Arriving at a fungicide decision

I debated writing about fungicides and plant diseases in this column, which will be published after most, if not all, fungicides have been sprayed this summer. On the other hand, now is a prime time to take a look back at the “great experiment” of foliar fungicide application to corn and soybean crops over the last decade or so.

Arriving at a fungicide decision

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CSI: Iowa

I debated writing about fungicides and plant diseases in this column, which will be published after most, if not all, fungicides have been sprayed this summer. On the other hand, now is a prime time to take a look back at the “great experiment” of foliar fungicide application to corn and soybean crops over the last decade or so.

It’s a chance to examine and fine-tune our decisions using industry and university information. Since we are far from having consensus on the short- or long-term pros and cons of fungicide use on commercial corn and beans, I’ll stir things up so you can ponder this from now until harvest, and as you plan for 2015 crops.

The original role of foliar fungicides was to protect plants (in our case primarily soybeans and corn) from disease. Think of corn and bean plants as photosynthetic factories, with healthy leaf tissue being a key part of the factory that helps process raw materials like sunlight, water and nutrients into the final product: grain. Plant diseases — we’ll focus on leaf diseases — can compromise the performance of the factories and limit production.

Fungicides slow the progress of the disease to protect our healthy factory leaf tissue, which should mean better production. You probably know this already; our products are rock solid, and when used right they do a great job managing the fungal diseases they are targeted at.

Soon after we started using fungicides on our crops on a larger scale, a secondary role for fungicides emerged. This secondary role encompasses various concepts and terms: plant health, yield bump, greater stalk strength, increased water and nitrogen use efficiency, improved chlorophyll retention, delayed leaf loss and plant maturity (longer grain fill time), and some other concepts that in general are viewed as potentially increasing yields even in absence of significant plant disease.

I’m not qualified to get too deep into all this, so I’ll keep it simple. We do see this going on in the absence of disease; lots of really smart people are working on figuring out how, when, where and why. We need to understand it better so we can help predict these responses and target fungicides more efficiently in situations where disease isn’t the driving factor, while balancing fungicide resistance and environmental issues as well. To keep from getting further over my head, let me wrap up the secondary role this way: It’s real. But don’t forget to couple it with this: Research by pathologists across the Midwest suggests that return on investment from foliar fungicide application is more likely when conditions are favorable for disease.

I admit I’ve left the secondary concept of foliar fungicides pretty open to interpretation. The first concept, protecting plants from disease, seems pretty easy, right? Scout for disease. If we find it at the right levels, we spray it. Fungicides work, and everyone lives happily ever after.

Well, the concept is easy, but the reality is much more ambiguous. We talk about the “disease triangle” being the driver if we have disease and how aggressive it may become. The “triangle” means:

Is the disease host present (susceptible corn or soybean plants)?

Is the disease pathogen present?

Is the weather right for pathogen?

Consider disease triangle

Consider Side 1 of the triangle; genetics change continually so evaluating and tracking disease resistance of what we plant isn’t easy. Resistance ratings of corn hybrids and soybean varieties aren’t universal and are open to interpretation, so that system isn’t perfect either.

We don’t have a lot of control over Side 2 other than some pathogen levels can be reduced with tillage or crop rotation — factors that may or may not fit in with our conservation or marketing plans.

Side 3 of the triangle is weather, which involves the challenges of predicting what comes next. From a research perspective, a lot of large-scale corn and soybean disease work has been carefully planned and put in place in the last decade; the weather doesn’t cooperate very well.

From a production viewpoint, it is just as challenging. With foliar fungicides, we have to be on the front end of the infection. If we apply too late, the disease is too far ahead for the products to work properly. A great example is this crop year. When the decisions to spray or not were made in mid- to late July, the disease triangle had signs pointing to a big disease year.

We had saturated soils; warm humid weather hit and was predicted to continue; and pathogens like northern corn leaf blight, common rust and gray leaf spot were starting to take off in many areas. We saw issues in soybeans as well. But the triangle can fall apart nearly as quickly as it comes together. As I write this, we are trying to decide which fields to treat and which to watch. A cool, dry stretch — or a hot, dry spell — can limit disease.

Trying to prioritize fields and make decisions isn’t easy. When put on the spot, even I start to sweat; we just can’t tell with much certainty if that $30-plus-per-acre investment will pay or not. Farmers like to drive the point home. A good grower and friend of mine said, “You know, spraying the susceptible hybrids means the price of a new pickup, right?” He followed that up with, “And if we pull the trigger on the questionable acres, I just spent enough to pay my kid’s college tuition for a few semesters.”

We are making our best recommendations based on a wide range of fungicide information on disease control and yield impact, and what the weather services predict for the next two to four weeks. Did I mention that for certain diseases, like northern corn leaf blight, Gulf Coast weather such as hurricanes can play into the mix? Throw in factors like hail, APHs, crop insurance and other relevant issues, and it is a darn tough decision.

Most agronomists get wrapped around the axle trying to help growers make the right decision. I know it isn’t just me who gets heartburn; I’m part of those conversations with a lot of growers, retailers, and seed and chemical industry folks going through the same thing. They don’t just recommend fungicides across the board to “sell product.” Agronomists know growers want the most economically sound advice; it is part of the “value package” that helps retain customers.

McGrath is the ISU Extension field agronomist at Harlan in western Iowa.

Growers on the same page

Planning and thought goes into making crop input decisions. Growers are on the same page as the rest of us in this. They have to manage many risks; disease is just one of them. Putting a crop in costs big bucks. If we use an ISU Ag Decision Maker ballpark of $800 per acre for corn on beans, a fungicide application is another 4%.

Spending another 4% or roughly $30 per acre may or may not seem like a big deal depending on your situation and experiences, but the $30 is only part of the issue. Growers are trying to decide if they need to spend that $30 to protect their $800 investment from diseases that may or may not take off in fields. If disease shows up, it could take from a few bushels per acre to over 50% of yield. Seems like an easy call, right?

Maybe when markets are favorable and margins are larger it is. But doing the math today, that roughly $30 per acre spent could eat away the small projected returns, or just add to the red ink. So more than any other fungicide application year I can recall, farmers are really drilling their advisers for insight and have walked their fields to try to make the right call. I’ll bet the information and experiences they have this year with disease scouting and discussions on the disease triangle, genetics, rotations, tillage, application timing, additives, residual control, integrated pest management, resistance management, proper use rates and more will enhance their future fungicide decision-making.

We won’t know what our fungicide application returns are or what the cost of not applying them will be until we crunch harvest and grain marketing data after harvest is over. On a side note, trying to explain the general trend in yield returns from fungicide applications over the last 10 to 12 years could take me from now until harvest is over — it’s complicated. Meantime, we all keep learning and gaining experience to help with these decisions in the future.

Scouting treated and untreated fields can provide insight into disease progression and fungicide performance to help put more context to what the final yield numbers tell us about our decisions.

— Clarke McGrath

 

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GOSS’S WILT: This is a bacterial disease, which fungicide can’t control. Often confused with northern corn leaf blight, Goss’s has distinguishing characteristics. For more information, visit www.extension.iastate.edu/CropNews.

 

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NORTHERN LEAF BLIGHT: The large elliptical lesions of northern corn leaf blight are caused by a fungus and can be controlled by a fungicide.

This article published in the August, 2014 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2014.

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