Gray leaf spot is one of the most common foliar diseases of corn. High levels of corn residue, moist conditions in the crop canopy, and susceptible hybrids are all factors that can contribute to yield loss from this disease2.
Research was conducted from 2006 to 2008 at the University of Tennessee Research and Education Center at Milan. The research site was specifically chosen due to a history of high gray leaf spot pressure. The plot area was in irrigated no-till corn production for four years prior to the start of the study, with a high level of gray leaf spot each year.
Three Pioneer® brand corn hybrids with varying levels of resistance to gray leaf spot were planted in eight-row plots, and were randomized and replicated four times (Table 1). Plots were planted at 30-inch row spacing, and no-till practices were maintained throughout the duration of the study.
|Pioneer® brand Hybrid
*Pioneer hybrids are rated for disease resistance on a 1-9 scale, with 9 being the most resistant.
Each main plot was split into two 4-row plots, with one half treated with either Headline® or Quadris® and the other half non-treated. Headline and Quadris were each applied at 6 oz/a with Penetrator® Plus as an adjuvant. Each fungicide was sprayed once at VT (tassel stage) with a tractor-mounted CO 2 -powered sprayer using 20 gallons of water per acre. Yield was determined by machine harvesting.
Gray leaf spot ratings were taken each year after the susceptible hybrid had reached its highest rating in the unsprayed plots. The gray leaf spot rating scale ranged from 0 to 10, where 0 indicated no disease spots present and 10 indicated the most disease possible.
Were disease severity ratings reduced with foliar fungicides?
Yes. Foliar fungicide treatment reduced the gray leaf spot severity rating in all three hybrids (Figure 2).
Fungicides reduced the rating of the susceptible hybrid from a three-year average of 8.2 (non-treated) to 4.3 (treated). For the moderately resistant hybrid, the rating was reduced from 4.7 (non-treated) to 2.2 (treated). For the tolerant hybrid, the disease rating decreased from 2.1 (non-treated) to 0.8 (treated). This shows that even with top hybrids and treatment, some disease damage to leaves may still occur.
How do the results of this study compare to on-farm observations of foliar fungicide yield benefits?
Pioneer recently published a survey of fungicide efficacy data from a large number of side-by-side field trials as well as several university research trials conducted over the past ten years (Jeschke, 2008). On-farm trials were conducted across the Corn Belt, and gray leaf spot was the principal foliar disease in the study area.
Trends observed in the survey were very similar to the results of this study (Figure 3). The yield response to the application of a foliar fungicide was the lowest for the more resistant hybrids. For moderately resistant hybrids, the average yield increase was great enough to likely cover the cost of application; however, the average yield increase with resistant hybrids was not. In general, yield responses in the survey were less than in the University of Tennessee study for hybrids with similar levels of gray leaf spot resistance. This would be expected since the University of Tennessee study was conducted at a site with a high level of gray leaf spot pressure, whereas the Pioneer survey included many locations with varying levels of disease pressure.
What are the most important points for producers to consider when applying a foliar fungicide?
Results of this study show that the probability of using a fungicide profitably is directly related to the susceptibility of a hybrid to the predominant leaf diseases. Pioneer has typically not recommended fungicide use on hybrids rating 6 or higher (on the Pioneer 1 to 9 rating system) for the disease in question (Munkvold, 2006). Most current Pioneer hybrids have at least some resistance (rating of 4 or 5) to gray leaf spot.
Although this research study shows some very positive results from the use of foliar fungicides, each producer will have to assess his or her own disease situation and decide whether to spray or not. Several factors must be present before foliar diseases can decrease yield. The most important factors that increase the chances for disease loss are: gray leaf spot susceptibility, continuous corn, no-till or minimum tillage, later than usual planting time, frequent rains or irrigation, and high humidity late in the growing season.
®Headline is a registered trademark of BASF
®Quadris is a registered trademark of a Syngenta Group Company.
®Penetrator Plus is a registered trademark of Helena Holding Company.