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Sweet Corn Disease Nursery
Stewart's Bacterial Wilt, Erwinia stewartii
Corn flea beetles, Chaetocnema pulicaria, vector Erwinia stewartii, the bacterium that causes Stewart's wilt of corn. The seedling wilt phase of Stewart's wilt substantially reduces sweet corn yield when infection is systemic (4). Yield is affected rarely when resistant or moderately resistant sweet corn hybrids are infected after the 3- to 5-leaf or 5- to 7-leaf stages, respectively. Yield of susceptible or moderately susceptible sweet corn hybrids can be affected if plants are infected before the 7- to 9-leaf or early tassel stages, respectively.
When flea beetles feed on plants prior to the 2-leaf or 3-leaf stage, the growing point of corn plants may be only a few cm from feeding wounds. When plants are infected by E. stewartii prior to the 3-leaf stage, main stalks may be killed because the growing point becomes infected (2). Tillers may grow profusely, but ears are not produced when main stalks are killed. Main stalk death occurs even in the most resistant sweet corn hybrids although the incidence of main stalk death decreases with increasing levels of resistance. In order to avoid yield reductions due to Stewart's wilt, initial infection must be prevented until resistant or moderate reactions are capable of inhibiting systemic infection and main stalk death.
Seed treatment insecticides reduce the incidence of Stewart's wilt, apparently by killing flea beetles before plants are infected by E. stewartii (1,3). In 11 field trials in 1999, seed treatment insecticides reduced the incidence of systemic Stewart's wilt infection and main stalk death by 50 to 80% of that observed in non-treated plots (3). The level of control and length of time that plants are protected from Stewart's wilt by seed treatments may depend on rates of insecticides applied to seed.
This report summarizes field trials in 2000 in which we examined the efficacy of different rates of seed treatment insecticides to control Stewart's wilt.
Four field trials at two locations (Urbana and Manito, IL) were planted early and late (26 and 27 April and 19 and 22 June) with a Stewart's wilt susceptible hybrid, Jubilee. Each trial included ten treatments. Seed were either not treated with an insecticide (i.e., non-treated check) or treated with Gaucho® 600FS (imidacloprid, Gustafson) at rates of 3.2, 4.8, and 6.4 oz. of product per CWT seed; Adage® (thiamethoxam, Novartis) at rates of 50, 100, 200, and 250 gal per 100 kg seed; or an experimental insecticide, which we refer to as G2, at rates 2.4 and 4.8 oz. of product per CWT seed. The experimental design was a randomized complete block with four replicates. Each experimental unit consisted of eight rows spaced 30 or 36 inches apart. Rows were about 27 ft long with about 1.5 plants per foot of row.
Plants were infected naturally by flea beetles. None were inoculated. The number of Stewart's wilt infected plants was counted twice in each trial at an early (e.g., 3-leaf) and late (e.g., 9-leaf) growth stage. Infected plants were counted three times in the trial planted 19 June in Urbana. Incidence (%) of Stewart's wilt was calculated as: (number of plants systemically infected or with main stalk death / total number of plants) x 100. Level of control (%) was calculated as: 100 - (the incidence of Stewart's wilt in insecticide treated plots as a percentage of Stewart's wilt incidence in non-treated plots). Incidence of Stewart's wilt and level of control were analyzed by ANOVA. Treatments were compared by BLSD values (k=100). High and low rates of seed treatment insecticides were compared by single-degree-of-freedom contrasts.
Incidence of Stewart's wilt in non-treated plots of the susceptible hybrid, Jubilee, ranged from 14% at the 3- to 4-leaf stage in the early-planted trial at Manito to 90% at the 9- to 10-leaf stage in the late-planted trial at Urbana (Figure 1, Figure 2, Figure 3, and Figure 4). Incidence of Stewart's wilt was significantly lower for all seed treated with insecticides than for non-treated seed. Levels of control ranged from 56% for the low rate of Adage (i.e., Adage 50) in the late-planted trial in Urbana to 94% for the high rate of the experimental compound (G2 - 4.8oz.) in the early-planted trial in Manito (Table 1).
Small differences occurred among rates of insecticides in some trials. The single-degree-of-freedom comparison of high and low rates of insecticides was significant for Stewart's wilt ratings in the two early-planted trials and for the first rating (10 July) in the late-planted trial in Urbana.
In the early trial at Urbana, control of Stewart's wilt at the later rating, 15 June, was slightly better for the high rate of Gaucho and the experimental compound (79% and 87% control, respectively) than for the two lower rates of Adage and the low rate of Gaucho (64%, 71%, and 70% control, respectively) [Table 1]. At the earlier rating, 1 June, the level of control was slightly higher (81%) for the high rate of the experimental compound than for some rates of Adage (70% or 71%). Differences among other seed treatments were not significant.
Results were similar in the early-planted trial in Manito. At the 2 June rating, both rates of the experimental compound and the high rate of Gaucho and Adage provided slightly more control (88% to 91%) than the low rate of Adage (76%). At the later rating, 22 June, control was slightly better for both rates of the experimental compound and the Adage 200 treatment (91% to 94%) than for the low rates of Adage or Gaucho (72% and 75%). In the late-planted trial in Urbana, control was higher at the first rating, 10 July, for the high rates of Gaucho and the experimental compound (76% and 79%) than for the low rate of Adage (64%).
The practical importance of these small differences in levels of control depends on the prevalence of Stewart's wilt. When Stewart's wilt incidence is high, these differences may be important. For example, in the late-planted trial in Urbana where incidence of Stewart's wilt on non-treated plants was about 50% at the 2- to 3-leaf stage, Stewart's wilt incidence was about 12% for the seed treatments with the best level of control and about 19% for the seed treatment with the worst level of control (Figure 2). A significant difference of 7% incidence of Stewart's wilt at this early growth stage probably results in economical differences in yield. Conversely, when Stewart's wilt is less prevalent and incidence is lower, differences in levels of control among treatments are of little consequence. For example, in the early-planted trial in Manito, levels of control were significantly higher for the high rates of all three compounds as compared to the low rate of Adage, but differences in the actual incidence of Stewart's wilt was only 2% among these treatments. Additional control from higher rates of seed treatment insecticides probably was economic-ally insignificant in this case.
All rates of these seed treatment insecticides reduced the incidence of systemic Stewart's wilt and main stalk death when compared to plants grown from non-treated seed. In some instances, levels of control were slightly higher when higher rates of insecticides were applied.
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