Mild winter weather has created conditions in Illinois that may be favorable to the development of Stewart's wilt in some areas. Growers need to be aware of disease symptoms that may appear on their corn. Stewart's bacterial wilt, caused by Erwinia stewartii, is spread by adult flea beetles that feed on corn. Although this bacterial disease is more serious on sweet corn, symptoms of the disease can be seen on field corn, especially during years when high flea beetle populations survive the winter. Two phases of this disease can occur. The first phase is a systemic phase infecting young corn plants. These bacteria can spread in the vascular system throughout the plant. Visible symptoms on young corn usually appear as wilting and linear green to yellow streaks following along the leaf's parallel veins. Unlike fungi, bacterial organisms that infect grasses typically create long, linear leaf lesions, because bacterial movement is restricted by the leaf's veins. Severely infected plants may develop cavities in the stalk pith near the soil line. Plants that are not killed may produce bleached, dead tassels. Typically, commercial field corn hybrids have general resistance to this phase of the disease, although in years when high flea beetle populations occur, symptoms may be visible and death of some individual young corn plants may occur simply due to an overload of the pathogen within the resistant plant. The second, more common phase of Stewart's wilt appears as leaf blight, frequently after tasseling. Similar linear green to yellow lesions develop along the veins, often originating from flea beetle feeding sites. The lesion areas on the corn leaf can die and may cause premature death of the entire leaf. The second phase may also predispose the corn plant to attack by fungal stalk rots, but the entire plant typically is not killed as in phase one. In most cases, no control measures are necessary on field corn. Simply be aware that the potential to see symptoms of this disease may be high this year. |
Under the microscope, Stewart's bacterial wilt can be distinguished from fungal pathogens by the presence of bacterial ooze. In the field, symptoms of Stewart's wilt may be confused with those caused by a fungal leaf blight called anthracnose, another early-season leaf blight that can produce long, linear lesions. Anthracnose leaf lesions will typically become tan in the center and have a red, brownish-red, or yellow-orange border. Older anthracnose lesions will develop dark, round, fungal fruiting bodies (acervuli) with the characteristic hair-like, upright structures called setae.
The potential for Stewart's wilt disease development depends on the population of adult flea beetle vectors. Several populations of beetles can develop in the growing season. The bacterium overwinters in the gut of the beetle. Survival of the flea beetle depends on winter temperatures during the months of December, January, and February. When the sum of the average temperatures for these three months is below 80°F, most of the adult beetles are killed. Therefore when the sum of the average mean temperatures for the three-month period is below 90°F, the potential for development of Stewart's bacterial wilt disease is low. When the sum of the mean temperatures for these three months falls between 90°F and 100°F, the potential for disease development is moderate. If the sum of these three months is above 100°F, then the potential for disease development is severe. Table 2 gives the average temperatures for the three winter months, the total temperature, and the disease potential for areas throughout Illinois. Temperature data was obtained from the Midwest Regional Climate Center.
For additional information on the flea beetle life cycle, scouting procedures for flea beetle damage, and control measures, refer to "Corn Flea Beetle: Expectations for Injury in 2000" by Mike Gray in the April 7, 2000, issue no. 2 of the Bulletin.--Loretta Ortiz-Ribbing