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Sweet Corn Disease Nursery
Stewart's Bacterial Wilt, Erwinia stewartii
Stewart's bacterial wilt, caused by Erwinia stewartii, is an economically important disease of sweet corn. Corn flea beetles, Chaetocnema pulicaria, vector E. stewartii (Elliot and Poos, 1940). Yield reductions due to Stewart's wilt are caused primarily by systemic infection. Levels of host resistance and plant growth stage determine whether infection is systemic (Suparyono and Pataky, 1989). Susceptible hybrids can become systemically infected as late as the 7- to 9-leaf stages but systemic infection of resistant hybrids is infrequent after the 3-leaf stage.
Stewart's wilt can be controlled with resistant hybrids or by controlling corn flea beetles. Seed treatment insecticides controlled flea beetles and decreased the incidence of natural, systemic Stewart's wilt infection by 60 to 80% in recent field trials (Pataky et al, 2000). The economic practicality of seed treatment insecticides depends on the level of host resistance and the amount of Stewart's wilt pressure, which is a result of the size of corn flea beetle populations.
Stewart's wilt reactions of more than 2,000 commercial sweet corn hybrids have been estimated since 1984 in disease nurseries at the University of Illinois (Pataky, 2000). Relationships between hybrid reactions to Stewart's wilt and incidence of natural, systemic infection could be useful in determining when to apply insecticide seed treatments.
This report summarizes relationships under various disease pressures between incidence of natural, systemic Stewart's wilt infection and reactions of sweet corn hybrids to this disease.
Incidence of systemic Stewart's wilt was assessed in seven trials planted 30 June 1998; 22 June and 6 July 1999; and 6, 18, and 29 June, and 6 July 2000. The experimental design for each trial was an RCB with 3 replicates. Plants were infected naturally by E. stewartii vectored by corn flea beetles. The number of plants systemically infected at the 3- to 7-leaf stage was counted once in six trials. Infected plants were counted at two growth stages in the trial planted 18 June 2000. Incidence of systemic Stewart's wilt infection was calculated by hybrid as: (systemically infected plants / total plants) * 100. For each trial, mean incidence was calculated for all hybrids in each category of Stewart's wilt reaction. Hybrid reactions were categorized from 1 (resistant) to 9 (susceptible) based on severity of infection following inoculation in disease nurseries (Pataky, 2000). Incidence was regressed on hybrids reactions for each trial.
Average incidence of natural, systemic Stewart's wilt infection ranged from about 5% (low Stewart's pressure) in the trial planted 6 June 2000 to about 50% (high Stewart's pressure) in the trial planted 22 June 1999 (Table 1, Figure 2, and Figure 3). Four trials with average incidence between 10% and 16% had moderate Stewart's wilt pressure (Table 1, Figure 4). Two trials with average incidence of 23% and 25% had moderately severe Stewart's wilt pressure (Table 1, Figure 5).
Relationships between incidence of Stewart's wilt and hybrid reactions to Stewart's wilt (i.e., resistant -1 to susceptible- 9) differed under various levels of Stewart's wilt pressure (Figure1). Curvilinear relation-ships occurred when Stewart's wilt pressure was low, moderate or moderately severe because incidence did not increase substantially until hybrid reactions were moderately susceptible, M/MR, or moderately resistant, respectively (Figure 2, Figure 4, and Figure 5).
When Stewart's wilt pressure was low (Figure 2), incidence was below 5% for hybrids with resistant to moderate (1 to 5) reactions. Incidence increased from about 5% to 10% when hybrid reactions ranged from moderate (5) to moderately susceptible (8). Incidence was about 25% for susceptible hybrids (9).
When Stewart's wilt pressure was moderate (Figure 4), incidence was about 5% or less for hybrids that were resistant (1) or R/MR (2). Incidence ranged from about 5 to 15% for hybrids with moderately resistant (3) to moderate (5) reactions. Incidence increased from about 10 to 50% when hybrid reactions ranged from M/MS to susceptible (6 to 9).
When Stewart's wilt pressure was moderately severe (Figure 5), incidence was about 10% or less for hybrids that were resistant (1) or R/MR (2). Incidence ranged from about 12% to 25% for hybrids that were moderately resistant (3) to moderate (5). Incidence ranged from about 25% to 70% for hybrids that were M/MS to susceptible (6 to 9).
When Stewart's wilt pressure was severe (Figure 3), there was a linear relationship between incidence and hybrid reaction. Incidence was about 15% for resistant hybrids (1). Incidence increased about 8% for each category of hybrid reaction to Stewart's wilt from 1 (resistant) to 9 (susceptible).
These relationships may be useful in determining conditions under which seed treatment insecticides will be economical on hybrids with various reactions to Stewart's wilt. Ear weight is reduced about 0.8% for each 1% incidence of plants systemically infected with Stewart's wilt (Freeman and Pataky, 2001). Seed treatment insecticides decrease the incidence of systemic Stewart's wilt infection by about 60% to 80% (Pataky et. al., 2000). If incidence were about 5% when plants were grown from non-treated seed, incidence would be about 2% if seed were treated with insecticides. Therefore, seed treatment insecticides would prevent a yield decrease of about 2% or 3% when Stewart's wilt incidence would have been 5% without the use of seed treatments (i.e., 5% incidence x 0.6 to 0.8 control x 0.8 yield reduction). If Stewart's wilt incidence were about 15%, seed treatment insecticides would prevent an estimated 7% to 10% yield decrease (i.e., 15% x 0.6 to 0.8 control x 0.8 yield reduction).
Relationships between incidence of systemic infection and hybrid reactions to Stewart's wilt differed under various levels of disease pressure. These relationships may affect the economical use of seed treatment insecticides to control of Stewart's wilt. The use of seed treatment insecticides on susceptible hybrids appears to be economical whenever Stewart's wilt occurs. The economic benefit of seed treatment insecticides varies for resistant to moderately susceptible hybrids under moderate and moderately severe Stewart's wilt pressure because relationships between incidence and hybrid reaction are curvilinear. Under high levels of Stewart's wilt pressure, seed treatment insecticides appear to be economical for all hybrids, including those with high levels of resistance to Stewart's wilt.
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