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The Triangle Effect of Plant Disease

The Triangle Effect of Plant Disease
A favorable environment, a virulent pathogen and a susceptible host can wreak havoc in your fields.

By Jennifer Bradley

In order for plant disease to wreak havoc on your fields, these three components must exist simultaneously: a favorable environment, a virulent pathogen and a susceptible host.

"All of those components have to come together, all at the same time, in order for that disease to take place," says Damon Smith, assistant professor of field crops pathology at University of Wisconsin-Madison. "If any one thing is missing then you break that triangle and you don't have disease."

The Triangle Effect of Plant Disease

The easiest component to manipulate in the plant disease triangle is the host, says Smith, who explains this step alone is the foundation of a successful pest management program. The practice of selecting and planting a more resistant hybrid of corn for instance, would eliminate the susceptible host component of the triangle. He recommends growers make sure resistance is as strong as it can be but also well-adapted to their area. This will make for a healthier plant with less disease risk overall.

The next easiest way to break the triangle and plant disease growth is to consider the environment. Smith says it's obviously not possible to control Mother Nature, but irrigation and planting patterns are two examples of things within a farmer's influence. To reduce the instance of white mold in soybeans, for example, wider spacing will increase the air flow and stop the conditions from being met for that particular pathogen to thrive.

That pathogen itself is very hard to manipulate, and one farmers won't target as much when it comes to preventing plant disease, says Smith. In the case of corn rust, the millions of spores being blown around are impossible to command. One option for soil-born organisms, however, is to rotate crops consistently, not allowing a pathogen too much time to grow but also leaving a long enough period for it to reduce.

The challenge with this comes in the trade-off between resistance and yield. This can be OK if there is not a history of high susceptibility in the field, says Smith. But if there is, there could be a problem.

"You put yourself in danger of losing the yield you were going to gain because you had the disease," he says.

Know the issues ahead of time
As farms get bigger and extra acres require effective management, good record keeping and intentional scouting for plant disease is more important, says Smith. Unlike with insects where treatment thresholds have been determined, plant disease won't be noticeable until the damage is done.

"With plant diseases, if we're scouting in season, a lot of the damage has already happened because we're dealing with a micro-organism, something we can't see," Smith explains. The records from previous seasons are invaluable tools for farmers to make educated decisions based on data rather than current conditions.

"We find that if you wait until you see the disease and then try to treat it, you usually don't gain much," he says. "If we had a crystal ball, it would make things a lot easier. Unfortunately we don't, and that's why a lot of people get really frustrated because it's not straightforward."

Here are Smith's top three tips for managing the plant disease triangle in 2014:
•Look at your records from the past, and choose a variety based on those records.
•Pick a variety that has good resistance and is adapted for your area of the state.
•Try to plant in a timely manner to minimize undue stress to plants. 

"If you do those things and can make it all come together, a lot of times you won't need to rely on fungicide applications," Smith concludes.

Bradley lives in East Troy.

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