Stewart's bacterial wilt, caused by Erwinia stewartii, is spread by adult flea beetles that feed on corn. While 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. The potential for Stewart's wilt disease development depends on the population of adult flea beetle vectors that survive the winter. This year several factors could influence flea beetle populations (refer to the flea beetle article by Mike Gray in this issue). |
Typically, survival of the adult flea beetle is based on the average winter temperatures during the months of December, January, and February. Please refer to the map (Figure 1) with average winter temperatures in the article by Mike Gray in this issue, when figuring the sum of average temperatures and the potential for Stewart's wilt disease development in your area. To reiterate, when the sum of the average temperatures for these 3 months is below 80°F, most of the adult beetles are killed. Therefore, the potential for development of Stewart's bacterial wilt disease when the sum of the average mean temperatures for the 3-month period is below 90°F is low. When the sum of the mean temperatures for these 3 months falls between 90 and 100°F, the potential for disease development is moderate. When the sum of these 3 months is above 100°F, then the potential for disease development is severe. However, the effect of snow as an insulator is unknown. At this point, the overwintering population of flea beetles remains a mystery.
Two phases of Stewart's wilt 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 lesions because the leaf veins restrict bacterial movement. 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 once again be high this year.
Stewart's wilt--foliar symptoms on corn.
Stewart's wilt--lower stalk decay.
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 hairlike, upright structures called setae.--Loretta Ortiz-Ribbing