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Recently, claims have been made that the use of glyphosate and transgenic crop traits increases plant susceptibility to pathogens. Transgenic traits used widely for years in dent corn are now available in commercial sweet corn cultivars, specifically, the combination of glyphosate resistance (GR) and Lepidoptera control (Bt). The objective was to assess the interactions of the GR+Bt trait, glyphosate, and Goss’s wilt on sweet corn. Nine treatments were tested under weed-free conditions at two sites in 2013 and 2014. Treatments included two isogenic cultivars differing only in the presence or absence of GR+Bt, with and without postemergence application of glyphosate, and inoculationwith the causal agent ofGoss’s wilt (Clavibacter michiganensis ssp. nebraskensis) before glyphosate application, after glyphosate application, or no inoculation. Results failed to show glyphosate or the GR+Bt trait influenced sweet corn susceptibility to Goss’s wilt. The only factor affecting Goss’s wilt incidence was whether plants were inoculated withC. michiganensis ssp. nebraskensis. In the absence of glyphosate application, yet under weed-free conditions, several yield traits were higher in sweet cornwith theGR+Bt trait. Results showed that theGR transgene confers the same level of tolerance to glyphosate in sweet corn as observed previously in dent corn. If true, recent claims about glyphosate and transgenic traits increasing plant disease would be of major concern in sweet corn; however, no relationships were found between the GR+Bt trait and/or glyphosate to Goss’s wilt incidence in sweet corn. Weed and insect pest management in agronomic crops has changed dramatically since commercialization of transgenic glyphosate [N-(phosphonomethyl)glycine]-resistant (GR) cultivars in the 1990s. The ability to use glyphosate, a nonselective herbicide, in previously sensitive crops made weed management in GR crops simpler and less expensive (Duke and Powles, 2008). In dent corn (Zea mays L.), developers of GR cultivars also engineered the plants to produce toxins effective against Lepidoptera larvae, commonly known as the Bt trait. Largely, because the GR+Bt combination appeared to simplify weed and insect management and at an economical advantage to pest management in non-Bt glyphosate-sensitive (GS) corn, transgenic cultivars were rapidly adopted in the United States (Green, 2012). The U.S. growers adopted GR+Bt dent corn on more than 80% of the corn area within 10 years of introduction (James, 2011). Using the same transgenic traits employed in dent corn, GR+Bt sweet corn cultivars were commercialized in 2011 (Seminis, 2015). Although adoption rates of GR+Bt sweet corn have yet to be published, many growers in the Midwest United States eagerly waited to embrace GR+Bt technology in sweet corn (Anonymous, 2015). In recent years, claims have been made that glyphosate use and the GR trait increases plant susceptibility to pathogens. Johal and Huber (2009) speculated a hypothetical glyphosate toxicity inGR cropswould enhance their vulnerability to fungal diseases following glyphosate application. Others have proposed increased disease susceptibility due to an adverse effect of glyphosate on plant mineral nutrition (Yamada et al., 2009). Although the preponderance of evidence indicates that disease susceptibility inGR crops is driven by the inherent level of disease resistance in the host plant and not by the presence of the GR gene or treatment with glyphosate (Duke et al., 2012), the vast majority of such research has been conducted in dent corn and soybean [Glycine max (L.) Merr.]. Susceptibility to a wide range of diseases has been problematic in sweet corn since the mid-19th century (Huelson, 1954). While improvement in tolerance to specific diseases has been observed (e.g., common rust), sweet corn germplasm remains susceptible to several other prevalent diseases because of the greater importance to breeders of improving yield and eating-quality traits (Pataky et al., 2011). One of the diseases many sweet corn and dent corn germplasm lines are susceptible to is Goss’s bacterial wilt and blight (caused by C. michiganensis ssp. nebraskensis). Goss’s wilt was first identified in Nebraska in 1969, Iowa in 1971, and Kansas in 1972 (Wysong et al., 1973).Within 12 years of the first discovery in Nebraska, Goss’s wilt was observed in additional states across the Plains and theMidwest, which included Colorado, Illinois, and South Dakota (Wysong et al., 1981). Incidence of Goss’s wilt in midwestern states, such as Illinois, continued to be very low until the late 2000s and early 2010s. During this period, Goss’s wilt was reported for the first time in Indiana (Ruhl et al., 2009),Minnesota (Malvick et al., 2010), and North Dakota (Friskop et al., 2014). During this time, Goss’s wilt also began to reemerge in the central High Plains in the United States (Jackson et al., 2007). Two phases of the disease can occur, a leaf blight and a systemic wilt. Both phases of the disease can occur in a field, but the systemic wilt phase is the most destructive (Suparyono and Pataky, 1989). Infections by the causal bacteria occur through wounds in plant tissue caused by injury such as hail or sandblasting (Claflin, 1999). The systemic wilt phase Received for publication 7 Oct. 2015. Accepted for publication 13 Nov. 2015. We thank Nick Hausman, Jim Moody, Kelsey Mehl, Adriane Boer, and the many students who assisted with this research. