Last week, we determined foliar pesticide applications to soybean in 2015 will likely be a profitable business decision. In fact, we discovered that insurance based applications would have, on average, been profitable over the last several years. So for corn, will insurance-based foliar fungicide applications pay in 2015? If not, can we target or “predict” environments where we suspect that they will?
- In a recent summary of 212 fungicide trials conducted by university plant pathologists, corn yields were, on average, increased 5.8 bu/ac with fungicides containing both strobilurin- and triazole-based active ingredients.
- In environments with some disease pressure (<5%), fungicides containing both strobilurin- and triazole-based active ingredients increased yields 9.8 bu/ac (Table 1).
- Based on these yield estimates, only targeted fungicide applications would increase net profitability. At cash corn prices ($4.05) and at average fungicide plus application costs ($29.80/ac), we would expect a net return just shy of $10/ac.
- Environments at high-risk for developing foliar diseases are those where no-tillage or strip-tillage is used and that have more years of corn in the rotation than soybean.
- Rotating corn with soybean, and burring corn residue with tillage, strongly reduces the probability of foliar disease development. However, hybrids with low levels of disease resistance, or fields with a recent history of foliar disease, are still candidates for disease development.
- If fungal inoculum is present in deteriorating corn residue, the above normal precipitation experienced across much of East-Central Iowa and North-West Illinois means fungal spores have had ample opportunity to be blown and splashed into the developing crop canopy.
- Corn yield increases observed in environments with low or no disease levels (3 bu/ac on average) suggests physiological effects from strobilurin-based fungicides on their own are not enough to consistently result in net profit returns.
Can past research help us make a prediction?
As reproductive growth draws near—and with corn prices creeping higher—we wonder if the abnormally wet conditions call for a blanket fungicide application just in case foliar diseases develop?
To answer this question, we turn to a recently published summary of 212 university fungicide trials. These trails were conducted for 7 years across 14 states and gave us a good estimate of what we might expect to happen on average (Paul et al., 2011).
Across all trials (years/environments) corn yields were increased 4 and 5.8 bu/ac from fungicides containing a strobilurin- or a strobilurin+triazole-based active ingredients. Based on these estimates, fungicides applied, for insurance, against disease development would not pay.
The good news is this research also uncovered that when disease is present (5% or more of the corn leaf area in the non-treated plots) foliar fungicides increased corn yields 6.6 and 9.8 bu/ac with fungicides containing a strobilurin- or a strobilurin+triazole-based active ingredients (Table 1). With the latter estimate of 9.8 bu/ac at $4.05 corn (cash today) and a total average cost of $29.80/ac for application plus a fungicide premix (strobilurin+triazole), yields a net return 9.89/ac.
With the above information, we concluded environments with foliar disease would, on average, produce a net return. However, environments with little or no foliar disease would not.
Recall the Disease Triangle
How do we predict when/where these environments will occur? If we reflect on the disease triangle, disease development requires a host (hybrid), inoculum (old corn residue containing the fungus), and a favorable environment. The hybrid is always present, but the old corn residue harboring the fungus—and the environment—are often moving targets. However, since the amount of residue on the soil’s surface and inoculum load are at least casually related, we would expect the probability of disease development to increase with the amount of corn residue remaining on the soil’s surface.
The link between surface residue and foliar diseases
To demonstrate the link between residue and foliar disease, we turn to a tillage-by-residue study. This study has been planted to corn for more than a decade at the University of Illinois under the direction of Dr. Emerson Nafziger. Within this study, there are tilled and no-tilled plots with three levels: partial, all, or no residue removal.
As we can see in Figure 1, either tilling or removing some—or all—of the residue lowers the percent of the canopy infected with grey leaf spot. Given that we can, to some extent, control the amount of residue and inoculum on the soil’s surface by manipulating crop rotation and tillage practices, we can begin to reach some good assumptions about where we would expect the risk of foliar disease development to be elevated.
High-risk environments would include: continuous corn or extended corn rotations with no-tillage, or strip-tillage, practices. Moderate-risk environments would include fields with corn-soy rotations where conservation tillage practices are used. Low-risk environments, however, would include those in corn-soy rotations with extensive tillage practices (Table 2).
In addition to our above assumptions, we at least have some evidence to suggest foliar fungicides are more effective at protecting yields in environments with increasing amounts of residue. The data in Figure 2 was adapted from Jeschke and Doerge (2010) and shows fungicides are more effective at increasing yields when corn follows corn as opposed to soybean (9.9 vs 5.2 bu/ac). This data also suggest that fungicides are less effective at increasing yields as tillage intensity and residue burial increases.
We do expect foliar diseases to be more prevalent than normal in 2015, given the Northern Corn Leaf Blight pressure in 2014 and what seems like daily rain providing extended periods of leaf wetness. We do not expect blanket applications to be necessary or profitable in all cases, particularly for those hybrids with good disease resistance and when corn follows soybeans in conventionally tilled fields.
When in doubt, scout. Scouting isn’t sexy, fun, easy, or clean, but it is without a doubt the best way to know when a foliar fungicide will pay.
Fungicide active ingredient groups and corn grain yield increases in encironments with greater than or less than 5% disease severity.
Crop canopy gray leaf spot severity with and without tillage and full, partial, or no corn residue removed at one location near Urbana, Il. in the growing season of 2014. Data courtesy of the University of Illinois and Dr. Emerson Nafziger.
Relative risk of corn developing foliar disease based on tillage and crop rotation.
Corn yield increases from foliar fungicides when corn is grown after corn or in
rotation with soybean, and when no-tillage/strip-tillage, conservation tillage, and conventional tillage is used. Data adapted from Jeschke and Doerge, 2010.
Jeschke, M. and T. Doerge. 2010. Manament of Foliar Diseases in Corn with Fungicides. Crop Insights. DuPont Pioneer. http://sdaba.org/agronomyconference/pdfs/OsbornHandout3.pdf (accessed 1 July 2015).
Paul, P. A., L. V. Madden, C. A. Bradley, A. E. Robertson, G. P. Munkvold, G. Shaner, K. A. Wise, D. K. Malvick, T. W. Allen, A. Grybauskas, P. VincellI and P. Esker. 2011. Meta-analysis of yield response of hybrid field corn to foliar fungicides in the U.S. Corn Belt. Phytopathology 101:1122-1132.