Stewart’s wilt bacterial disease of sweet corn has been especially prevalent in southern Michigan and other Midwest areas during the past two growing seasons. Many sweet corn growers have, for the first time, experienced the reduced yield and quality that Stewart’s wilt causes, leading them to ask: What is Stewart’s wilt? Why has this disease suddenly become a problem? How can Stewart’s wilt be prevented? Will growers experience Stewart’s wilt problems again in 2000?
What causes Stewart’s wilt?
Stewart’s wilt disease of corn is caused by a bacterium, erwinia stewartii. Once inside the plant, the bacteria multiply in the vascular system and plug it. When plants are infected as seedlings, they wilt and die. When older plants are infected the bacteria cause leaf blight, reduced size and yield, and poorly filled ears.
Although bacteria actually cause the disease, the real culprit is a small, black insect called the corn flea beetle (chaetocnema pulicaria). This insect is NOT the same kind of flea beetle found on cole crops or potatoes. Corn flea beetles feed only on wild and cultivated plants in the grass family. Corn flea beetles acquire Stewart’s wilt bacteria by feeding on an infected plant. The bacteria survive in the gut of corn flea beetles and are spread to new corn plants when flea beetles feed. Stewart’s wilt bacteria survive the winter by living inside corn flea beetles. Corn flea beetles pass the winter in soil and plant trash near field edges.
Why suddenly a problem?
We can blame El Niño for this one. Southern Michigan is at the northern extreme of the corn flea beetle’s range. In most winters, many corn flea beetles are killed by the cold weather. However, the winters of 1997/98 and 1998/99 were extremely mild. We presume more beetles than usual survived the winter, thus leading to higher infestations and an epidemic of Stewart’s wilt.
How can Stewart’s wilt be prevented?
If Stewart’s wilt and corn flea beetles are present, there are two basic lines of defense. First, sweet corn varieties vary considerably in how susceptible or tolerant they are to Stewart’s wilt. A tolerant corn plant does not kill the bacteria, rather it slows the growth and spread of the bacteria, so that the disease’s effect on yield and survival is reduced. Even so, tolerant varieties that are infected very early, or experience very heavy flea beetle populations can succumb to Stewart’s wilt. J.K. Pataky at the University of Illinois has been testing sweet corn varieties for resistance or tolerance to a number of diseases, including Stewart’s bacterial wilt. The results are listed each year in the Midwest Vegetable Variety Trial Report. Unfortunately, few early-season hybrids have enough tolerance to stand up to moderate to severe disease pressure.
The other strategy for combating Stewart’s wilt is attempting to control corn flea beetle populations with insecticides. For the most part, control of Stewart’s wilt is better when a resistant variety is planted than when an insecticide is used. Of the two types of insecticides, at-plant systemics are, in general, more effective than foliar sprays. There are three registered at-plant insecticides that may provide partial control of corn flea beetles: Counter, Thimet and Furadan.
Foliar insecticides may also provide some protection against Stewart’s wilt. Foliar insecticides that are registered in Michigan for control of corn flea beetle on sweet corn include: Sevin, Asana, diazinon and permethrin (Ambush, Pounce). Foliar insecticides are less expensive than at-plant insecticides, and can be applied as needed. The disadvantage of foliar insecticides over systemics is that young corn plants grow rapidly, leaving new foliage unprotected and susceptible to feeding and infection as corn flea beetles migrate into the field from the borders.
Recently, EPA granted a Section 18 crisis exemption that allows Gaucho 480F to be applied to sweet corn as a seed treatment in Idaho. Treated seed may be planted in Arkansas, Connecticut, Delaware, Illinois, Iowa, Maryland, Michigan, Missouri, New York, New Jersey, Ohio, Pennsylvania and Wisconsin. The active ingredient in Gaucho (imidacloprid) is the same as that in Admire and Provado and efficacy experiments have shown it to be very effective against corn flea beetle. Currently, Harris Moran will custom apply Gaucho to their seeds if growers request. The Section 18 permits a limited amount of seed to be treated, so if growers are interested in planting Gaucho-treated seed they should contact their seed company representative as soon as possible.
Wilt problems in 2000?
We do not know the answer to this question. Whether Stewart’s wilt will be a problem depends on two things: how many corn flea beetles survive the winter and what proportion of the surviving beetles carry the Stewart’s wilt bacteria.
While this past winter was, in general, colder than the previous two, we don’t fully understand why flea beetles survive or die during the winter. Several years ago, scientists developed a temperature index to predict corn flea beetle survival. According to this index, an average winter temperature during December, January and February of more than 33&Mac251; results in high flea beetle survival and severe Stewart’s wilt. Average temperatures of 30-33&Mac251; F are predicted to result in moderate Stewart’s wilt. When average temperatures are below 27&Mac251; F, the risk of Stewart’s wilt is expected to be minimal.
This index, however, is not always reliable. Many times the index has predicted lower Stewart’s wilt than what actually developed. There may be many reasons why the index is sometimes unreliable. The index, for example, bases its prediction on air temperature, and flea beetles may survive in higher numbers than would be expected in areas sheltered from the cold, such as hedgerows, tree lines and ditch banks. Heavy snow cover may also shelter corn flea beetles from the cold, resulting in higher than expected survival. Also, we don’t currently understand what proportion of corn flea beetles are infected with Stewart’s wilt bacteria, how this proportion changes over the season, and how it varies from location to location.
So what is a grower to do? First, if the traditional temperature index predicts moderate to severe Stewart’s wilt for an area, a grower should expect this and should use control methods mentioned above (resistant varieties, insecticides). If the traditional temperature index predicts low or nonexistent Stewart’s wilt, growers should still monitor fields for the presence of corn flea beetle and treat when populations are high. To be on the safe side, growers should use preventative methods if planting in or near sweet corn fields that were infested with Stewart’s wilt last year.
Because of the recent prevalence of Stewart’s wilt bacterial disease, researchers at several universities have increased their efforts to understand, control and predict the disease. In a few years, we may know better how to protect sweet corn fields from corn flea beetles and Stewart’s wilt. We may also have a more accurate way of predicting Stewart’s wilt severity. Until then, a grower’s best strategy is to find out the risk of Stewart’s wilt in the area based on the traditional temperature index (which will be available soon), to monitor fields for corn flea beetle, and to take preventative measures, if appropriate.
