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Foliar Fungicides for Gray Leaf Spot Management in Corn

 

Foliar Fungicides for Gray Leaf Spot Management in Corn


By Mark Jeschke and Greg Luce

Summary of Gray Leaf Spot

  • Gray leaf spot is common in most major corn-producing areas of the U.S. Planting hybrids with resistance and using foliar fungicides are the primary practices for managing this disease.
  • A study was conducted by the University of Tennessee to determine the yield benefit associated with foliar fungicide management of gray leaf spot in three hybrids with differing levels of gray leaf spot resistance.
  • At this site with a history of high gray leaf spot pressure, treatment with a foliar fungicide increased yield of the most susceptible hybrid by over 23 bu/acre, compared to an increase of only 7 bu/acre with the resistant hybrid.
  • Foliar fungicide treatment reduced the gray leaf spot severity rating in all three hybrids, but even the most resistant hybrid still showed some leaf damage.
  • 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 in the field.
 

Introduction of Gray Leaf Spot

Gray leaf spot ( Cercospora zeae-maydis ) is a common foliar disease that is well established in most U.S. corn growing states and can cause substantial reductions in yield. The disease has become especially prevalent where continuous corn is produced in conjunction with minimum tillage. Corn residue is the primary source of gray leaf spot inoculum, so a corn crop with high levels of residue on the soil surface from the previous season will be at a greater risk of developing gray leaf spot.

Gray leaf spot has become so well established in the Mid-South that even practices intended to minimize crop residue, such as conventional tillage and crop rotation, will not totally control the disease. In many cases, hybrids with resistance can reduce yield loss due to gray leaf spot; however no hybrid is immune to the disease. Even the most resistant hybrids can suffer yield loss under high gray leaf spot pressure.

Foliar fungicides provide an additional management option in hybrids susceptible or moderately resistant to gray leafspot, and in cases where agronomic practices that may reduce the inoculum load are impractical. Two of the most common fungicides used for control of gray leaf spot in corn are the strobilurin fungicides Quadris® and Headline®.

 

Gray leaf spot is one of the most common diseases in corn. Typical gray leaf spot lesions are rectangular with straight edges.
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.

Objectives

Previous research studies have shown that foliar fungicides are often practical, effective tools in managing gray leaf spot in corn, especially when growing susceptible hybrids. Dr. Melvin Newman at the University of Tennessee conducted a three-year study to determine the yield benefit associated with foliar fungicide management of gray leaf spot in hybrids with differing levels of resistance.

Methods

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.

Table 1. Gray leaf spot resistance ratings of Pioneer hybrids used in a three-year foliar fungicide study at the University of Tennessee.

Pioneer® brand Hybrid GLS Rating* GLS Resistance
32T22 3 susceptible
33R76 5 moderately resistant
33V14 7 resistant

*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.

Results

Three years of field studies showed a high degree of control of gray leaf spot with two strobilurin fungicides, Headline and Quadris. Yield and disease reduction effects of the two fungicides were generally similar and are reported as an average of both products.

Applied Questions

Did foliar fungicides increase corn yields?
Yes. The average yield increase associated with foliar fungicide treatment was related to the gray leaf spot resistance of the hybrid (Figure 1). For the susceptible hybrid, the three-year average yield increase was 23.5 bu/acre over the non-treated check. For the moderately susceptible hybrid, the yield increase was 12.5 bu/acre, and for the resistant hybrid - 7 bu/acre.

These results demonstrate the potential for gray leaf spot to cause substantial reductions in yield when disease pressure is very high. Hybrid resistance was effective in mitigating a large portion of yield loss due to gray leaf spot; however, even with the most resistant hybrid, the yield benefit of the foliar fungicide application was sufficient to pay for the application (with then current corn prices).

Chart: Average yield increase with foliar fungicide application over three years in hybrids susceptible, moderately resistant, and resistant to gray leaf spot.

Figure 1. Average yield increase with foliar fungicide application over three years in hybrids susceptible, moderately resistant, and resistant to gray leaf spot.

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).

Chart: Gray leaf spot severity ratings with and without foliar fungicide treatment in hybrids susceptible, moderately resistant, and resistant to gray leaf spot.

Figure 2. Gray leaf spot severity ratings with and without foliar fungicide treatment in hybrids susceptible, moderately resistant, and resistant to gray leaf spot.

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.

Chart: Average yield response of Pioneer® brand hybrids to foliar fungicide application according to gray leaf spot disease rating (300 trials, 1999-2008).

Figure 3. Average yield response of Pioneer® brand hybrids to foliar fungicide application according to gray leaf spot disease rating (300 trials, 1999-2008).

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.

References

Jeschke, M. 2008. Foliar Fungicide Effects on Corn Yield. Crop Insights. Vol. 18, no. 17. Pioneer Hi-Bred Int'l, Inc. Johnston, IA.

Munkvold, G. 2006. Foliar Fungicide Use in Corn. Crop Insights, Vol. 16, no. 7. Pioneer Hi-Bred Int'l, Inc. Johnston, IA.

1The Pioneer Crop Management Research Awards (CMRA) Program provides funds for agronomic and precision farming studies by university and USDA cooperators throughout North America. The awards normally extend for three years and address crop management information needs of Pioneer agronomists, sales professionals and customers.
2Photos courtesy of Donald Specker.

 

®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.
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