Rates of Seed Treatment Insecticides and Control of Stewart's Wilt in Sweet Corn.

Neonicotinoid insecticides applied as seed treatments reduce the incidence of Stewart's wilt. The objectives of this study were to examine the efficacy of different rates of seed treatment insecticides to control Stewart's wilt on susceptible sweet corn hybrids and to compare the economic value of Stewart's wilt control in sweet corn grown for processing and fresh market. Clothianidin (Poncho), imidacloprid (Gaucho), and thiamethoxam (Cruiser) applied to seed at rates ranging from 0.125 to 1.25 mg a.i. per kernel were evaluated in 11 field trials in Illinois and Delaware from 2000 to 2003. Incidence of Stewart's wilt was significantly lower when seed was treated with insecticides than when plants were grown from nontreated seed in all but one trial. The level of control usually was between 50 and 90%. Small but statistically significant differences in incidence of systemically infected plants occurred among rates of insecticides in all trials except those in 2001. Usually, incidence of systemic Stewart's wilt was lower when higher rates of insecticides were applied; however, increasing the rate of insecticides from 0.125 mg a.i. to 1.25 mg a.i. per kernel had a relatively small effect on the level of Stewart's wilt control compared with the difference between treated and nontreated sweet corn seed. Based on a regression analysis, the lowest rates of the insecticides provided 64 to 72% control. The level of control increased about 1.85% with each additional 0.1 mg a.i. of insecticide per kernel from 0.125 mg a.i. to 1.25 mg a.i. Clothianidin provided an 8 or 9% higher level of control than thiamethoxam or imidacloprid at the same rate. Recommendations for application of seed treatment insecticides to processing and fresh market sweet corn differed somewhat due to substantial differences in the value of the crops. Based on estimated costs of $6 to $12 per 0.4 ha for the seed treatments, the economic break even point (i.e., cost of control = value from control) occurred in the range of 3 to 6% Stewart's wilt incidence for processing sweet corn valued at $325 per 0.4 ha and at about 1% Stewart's wilt incidence for fresh market sweet corn valued at $1,625 per 0.4 ha. Relatively small differences in levels of control conferred by commercially available rates of clothianidin (0.25 mg a.i. per kernel) and thiamethoxam (0.125 mg a.i. per kernel) were of little consequence in processing sweet corn but had considerable economic value in fresh market sweet corn.

Ability of an ELISA-Based Seed Health Test to Detect Erwinia stewartii in Maize Seed Treated with Fungicides and Insecticides.

Two sets of experiments were done to examine whether seed-treatment chemicals affected the ability of an enzyme-linked immunosorbent assay (ELISA)-based seed health test to detect Erwinia stewartii. The chemicals evaluated included Actellic, Apron, Captan, Cruiser, Gaucho, Maxim, Poncho, Thiram, and Vitavax in 11 seed-treatment combinations. In one experiment, seed-treatment chemicals were evaluated quantitatively in a critical region of ELISA absorbance values near 0.5 using maize seed that were spiked with uniform quantities of a liquid suspension of E. stewartii. The number of bacteria in each sample was estimated from ELISA absorbance values using standard curves. Log CFU of E. stewartii per sample were not significantly different among the untreated control and the 11 seed treatments compared with Tukey's Studentized Range Test (P = 0.05). Means of log CFU/ml for all treatments were tightly clustered around 5.70 which corresponded to an absorbance value of 0.440 and a bacterial population of about 500,000 CFU/ml. In a second set of experiments, seed treatment chemicals were evaluated based on qualitative decisions that resulted from the ELISA-based seed health test of seed lots of Jubilee and A632 infected with E. stewartii. The number of negative samples was not substantially greater than expected based on binomial probabilities except for samples of Captan/Vitavax-treated A632, which we considered to be a type I error. The mean absorbance values of positive samples ranged from 1.42 to 1.72 for A632 and from 1.51 to 1.91 for Jubilee and did not differ significantly among the seed treatments. There was no consistent evidence from these experiments that fungicide or insecticide seed treatments interfered with the sensitivity of the ELISA or altered low (e.g., 0.5) or high (e.g. 1.4 to 1.9) absorbance values. The ability of the ELISA-based seed health test to detect E. stewartii in maize seed was not affected by these seed treatments.

Relationships Between Reactions of Sweet Corn Hybrids to Stewart's Wilt and Incidence of Systemic Infection by Erwinia stewartii.

