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Stewart’s Bacterial Wilt — Still a Problem After 107 Years Thomas A. Zitter Department of Plant Pathology Cornell University Ithaca, NY 14853 March, 2002 Stewart’s bacterial wilt of sweet corn was first reported in the United States on Long Island in 1897. This disease remains important in most sweet corn producing areas of the Northeast, Mid-Atlantic, and Mid-Western states. Stewart's wilt is caused by a bacterium Erwinia stewartii (syn. Pantoea stewartii) that can spread systemically throughout the plant.

Plant Damage and Time of Infection

There are two main phases to the disease, although symptoms may appear at any stage of plant growth. At the seedling wilt phase, plants are infected at or before the 5-leaf stage. The bacterium enters through feeding wounds on leaves created by the corn flea beetle (Chaetocnema pulicaria) and spreads to the developing stalk of the plant, where it kills the growing point (dead tassels). Gradually the seedling wilts and dies. Leaves may show linear pale green to yellow streaks. Infections of older plants usually result in the development of another phase, the leaf blight phase. This phase is characterized by long whitish to chlorotic streaks along the leaves. These streaks usually have irregular wavy margins, running parallel to the veins, and may extent the entire length of the leaf. In some cases the entire leaf my dry up and die suggesting drought damage or nutritional deficiencies. Vascular discoloration in the stem and at the plant nodes can occur in highly susceptible varieties. The loss of leaf tissue can result in smaller, poorly filled, and unmarketable ears.

The Stewart's wilt bacterium can infect plants at any age, but as with most diseases, the greatest damage to corn occurs when plants are infected as seedlings (prior to the 5-leaf-stage). If infected in the seedling stage, susceptible hybrids may be killed by the disease, resulting in a total yield loss. As plants mature, the amount of damage caused by Stewart's wilt is reduced. Hybrids with greater levels of resistance can tolerate earlier infections with less yield reduction (Table 1).

Table 1. Potential yield loss (%) from Stewart’s wilt based on the time of infection.

Insect Occurrence and Transmission

The bacterium that causes Stewart's wilt survives through the winter in the alimentary tract or "gut" of adult corn flea beetles, and is spread to sweet corn plants in the spring as the beetles feed. Bacteria do not overwinter in soil or plant debris. Beetles deposit the bacteria in the feeding sites, and the bacteria then colonize the leaf tissue. Eventually the bacteria enter the vascular system and then spread throughout the plant. The severity of Stewart's wilt depends on three factors: (1) the winter temperatures prior to planting, (2) the amount of Stewart's wilt the previous season, and (3) the susceptibility of the hybrid to the disease. If cold winter temperatures prevail, then fewer flea beetles survive to emerge in the spring, then fewer beetles are available for transmitting the disease. Only 10-30 % of the emerging beetles carries the bacteria. Generally, the prevalence of the disease in the previous season determines the range of percentage of emerging beetles carrying the Stewart's wilt bacterium.

The potential disease severity for the coming season can be estimated by calculating the average temperature for December, January, and February and comparing that average with the temperature ranges as shown in Table 2.

Control Methods

There are two methods for the control of Stewart’s wilt of corn and the approaches can be interrelated depending on the susceptibility of the variety grown. The first approach is to grow varieties that are resistant to the disease. Dr. Jerald Pataky at the University of Illinois has rated the susceptibility of popular hybrids to bacterial wilt for many years. The susceptibility of some hybrids as shown in Table 3, using the rating scale where 1 = most resistant and 9 = most susceptible

Table 3. Susceptibility of sweet corn hybrids to Stewart’s bacterial wilt.

Resistance to Stewart's wilt in sweet corn restricts the movement of the pathogen in the plant. Even highly resistant plants are not immune, but the bacteria never move far from the site of infection and therefore do not cause as much damage. Many sweet corn hybrids fall into the intermediate range of resistance to Stewart's wilt. Unfortunately, very few early-season hybrids are available that have levels of resistance that would be effective under moderate to severe disease conditions.

The use of insecticides to control the flea beetles, especially on susceptible hybrids during the seedling stage, is the second method for controlling Stewart’s wilt. The use of insecticides to control Stewart's wilt by killing flea beetles is never as effective as using resistant hybrids, but where susceptible hybrids must be planted, insecticides can reduce losses. One approach is to use an in-furrow treatment at planting, such as Furadan 4F. Uptake and insect control may be reduced under dry conditions. Several insecticides may be used as foliar sprays for corn flea beetle control; trade names include Sevin, Asana, Pounce / Ambush, and Warrior. Although some of these products persist a little longer than others, rapid growth of leaf tissue means untreated surfaces are available to flea beetles that migrate into fields a few days after treatment. A key for suppressing Stewart's wilt is to scout frequently for flea beetles (2 or 3 times per week) and re-apply insecticides if populations are rebuilding.

Research conducted in Illinois by Dr Pataky demonstrated that Gaucho (imidacloprid) seed treatments provided systemic control of corn flea beetles and reduced incidence of Stewart's wilt in treated plots. The degree of control provided by these treatments roughly equaled using a hybrid with one higher level of resistance. In the past the US EPA approved a Section 18 emergency exemption to allow seed suppliers in Idaho to use Gaucho on sweet corn seed destined for sale in other states. Check the status of treated seed for use in New York prior to purchase and use.

Acknowledgement: Data provided in this report was adapted from reports from Dr. Pataky and from vegetable newsletters from Illinois and Ohio, and from current hybrid and insecticide recommendations from several states.