cause to be concerned this spring for susceptible inbreds and hybrids. This is especially important since this insect transmits the bacterium that causes Stewart’s disease in corn. The severity of the disease correlates with last season’s beetle abundance and this winter’s temperatures. This is because the Stewart’s wilt bacterium survives in the gut of the overwintering beetles and depends upon the beetle to infect corn. Warmer temperatures result in higher beetle survival, and therefore a greater potential for Stewart’s disease.

To determine the potential severity of Stewart’s disease, add the average daily temperatures for the months of December, January, and February. If the sum is below 90, the potential for disease problems to develop is low. If between 90 and 100, moderate disease activity is a possibility. Sums above 100 indicate a high probability that beetles will survive the winter and vectoring of Stewart’s disease will occur. To help you better gauge the potential for corn flea beetle activity in your area (and the potential severity of the threat of the disease) in 2013, we have created the state map shown below. According to the temperature model, there is low probability of corn flea beetle activity and subsequent disease in northern and central Indiana, moderate in areas south of Interstate 70, and higher south of US 50.

This temperature model for corn flea beetle has been in use for many years and has been fairly accurate in predicting the activity of this pest the following spring. However one inherent flaw is that the model is based on ambient air temperatures, not temperatures under leaf litter and grass clumps where this pest is actually overwintering. If snow cover is present, this provides an insulating blanket for the insect, and may protect some beetles from winterkill. Even with this “disclaimer” statement, we think the 2012/2013 winter was cold enough to have negatively impacted overwintering beetles in most of Indiana. Also, flea beetle numbers have been low statewide for the last several years.

As for the disease, there are two phases of Stewart’s wilt: a seedling wilt phase and a leaf blight phase. In the wilt phase, plants wilt rapidly, usually at an early stage of growth. Leaves emerging from the whorl of infected plants are often the first to wilt. Internal tissues at the growing point are discolored or hollowed out. Faint green to yellow streaks containing corn flea beetle feeding marks are visible on one or more leaves. If stalks of wilted plants are cut, it may be possible to see yellow beads of bacteria ooze from the vascular tissue. Sweet corn hybrids are especially susceptible. Some dent corn inbreds, and occasionally dent corn hybrids, and some popcorn lines are susceptible as well. Dent corn hybrids rarely show symptoms of the wilt phase after growth stage V5.

The leaf blight phase can occur at any time during the growing season, but often does not appear until after tasseling. Lesions are typically long and narrow, with greenish-yellow streaks and irregular or wavy-margins. Lesions will become straw-colored, and infected leaves may die prematurely. Hybrids with resistance to Stewart’s wilt may have smaller lesions that are limited to the tissue surrounding the feeding site of the beetle. These lesions can be confused with the fungal diseases gray leaf spot or northern corn leaf blight. Stewart’s wilt is also commonly confused with another bacterial disease, Goss’s wilt. One way to differentiate between these diseases and Stewart’s wilt is to look for the beetle feeding scars associated with Stewart’s wilt.

Management decisions made now should be based on the corn’s susceptibility to the disease and anticipated risk. With virtually all corn seed treated with insecticide (Poncho or Cruiser), the only decision to be made is whether a low or higher insecticide rate should provide sufficient protection. Those decisions have likely been made months ago, and even the lowest available rates of seed treatments (Poncho 250) are expected to provide protection from emergence to 2-leaf corn, whereas the higher rate (eg. Poncho 1250 and Cruiser 1.25, also called the “rootworm rate”) should protect corn through the 5th leaf stage. If seed-applied insecticide is not an option, broadcast application of foliar insecticides at the time when corn spikes should provide 7-10 days of residual protection from beetle feeding.

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