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Disease Management

Management of Stewart's wilt of corn

Various disease control tactics can be integrated to manage Stewart's wilt, including: host resistance, protection/eradication, and exclusion.

Host resistance

Throughout North America, Stewart's wilt is controlled effectively by planting resistant corn hybrids. Resistance restricts the movement of Erwinia stewartii in the vascular system of plants and prevents plants from becoming infected systemically. Thus, levels of resistance affect the frequency of systemically infected plants (Figure 17). Most field corn hybrids are highly to moderately resistant to Stewart's wilt, whereas reactions of sweet corn hybrids range from resistant to highly susceptible. Stewart's wilt resistance is inherited relatively simply and can be selected easily. Plant breeders and pathologists typically inoculate plants with E. stewartii or rely on natural infection to select plants with mild symptoms (Figure 18). If a hybrid has a level of resistance that prevents systemic infection, economic damage (e.g., yield losses) is rare (Figure 19).

Figure 17	Figure 18
Figure 19

Protection/Eradication

In some situations, growers prefer to plant hybrids with moderate or susceptible reactions to Stewart's wilt because these corn hybrids are superior for other important characteristics. For example, many fresh market sweet corn growers plant early-maturing hybrids that are moderately susceptible or susceptible so that crops can be harvested early when prices are high. In this situation, Stewart's wilt can be managed by controlling corn flea beetles. Seed treatment insecticides (e.g., clothianidin, imidicloprid, thiamethoxam) protect plants from infection by killing the insect vector before the bacterium is transmitted (Figure 20). Insecticides applied in-furrow at planting or insecticides applied as foliar treatments after seedlings emerge also control flea beetles, although the ability of in-furrow or foliar insecticides to manage Stewart's wilt has been somewhat more erratic than seed treatment insecticides.

Figure 20

Exclusion

Infected seed is unimportant in the epidemiology of Stewart's wilt in areas where the disease is endemic; however, the bacterium can be excluded from areas where it does not already occur by ensuring that seed is pathogen-free. The probability of transmitting E. stewartii in seed is extremely low, but over 60 countries place some type of quarantine restriction on corn seed to prevent the introduction of E. stewartii.

Producing seed in areas where Stewart's wilt does not occur ensures that E. stewartii will not be introduced on seed. For example, most sweet corn seed is produced in Idaho where Stewart's wilt normally does not occur. In the Midwest where most field corn seed is produced, visual inspections of seed production fields for symptoms of systemic Stewart's wilt infection can provide a qualitative assessment of whether seed may harbor E. stewartii. The probability of introducing E. stewartii on seed is nearly zero except when seed parent plants are infected systemically. When plants are systemically infected, an approved seed health test can be used to detect E. stewartii-infected seed. A seed health test based on an enzyme-linked immunosorbent assay (ELISA) and an appropriately large sample of seed has been approved by the United States Department of Agriculture (USDA) National Seed Health System to ensure at a known probability that E. stewartii-seed infection is below an accepted threshold (Figure 21).

Figure 21