Assessing Fungicide Efficacies for the Management of Fusarium Head Blight on Spring Wheat and Barley

Small grains crop yield and quality losses resulting from Fusarium head blight (FHB) continue to threaten the economic sustainability of many small grains producers in Minnesota. Spring wheat breeders have made some progress in developing cultivars with moderate levels of disease resistance, but increased resistance in barley has not been achieved. Crop rotation and a timely application of fungicide remain the most important disease management strategies for managing the disease on both cropping species. Fungicide efficacy trials were conducted during 2003 and 2004 to compare the current industry standard (tebuconazole) efficacy with those of two experimental fungicides. Experimental products with active ingredients of metconazole or tebuconazole + prothioconazole significantly reduced percent FHB severity of spring wheat. Disease severity means with these experimentals averaged 28.5% less than tebuconazole, and percent visually scabby kernel means were 47% less with the experimentals compared with tebuconazole. Results were not as definitive for spring barley. Numerical trends from fungicide treatments were similar to those in spring wheat, but data were not statistically significant. These data indicate increased FHB management in Minnesota can be expected when experimental fungicides with active ingredients of metconazole or prothioconazole are registered for use on spring wheat by the EPA. The results for spring barley emphasize the urgency of achieving an effective disease management strategy for FHB and underscore the need for additional research on the disease in Upper Midwest states. Introduction Fusarium head blight (FHB, scab) of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) is caused primarily by Fusarium graminearum Schwabe [teleomorph Gibberella zeae (Schwein) Petch] in the US. The pathogen is known to cause widespread and repeated disease epidemics in regions with extended periods of rain, fog, or high humidity during critical plant growth stages (8,10). Wheat is known to be most susceptible to infection during and immediately following flowering (1,8,10), while barley is most susceptible to infection following spike emergence. Typically, culms of wheat are fully extended and spikes are emerged when plants flower (Feekes growth stage 10.51), but barley plants flower within the ‘boot,’ prior to spike emergence (Feekes growth stage 10.5). Fusarium head blight has consistently reduced spring sown small grain yields and kernel quality in Minnesota and North Dakota since the early 1990s. Nganje, et al. (9) estimate disease-related losses at $5.2 billion from the two states between 1993 and 2001. During 2005, Minnesota and North Dakota experienced yet another FHB epidemic, resulting in estimated losses of $248 million. 6 September 2006 Plant Health Progress A moderately resistant hard red spring wheat cultivar, ‘Alsen,’ is widely grown in northwest Minnesota and eastern North Dakota, but it has inherently lower yields than FHB-susceptible cultivars in the absence of a FHB epidemic. Currently, a small number of newly developed spring wheat cultivars with moderate disease resistance are being made available commercially. Barley breeders have been unable to duplicate the successes of wheat breeders. Resistance levels of commercial malting barley cultivars remain inadequate for use as a disease management tool. Employing a number of disease management strategies has been shown to enhance FHB management during years when the disease is not severe. Small grains producers routinely: (i) rotate with a broadleaf crop; (ii) grow varieties with resistance; and (iii) track weather conditions as crops approach susceptible growth stages for determining whether fungicide application is needed. Since 1998, many Minnesota and North Dakota wheat producers have applied tebuconazole (Folicur) at 126 g/ha during the early flowering crop growth stage. Some states annually submit federal specific exemption requests to the US Environmental Protection Agency (EPA) for emergency use registrations (Section 18s) to annually permit use of tebuconazole on wheat and barley. A second generation of experimental triazole fungicides appear to offer superior disease management of FHB compared with tebuconazole. The object of this study was to compare disease management efficacies of systemic, experimental fungicides with the efficacy of tebuconazole, the current industry standard. This data will establish whether the next generation of products can significantly and consistently increase the level of FHB management for spring wheat and barley crops in Minnesota. Protocol for Producing F. graminearum Corn Grain Inoculum Four kg of corn kernels and approximately 4 liters of reverse-osmosestreated water were placed into each of two stainless steel pans (30 cm × 51 cm × 10 cm). Pans were covered with aluminum foil and autoclaved twice for 120 min each cycle. Ten F. graminearum isolates (Fg1 Fg10), collected from diseased grain harvested between 1996-1997 in Minnesota and North Dakota, were stored at -80°C on silica gels. Isolates were grown on Difco potato dextrose agar (Becton-Dickson and Co., Sparks, MD) in Petri dishes (100-mm diameter) on a laboratory bench for 7 to 10 days with a 12-h photoperiod under cool white fluorescent lighting. Half the growth medium and fungal mycelium from one dish of all 10 isolates was subdivided into smaller pieces and placed, fungal side down, into each pan containing sterile, room-temperature corn grain. Pans were recovered with aluminum foil and maintained on a laboratory bench until grain was completely colonized by isolates (approximately 14 days). Colonized grain was mixed by hand and transferred to burlap bags (Fig. 1). Bags were placed in an oven and maintained at 33 to 38°C from 48 to 72 h until dry. Dried inoculum was stored in burlap bags for 8 to 10 weeks at room temperature until trials were inoculated. 6 September 2006 Plant Health Progress 2003 Field Trials of Spring Wheat and Barley Hard red spring wheat cultivar ‘Oxen’ (FHB susceptible to moderately susceptible), and spring barley cultivar ‘Robust’ (FHB susceptible), were planted into a Wheatville loam soil with oat residue present on 28 April at the Northwest Research and Outreach Center (NWROC) near Crookston, MN. Each trial was a randomized complete block design with four replicates. Plots were 1.5 m × 4.6 m with rows 19 cm apart. Weeds were controlled by hand labor and a 29 May tankmixed application of Puma (fenoxaprop-p-ethyl) at 585 ml/ha and Bronate Advanced (bromoxynil) at 1.2 liter/ha. On 4 June, five weeks after planting, dried F. graminearum inoculum was spread evenly throughout the test and bordering areas at a rate of 370 kg/ha. Full-canopy, night cycle mist irrigation was initiated following inoculation. Rondo mini-sprinkler misting heads with 0.8 mm nozzle orifice size and 40 liter/h nominal flow rate (Bowsmith, Inc., Exeter, CA) were situated atop fiberglass dowels at 3-m intervals and supplied water via plastic feeder pipe. Fiberglass poles were used in both trials as well as the nontreated border plots to ensure a uniform misting canopy throughout the test area. Beginning June 5, trials were misted at 15-min intervals each night at 10:00 pm, 11:35 pm, 1:10 am, 2:45 am, 4:20 am, 5:55 am, and 7:30 am. Misting was temporarily discontinued if rain events caused saturated soil in the test area. Fungicide applications were made on barley test plots at the Feekes 10.5 plant growth stage (27 June) and on wheat plots at the Feekes 10.51 plant growth stage (1 July) with a CO2 backpack-type applicator with XR Teejet flat fan 8001VS nozzles spaced 51 cm apart on a 1.5-m boom angled 60° from vertical positioned forward and backward to the direction of travel at 187 liter/ha and 276 kPa (Table 1). Table 1. Fungicide treatments applied to hard red spring wheat and spring barley during 2003 and 2004 near Crookston, MN. x Each fungicide treatment included Induce, a nonionic surfactant, at 0.125%. Fig. 1. Corn grain that has been colonized by isolates of F. graminearum is transferred to a burlap bag to be dried. After the inoculum is dry, it is stored until small grains plants in field tests are at the appropriate growth stage for inoculation. Treatmentx Rate a.i. (g/ha)