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Sweet Corn Disease NurseryStewart's Bacterial Wilt, Erwinia stewartii |
Stewart's bacterial wilt, caused by Erwinia stewartii, can be a serious problem when susceptible sweet corn hybrids are grown in the eastern and midwestern United States. Yield reductions due to Stewart's wilt are associated with levels of disease development which are affected by the growth stage at which plants are infected and the resistance or susceptibility of the hybrid (4). Stewart's wilt susceptible sweet corn hybrids are sometimes grown where the disease occurs because they have to superior agronomic and horticultural qualities. In the past decade, several new hybrids with resistant or moderately resistant reactions to Stewart's wilt have been introduced. It is doubtful that Stewart's wilt will affect yield of resistant hybrids, but the effect of the disease on hybrids with moderate reactions needs to be evaluated further.
This report summarizes a field trial in 2000, in which we determined the relationship between Stewart's wilt severity and sweet corn yield in order to evaluate the level of Stewart's wilt resistance necessary to prevent yield reductions.
Sixty sweet corn hybrids with Stewart's wilt reactions ranging from susceptible to resistant were planted on 5 May 2000 at the University of Illinois South Farms. These hybrids included several grown by food processors in the Midwest and several check hybrids with known reactions to Stewart's wilt (2, Table 1). The experimental design was a split-plot of an RCB with four replicates. Each experimental unit was a 2-row plot that was 4.9 m long with 24 plants per row. Hybrids were planted in main plots and plants in sub-plots were inoculated or non-inoculated. All seed were treated with Adage® (thiamethoxam, Novartis) at a rate of 250 gai per 100 kg seed to reduce natural Stewart's wilt infection (3).
Seedlings at the 3- to 6-leaf stages were inoculated on 30 May, and 6 and 8 June with E. stewartii using the pinprick method (1). Disease reactions were rated on a 1 to 9 scale (Figure 1) on 22 June and 20 July, about one week prior to harvest. Naturally infected plants with systemic Stewart's wilt were counted on 23 June. Ears from 30 plants per experimental unit were harvested and weighed 21 days after mid-silk.
The effects of treatments were assessed by analysis of variance (ANOVA). Yield losses were assessed by regression. Percent yield (based on ear weight) was calculated by dividing ear weight from inoculated sub-plots by those values from non-inoculated sub-plots and multiplying by 100. Percent yield was regressed on means of Stewart's wilt ratings (1 to 9) and incidence (%) of systemic infection for each hybrid.
Stewart's wilt ratings in inoculated plots ranged from 2.0 to 7.3 (Table 1). Ratings in this trial and hybrid reactions to Stewart's wilt based on previous disease nurseries (2) were correlated, r = 0.83 (Figure 2). Incidence of systemic infection ranged from 0 to 8% in non-inoculated sub-plots and from 0 to 86% in inoculated sub-plots (Table 1). The relationship between incidence of systemic Stewart's wilt infection and severity ratings in inoculated sub-plots was best described by a quadratic regression (Figure 3). Incidence of systemic infection was less than 10% when Stewart's wilt ratings were below 3.5. When Stewart's wilt ratings were above 4 incidence of systemic infection increased dramatically (Figure 3).
Ear weight ranged from 1.5 to 8.9 tons/acre in inoculated sub-plots and from 3.2 to 9.1 tons/acre in non-inoculated sub-plots. Percent yield based on ear weight ranged from 26 to 114% among hybrids. The relationship between percent yield and Stewart's wilt rating was described best by a quadratic regression (Figure 4). When Stewart's wilt ratings were below 3.5 (non-systemic infection) yield reductions were minimal. When Stewart's wilt ratings were above 4, yield decreased dramatically (Figure 4). The relationship between percent yield and incidence of systemic Stewart's wilt in inoculated sub-plots was described best by a linear regression (Figure 5). Yield decreased 0.8% for each 1% increase in incidence.
The difference between the two yield loss relationships (Figure 4 and Figure 5) and the incidence-severity relationship (Figure 3) illustrate the importance of systemic Stewart's wilt infection. When Stewart's wilt ratings are below 3.5, few plants are infected systemically and effects on yield (ear weight) are minimal. As Stewart's wilt ratings increase above 4, incidence of systemic infection and yield reductions increase substantially.
In this trial, yield reductions averaged 60% for susceptible hybrids with Stewart's wilt reactions of 8 and 9 (Table 2). Stewart's wilt ratings were between 4.7 and 7.3 for these hybrids. Yield reductions averaged 26% and ratings ranged from 4.3 to 5.4 for moderately susceptible hybrids with reactions of 7. Moderate hybrids with reactions scored from 4 to 6 had severity ratings in this trial ranging from 3.2 to 6 and yield reductions averaged 15%. For moderately resistant and resistant hybrids, yield reductions averaged 9% and 1%, respectively, and ratings ranged from 2 to 5.4 and 2 to 4, respectively.
Yield reductions (based on ear weight) due to Stewart's wilt occurred when infection was systemic. Yield of hybrids with resistant or R/MR reactions were not affected because Stewart's wilt infection was not systemic. Yield reductions averaged less than 10% for hybrids with moderate resistance and about 15% for hybrids with moderate reactions. Substantial reductions in yield can be avoided if sweet corn hybrids have levels of Stewart's wilt resistance which prevent systemic infection.