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable. Current address: Department of Plant Pathology, University of Kentucky, 1205 Hopkinsville Street, Princeton, KY 42445. Corresponding author. E-mail: martin.williams@ ars.usda.gov. HORTSCIENCE VOL. 50(12) DECEMBER 2015 1791 occurs when the bacteria reaches the xylem tissue where they spread, limiting water movement. Symptoms of leaf blight can appear as large lesions with wavy margins and irregular, dark spots within the lesions (typically referred to as ‘‘freckles’’). Yield of sweet corn (both ear weight and number of ears) can be reduced greatly, especially when infections occur in the early stages of plant development (Suparyono and Pataky, 1989). Although scientific evidence of glyphosate applications having an effect on Goss’s wilt has not been published (Duke et al., 2012), Langemeier (2012) reported that a significant, positive correlation was observed between glyphosate application and detection of C. michiganensis ssp. nebraskensis in corn leaf samples. In addition, Huber (2011) speculated that surfactants applied with herbicides could change the host and facilitate penetration by Clavibacter. The objective of the present study was to assess the interactions of the GR+Bt trait, glyphosate, and Goss’s wilt on sweet corn. Materials and Methods A single protocol was used to conduct field experiments in 2013 and 2014 near Urbana, IL. Experiments were conducted at two locations at the University of Illinois Crop Sciences Research and Education Center near Champaign–Urbana, IL; one at the Fruit Research Farm and the other at the Vegetable Crop Research Farm. Different fields were used each year. The soil at the Fruit Farm was a Dana silt loam (fine-silty, mixed, superactive, mesic Oxyaquic Argiudolls) averaging 6.9% organic matter and a pH of 6.4. The soil at the Vegetable Crop Farm was a Drummer silty clay loam (finesilty, mixed, superactive, mesic Typic Endoaquolls) averaging 3.3% organic matter and a pH of 6.0. Within two weeks of planting, 135 kg N/ha was applied as urea and immediately incorporated with a field cultivator. Sweet corn was planted on 76-cm rows at 69,000 plants/ha on 16 May 2013 (Vegetable Crop Farm), 17 May 2013 (Fruit Farm), and 21 May 2014 (both sites). As a preventative measure for control of corn rootworms (Diabrotica spp.) in all plots, tefluthrin {(2,3,5,6-tetrafluoro-4-methylphenyl) methyl (1R,3R)-rel-3-[(1Z)-2-chloro-3,3,3trifluoro-1-propenyl]-2,2-dimethylcyclopropanecarboxylate} was applied in a t-band at planting. The entire site was maintained weed free using a preemergence application of smetolachlor {2-chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl] acetamide} + atrazine (2-chloro-4-ethylamino6-isopropylamino-1,3,5-triazine) and weekly hand weeding. Any sign of drought stress (e.g., leaf rolling) was alleviated with supplemental sprinkler irrigation, which occurred twice in the 2013 Vegetable Crop Farm field. Experimental approach. The experimental design was a randomized complete block with four replications. Nine treatments (Table 1) were assigned to experimental units of four crop rows spaced 76 cm apart and measuring 9.2 m in length. Cultivars were isogenic, differing only in the presence (‘Passion II’) or absence (‘Passion’) of two transgenes; one transgene (cp4 epsp) conferring resistance to glyphosate and another transgene (cry3 Bb1) conferring resistance to corn rootworms (Monsanto, St. Louis, MO). In treatments receiving glyphosate, the formulated product was applied using a compressed air backpack sprayer at a rate of 1.68 kg a.e./ha in 187 L·ha of spray volume when the crop had 4 to 5 visible collars (V4–V5). For treatments that included inoculation with C. michiganensis ssp. michiganensis, the pinprick inoculation method was used (Blanco et al., 1977; Pataky, 1985). This method involved growing C. michiganensis ssp. nebraskensis (originally isolated from a corn leaf collected in Champaign County, IL, in 2011) in nutrient broth (Becton, Dickinson and Company, Franklin Lakes, NJ) on a shaker table for 2 d at room temperature ( 23–25 C) under constant light. Bacterial suspensions were adjusted to 1 · 10 cfu/mL using a 0.1 M NaCl solution before inoculations. This inoculum was used to inoculate early planted ‘Jubilee

Goss’s Wilt Incidence in Sweet Corn Is Independent of Transgenic Traits and Glyphosate