Relationships between the reactions of sweet corn hybrids to Stewart's wilt and the incidence of natural, systemic infection by Erwinia stewartii differed among trials in which the prevalence of Stewart's wilt differed. Systemic Stewart's wilt infection was assessed for 262, 296, and 245 hybrids planted in seven trials in central Illinois in June and July 1998, 1999, and 2000, respectively. Incidence of systemic infection was calculated in each trial for all hybrids in each of nine categories of Stewart's wilt reactions (i.e., 1 = resistant and 9 = susceptible). When mean incidence was about 5%, incidence ranged from about 1 to 8% on resistant to moderately susceptible hybrids, but incidence was nearly 30% on susceptible hybrids. When mean incidence ranged from 10 to 16%, the relationships between hybrid reactions and incidence were explained by exponential or polynomial regressions. Incidence was less than 10% for hybrids with resistant and moderately resistant reactions, and incidence was greater than 15% for moderately susceptible to susceptible hybrids. When mean incidence was near 50%, the relationship was linear. Incidence was about 18% for resistant hybrids and about 80% for susceptible hybrids. Incidence increased about 8% for each class of hybrid reaction from 1 to 9. The influence of resistance on the development of systemic infection at very early seedling growth stages also was evaluated in six greenhouse trials. A highly resistant hybrid, Bonus, was systemically infected in two of six greenhouse trials when seedlings were inoculated prior to the V3 growth stage; however, systemic infection was not as severe as on a susceptible hybrid, Jubilee. Systemic infection was more severe on Bonus when plants were inoculated at earlier growth stages between VE and V3. The resistant hybrid Bonus was not systemically infected when inoculated after the V4 growth stage except for one greenhouse trial when all Stewart's wilt ratings were higher than usual. Hybrid reactions to Stewart's wilt affected the incidence of systemic infection in field situations and they affected the growth stage at which resistance effectively prevented systemic movement of E. stewartii within plants in greenhouse trials. This information can be used to determine more effectively when to apply other control measures, such as insecticidal seed treatments.

Rates of Transmitting Erwinia stewartii from Seed to Seedlings of a Sweet Corn Hybrid Susceptible to Stewart's Wilt.

Rates of transmitting Erwinia stewartii from seed to seedlings were estimated from field grow-outs of seedlings grown from seed infected with E. stewartii. Infected seed were produced in 1998, 1999, and 2000 on a Stewart's wilt-susceptible sweet corn hybrid, Jubilee. Seedlings were inoculated repeatedly with pinprick inoculators and suspensions of E. stewartii were injected into ear shanks of the primary ears of each adult plant. Seed from inoculated plants were harvested and bulked. Single kernels were assayed for E. stewartii to estimate the proportion of kernels infected with E. stewartii. Estimates of E. stewartii-infection were 15.6 ± 4.3, 49.4 ± 3.9, and 12.5 ± 2.4% for seed produced in 1998, 1999, and 2000, respectively. Approximately 61,800 seedlings were grown in DeKalb, IL in 1999 and 83,400 and 60,000 seedlings were grown in Plover WI in 2000 and 2001, respectively, from infected seed lots produced the previous year. Approximately 10,000, 12,200, and 29,400 seedlings of susceptible sweet corn hybrids also were grown each year from commercial seed produced in Idaho where Stewart's wilt does not occur. Based on estimates of kernel infection in each seed lot and plant populations in each grow-out trial, about 9,600, 41,200, and 7,500 seedlings were grown from infected kernels in 1999, 2000, and 2001, respectively. Seedlings at the two- to three-leaf stage were examined for symptoms of Stewart's wilt. Infected plants were confirmed by microscopic observations of bacterial ooze and by enzyme-linked immunosorbent assay. When data were combined from all three trials, 59 of approximately 58,300 seedlings grown from infected seed were infected with E. stewartii based on symptoms of Stewart's wilt and E. stewartii-positive leaf tissue samples. Of these 59 seedlings, 22 probably were infected from seed-to-seedling transmission of E. stewartii and 37 probably were the result of natural infection due to the presence of flea beetles in DeKalb in 1999. Twenty-two infected seedlings from 58,300 infected kernels corresponds to a seed-to-seedling transmission rate of 0.038%. This rate of seed-to-seedling transmission of E. stewartii is substantially lower than seed transmission rates reported in the first half of the twentieth century; however, it is similar to seed-to-seedling transmission rates reported from other recent research.

