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Disease Cycle and Epidemiology

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Pathogen survival and dissemination

Erwinia stewartii survives primarily in two hosts, corn and the corn flea beetle, Chaetocnema pulicaria. The bacterium is transmitted almost exclusively by corn flea beetles. There are no known examples of widespread, prolonged occurrences of Stewart's wilt in the absence of this insect. Charlotte Elliot and F. W. Poos tested over 28,500 insect specimens representing 94 species and 76 genera for E. stewartii and concluded that C. pulicaria was the only species of importance in harboring the bacterium and spreading the disease. The bacterium occasionally has been isolated from Chaetocnema denticulata and other insects, but none is a vector of importance under field conditions.

Erwinia stewartii overwinters in the alimentary tract of corn flea beetles. These insects emerge from hibernation in the spring when soil surface temperatures reach about 18° to 21°C (65° to 70°F). Flea beetles transmit E. stewartii while feeding on corn. The bacteria are unable to spread from plant to plant without the vector. Although bacteria are present in regurgitated material and in feces of C. pulicaria, the most probable mode of infection appears to be from fecal contamination of feeding wounds. Two or more summer generations of flea beetles develop during the growing season. The first summer generation peaks about mid-June and subsequent generations occur about four or more weeks later. Two cycles of Stewart's wilt infection coincide with flea beetle population dynamics. The first cycle of Stewart's wilt is the most damaging because seedlings are infected by E. stewartii transmitted by the overwintering generation of flea beetles. The second cycle of infection occurs when the bacteria are transmitted by the first summer generation of the insect. The second cycle of infection usually causes the leaf blight phase of Stewart's wilt in field corn, but it can cause the seedling wilt phase in late-planted sweet corn. Subsequent generations of flea beetles acquire the bacterium from infected plants and become the overwintering population.

Although E. stewartii can be seed-borne, seed transmission plays an insignificant role in the epidemiology of Stewart's wilt in North America. Similarly, the chance of introducing E. stewartii into other regions of the world by seed transmission is virtually zero when seed is produced on resistant inbred seed parents. The susceptibility of the seed parent plant greatly affects whether seed harbors E. stewartii. If seed parent plants are infected systemically during the seedling wilt phase of the disease, seed may be infected with E. stewartii. If the leaf blight phase of Stewart's wilt occurs but seed parent plants are not infected systemically, seed infection is extremely unlikely. Rates of seed transmission were overestimated for nearly 60 years because initial research on seed transmission of E. stewartii in the first half of the 20th century did not consider reactions of seed parent plants and/or the occurrence of vectors. Based on recent research, rates of transmitting E. stewartii in seed appear to be about 1 in 50,000 for seed produced on systemically infected, susceptible seed parent plants and 1 in 20,000,000 for seed produced on resistant plants with symptoms of the leaf blight phase of Stewart's wilt.

Geographic Distribution

Although the disease has been reported infrequently from various parts of the world, the bacterium has never become established outside of the region of North America where the vector occurs. Over 60 countries place quarantine restrictions on corn seed to prevent the introduction of E. stewartii. In the United States, Stewart's wilt is endemic in the mid-Atlantic states, the Ohio River Valley, and the southern portion of the Corn Belt (Figure 15). The occurrence of Stewart's wilt in other eastern and midwestern states and in portions of Canada coincides with the occurrence of corn flea beetles.

Figure 15

Stewart's wilt forecast

Stewart's wilt was one of the first plant diseases for which a disease forecast system was developed. By comparing maps of winter temperatures and Stewart's wilt in the early 1930s, N.E. Stevens developed a system to predict Stewart's wilt based on a winter temperature index. Fifteen years later, G. H. Boewe modified Steven's forecast to more accurately predict the leaf blight phase of Stewart's wilt (Table 1). The Stevens-Boewe forecast can be simplified further by dividing the winter temperature index by 3 to approximate the average winter temperature. If the average daily temperature for December through February is above freezing, >1°C (33°F), flea beetles survive and Stewart's wilt is likely to be severe on susceptible hybrids. If the average daily temperature is less than -3°C (27°F), flea beetles are not likely to survive, and it is unlikely that Stewart's wilt will be severe. The Stevens-Boewe system is an imperfect predictor, but it can be used as a guideline to advise growers of the potential for Stewart's wilt on susceptible varieties (Figure 16). Severe outbreaks of Stewart's wilt in the Midwest and eastern United States in the early-1930s, early-1950s, and 1990s followed winters that were among the warmest of the 20th century.

Table 1. Stevens-Boewe forecast for Stewart's wilt. Developed by N. E. Stevens in the 1930s and revised in the 1940s by G. H. Boewe at the Illinois Natural History Survey, University of Illinois.
Winter Temp index	Seedling wilt phase	Leaf blight phase
100 or more	destructive	severe
90 to 100	light to severe	severe
85 to 90	nearly absent	moderate
80 to 85	nearly absent	light
below 80	nearly absent	trace
Winter temperature index = sum of average temperature (°F) for December, January, and February.

Figure 16