Stewart's wilt, caused by the bacterium Erwinia stewartii, can result in serious losses for corn producers in Kentucky. Stewart's wilt is also known by other names such as bacterial leaf blight, Stewart's leaf blight, or maize bacteriosis.
Although sweet corn and popcorn are more susceptible to Stewart's wilt than field (dent) corn, some very susceptible inbreds and hybrids are on the market. Stewart's wilt can cause yield reductions directly through stand reductions and the production of fewer and smaller ears, and indirectly through an increased susceptibility of wilt-infected plants to stalk rotting organisms.
The Stewart's wilt bacteria overwinter within the body of the corn flea beetle, (Chaetocnema pulicaria), the primary vector of Stewart's wilt. Bacteria can also be transmitted by both adult and larva of the 12-spotted cucumber beetle, the toothed flea beetle, as well as larva of the seedcorn maggot, wheat wireworm and the May beetle. These additional insects, however, are not important in the disease organism's overwintering. Additionally, Stewart's wilt is spread in infected sweet corn seed and rarely in seed of infected field corn.
The severity of Stewart's wilt in any given year is closely aligned with the weather. Specifically, warm winters favor the beetle's survival and subsequent disease development. Research and experience have shown that if the sum of the mean monthly temperatures for December, January and February totals 100°F or more (typical of a Kentucky winter), Stewart's wilt is likely to be a problem the following season. Conversely, if the sum of the mean temperatures totals less than 90°F, little disease can be expected to develop.
Stewart's wilt is transmitted to corn as the beetles (and others) begin to feed on corn seedlings in late spring and early summer. Beetles continue to feed and transmit the disease throughout the growing season by feeding on infected plants and then moving to healthy ones. As the beetles feed, bacteria are placed into corn leaf tissue. Once inside the leaf, the bacteria multiply rapidly and fill the water-conducting vessels and intercellular spaces of the leaf. The typical foliar symptoms then develop, as described below. From the leaves, bacteria may move to the stalk or ear, causing wilt and blight. Systemic bacterial movement is more likely to occur in sweet corn and popcorn than in field corn.
Stewart's wilt is most noted early in the seedling to pre-tassel (whorl) stage and later after tasseling on the upper leaves. Early season disease frequently results in the bacterium infecting the growing point of the plant, thus causing discoloration followed by decay of the interior lower stalk and finally by wilting and plant death. When post-tasseling symptoms develop, the stage is often referred to as the leaf blight or late infection stage. Additionally, mineral nutrition influences corn hybrids' susceptibility to E. stewartii. High levels of ammonium nitrogen and phosphorus increase susceptibility, while high levels of calcium and potassium tend to decrease it. Moreover, disease severity is generally aggravated by high temperatures.
Highly susceptible sweet corn hybrids affected by Stewart's wilt may wilt rapidly and resemble plants suffering from drought, nutrient deficiencies or insect injury. Infected leaves show linear pale green to yellow-tan streaks with wavy margins which run parallel to leaf veins and may extend the length of the leaf. When examined closely, these streaks can be seen to originate at flea beetle feeding sites. Streaks usually turn a tan-brown color as the areas die and it is not uncommon for entire leaves to wither and dry up. In severely infected plants, brown cavities may form in the base of the plant's stalk. When such cavities occur, plants may wilt and die or bacteria may spread throughout the vascular system of the plant, sometimes passing into and infecting the kernels. Infected plants may produce bleached and dead tassels.
The symptoms expressed by highly susceptible field corn types are much like those described for sweet corn. The degree of symptom expression in most infected field corn hybrids is relatively mild, and many show only late-season leaf blight symptoms.
Once Stewart's wilt is established in a planting, nothing can be done to moderate the disease's effects. Growing resistant hybrids affords the best means of disease control. Generally, early-maturing hybrids are more susceptible to Stewart's wilt than are late hybrids. Early applications of insecticides to kill corn flea beetles may also be beneficial following mild winters. Control of early-generation flea beetles often lessens the seriousness of the infection and delays its development. However, controlling beetles throughout the season is not practical in most situations. Lastly, maintaining balanced soil fertility by avoiding excessively high levels of ammonium nitrogen and phosphorus, should help in controlling Stewart's wilt.