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Symptoms and signs

Two phases of Stewart's wilt occur on corn. The seedling wilt phase occurs when young plants are infected systemically (Figure 2). The leaf blight phase occurs when plants are infected after the seedling stage (Figure 3). Infection occurs in plant tissues that are wounded during feeding by an insect (Figure 4), the corn flea beetle (Chaetocnema pulicaria) (Figure 5). The corn flea beetle is the overwintering host and vector of Erwinia stewartii, the bacterium that causes Stewart's wilt.

Figure 2	Figure 3
Figure 4	Figure 5

Initial symptoms are similar for both phases of the disease. Leaf tissue surrounding feeding wounds initially become water-soaked (Figure 6). Pale-green to yellow linear streaks with irregular or wavy margins develop parallel to leaf veins (Figure 7). These lesions become necrotic with age and may extend the entire length of the leaf on susceptible cultivars. When plants are infected systemically, symptoms appear on new leaves emerging from the plant whorl (Figure 8), and cavities may form in the stalks near the soil line (Figure 9). Bacteria spread throughout the vascular system of systemically infected plants and occasionally infect kernels. A range of reactions to Stewart's wilt occurs among different cultivars of corn (Figure 10). In resistant cultivars, symptoms usually are limited to within 2 to 3 cm (1 to 2 in.) surrounding flea beetle feeding wounds (Figure 6), and systemic infection occurs rarely, if ever. If infection occurs when seedlings are emerging, main stalks can be killed which may result in profuse growth of tillers (Figure 11). Although it has not been demonstrated experimentally, main stalk death probably results from infection of the primary growing point when flea beetles transmit E. stewartii while feeding on emerging coleoptiles (the tissue sheathing the first true leaf) or very young seedlings.

Figure 6	Figure 7
Figure 8	Figure 9
Figure 10	Figure 11

Foliar symptoms of the leaf blight phase are similar to those of the seedling wilt phase. Chlorotic or necrotic tissues may extend the entire length of leaves, or symptoms may be limited to a few centimeters (inches) depending on the susceptibility of the cultivar (Figure 3). Premature leaf death due to Stewart's wilt may predispose the weakened plant to stalk rot resulting in reduced yields.

Multiple, coalesced lesions of northern corn leaf blight (NCLB), caused by the fungus Exserohilum turcicum, may resemble necrotic leaf symptoms of the seedling wilt or the leaf blight phases of Stewart's wilt (Figure 12). Plants suffering from drought stress, insect injury, nutritional deficiencies (such as iron deficiency) or genetic stripe (a non-infectious disorder) also may have foliar symptoms that are similar to the seedling wilt or leaf blight phase of Stewart's wilt. A simple microscopic examination of leaf tissue for bacterial ooze (Figure 13) can differentiate Stewart's wilt from non-bacterial disorders with similar symptoms. Differentiating Stewart's wilt from Goss's wilt, caused by Clavibacter michiganensis subsp. nebraskensis, may require additional tests.

Figure 12

Figure 13

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