Transmission of Erwinia stewartii from Plants to Kernels and Reactions of Corn Hybrids to Stewart's Wilt.

Stewart's wilt reactions of 98 food-grade, white corn hybrids, 3 yellow dent corn hybrids, and 23 sweet corn hybrids and infection of kernels by E. stewartii were evaluated in 1998, 1999, and 2000. Stewart's wilt symptoms were rated from 1 (no appreciable spread of symptoms) to 9 (dead plants) following inoculation. The mean Stewart's wilt ratings for the food-grade, white corn and yellow dent corn hybrids were 1.9, 2.4, and 2.9 in 1998, 1999, and 2000, respectively. The mean Stewart's wilt ratings for the sweet corn hybrids were 3.8, 4.2, and 4.6 in 1998, 1999, and 2000, respectively. Hybrids with ratings less than 3 were classified as resistant. Hybrids with ratings between 3 and 4.5 were classified as moderate. Hybrids with ratings greater than 4.5 were classified as susceptible. Ears harvested from each row in 1998 and 1999 were assayed for E. stewartii using an enzyme-linked immunosorbent assay (ELISA)-based seed health test. Kernels from 16 hybrids were positive for E. stewartii in 1998. Kernels from 11 hybrids were positive for E. stewartii in 1999. Kernel infection by E. stewartii was affected considerably by the level of host resistance (i.e., reactions of seed parent plants). For hybrids classified as resistant, estimates of kernel infection were 0.024 and 0.0007% in 1998 and 1999, respectively. For hybrids with moderate reactions to Stewart's wilt, estimates of kernel infection were 0.19 and 0.07% in 1998 and 1999, respectively. For hybrids with susceptible reactions to Stewart's wilt, estimates of kernel infection were 11.6 and 7.8% in 1998 and 1999, respectively. Based on high levels of Stewart's wilt resistance in food-grade, white corn hybrids, and low rates of kernel infection by E. stewartii in resistant and moderate hybrids, there is an exceedingly low probability of introducing E. stewartii to areas where it does not occur by transmitting the bacterium in grain of the food-grade, white corn hybrids evaluated in this study. Although all of the kernels harvested in these experiments were produced as grain on open-pollinated F1 hybrids, the rates of kernel infection observed for hybrids with resistant, moderate and susceptible reactions to Stewart's wilt are applicable to seed produced on inbred lines with equivalent Stewart's wilt reactions.

Control of Stewart's Wilt in Sweet Corn with Seed Treatment Insecticides.

Corn flea beetles, Chaetocnema pulicaria, vector Erwinia stewartii (synamorph Pantoea stewartii), which causes Stewart's bacterial wilt of corn (Zea mays). A seed treatment insecticide, imidacloprid, killed flea beetles and reduced the number of feeding wounds and Stewart's wilt symptoms per leaf in greenhouse studies. The objective of our research was to evaluate the ability of imidacloprid and thiamethoxam seed treatments to control Stewart's wilt on sweet corn hybrids under field conditions with naturally occurring populations of the corn flea beetle. Six field trials were planted at four locations in 1998. Eleven field trials were planted at nine locations in 1999. The treatment design was a factorial of sweet corn hybrids and seed treatments. Stewart's wilt incidence ranged from 0 to 54% in the 1998 trials. Incidence of Stewart's wilt in nontreated plots of the susceptible hybrid Jubilee ranged from 2% at the 8-leaf stage to 77% 1 week after mid-silk in the 1999 trials. Seed treatment insecticides reduced the incidence of Stewart's wilt by ≈50 to 85% relative to nontreated controls. The level of control was ≈75 to 85% in seven trials planted before 1 June 1999, when incidence of Stewart's wilt on nontreated Jubilee ranged from 4 to 71%. The level of control was ≈50 to 70% in the three trials planted after 1 July 1999, when incidence of Stewart's wilt on nontreated Jubilee ranged from 44 to 73%. Although comparisons varied, the level of control gained from seed treatment insecticides was similar to the next higher level of host resistance. Seed treatment insecticides appear to control Stewart's wilt during very early growth of corn plants, when foliar applications of insecticides are ineffective and the effectiveness of host resistance varies depending on the proximity of flea beetle feeding sites to the plant's growing point.