A farmer recently asked Ken Ferrie this two-part question: Can a soil test help determine the need for a fungicide application, and does healthy soil correlate with less need for fungicide?
“The answer to these questions is yes, maybe and sort of,” says Ferrie, Farm Journal Field Agronomist.
The final determination is based on the disease triangle – you must have the disease, a host and the right conditions to trigger a disease outbreak. Soil health falls under the area of conditions, and soil tests can help identify conditions.
What a soil test doesn’t do is detect the presence of disease in a field – an insight that is valuable to know if you experienced heavy disease pressure in corn this season.
“Healthy soils do not prevent disease from moving into a field, but we do know healthy plants handle stress better than unhealthy plants,” Ferrie explains.
That fact was evident in Farm Journal corn fertility test plots in 2021 and again this season in Midwest farmers’ fields as Ferrie and his agronomic team helped corn growers deal with a toxic mixture of multiple diseases ranging from southern rust to northern corn leaf blight and tar spot.
“In some of our nitrogen (N) plots, an additional 30 pounds of N looked like a fungicide application when it came to keeping corn greener longer, packing more starch in before disease shut down the plants,” he recalls.
What Farmers Need To Consider For 2026
Soil tests done this fall can give farmers some indication of which fields could be at risk to any disease pressure that shows up next season. Ferrie offers several scenarios as examples:
“Low potash testing fields will have more trouble than fields where the potash is adequate. Fields that are acid and need limestone will be more susceptible to disease pressure,” he says.
“Fields that run out of nitrogen during grain fill are more susceptible to disease pressure. In our test plots where we pulled N rates back and disease was an issue (in 2021), some hybrids died a month early, knocking 30 to 50 bushels per acre out of the yield,” he adds.
Fungicides and good fertility levels will lessen the impact of a disease outbreak, but they will not eliminate it. “Therefore, we want to be careful pulling back too far on our fertility, especially in those fields that aren’t at the optimum levels to begin with,” Ferrie says.
Along with that recommendation, Farm Journal Field Agronomist Missy Bauer encourages growers to keep some level of nutrients in the field.
“So even if you put reduced rates of fertilizer on, keep soluble nutrients in front of your crop,” she advises.
Crop-Tech Consulting Field Agronomist Isaac Ferrie says to manage pH levels based on what soil tests show. Even small changes can have a significant impact on plants, nutrient availability and soil microbial activity.
“Keeping your pH in check will help keep other nutrients more available, so make sure your pH levels are in good shape and lime where needed,” he advises.
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