Corn Fungicide Use

corn field


With humid wet weather occurring comes the concern for corn fungal diseases. The goal of using fungicides is to optimize plant health and keep your corn crop alive to optimize crop yield. Healthy biologically active soils should be your foundation for healthy plants and yields. Fungicides should be used to enhancement and aid plant health—not a primary tool for growing healthy crops. There may not be a right answer to whether applying fungicides to healthy soils and plants is really needed or even profitable.

Fungicides are commonly used to control of fungal pathogens by changing the activity and the number of soil microbes. Beneficial fungus breakdown and mineralize nutrients, making these nutrients plant available. Some foliar applied fungicides reach the soil and may have a negative effect on the soil microbial population. Crop scouting is critical so that fungicides are only used when necessary and fungus resistance does not occur. Most fungicides are broad spectrum meaning they are designed to suppress or terminate most fungi, even the beneficial ones.

Some beneficial fungi suppress plant root diseases and promote healthier plants by attacking plant pathogens with fungal enzymes. Fungi decompose soil organic matter and efficiently supply nitrogen, phosphorus, many micronutrients, and even water to the plant better than the plant roots themselves, since fungi hyphae are so small and numerous. Beneficial fungi also produce antibodies to reduce or suppress other harmful microbes. Over applying fungicides carelessly may be harmful to beneficial microbes, especially beneficial fungi. Corn tar ppot is a relatively new corn fungus disease that has some farmers concerned and thinking about applying a fungicide.

Corn tar spot shows up as small, raised, black spots scattered across the leaf surface. The leaf is rough to touch and looks similar to rust. Currently, there are no corn hybrids that are resistant to corn tar spot but some hybrids are more tolerant than others so the disease may progress slower and the yield loss may be more limited. Corn tar spot exists in the crop residue and is carried on air currents and can travel several miles. Healthy soils with good residue decomposition may be beneficial in reducing this fungal disease. Corn tar spot thrives when summer growing conditions are wet and humid.

There are many fungicides labeled for corn tar spot suppression. Fungicides can reduce the incidence of tar spot but this disease reproduces quite rapidly. Timing of corn fungal application is critical and should be applied to corn during R1 (Silking) to R3 (milk stage). There is a natural fungal parasite that infects tar spot but not much is known about its effectiveness yet. Increasing corn spacing and reducing corn population may help reduce canopy humidity also help. In Mexico where this disease is common, corn experts suggest a good practice is to avoid over fertilizing corn.

Crop rotation may help some but the inoculant can easily travel several miles from a neighbor’s infected fields. It has been observed that many fields that had soybeans then rotated to corn had as much disease as fields planted corn back to corn. Crop rotation helps with other diseases like grey leaf spot, northern corn leaf blight, and anthracnose which are also common in corn.

Grey leaf spot can rapidly increase with warm temperatures 70–90°, high humidity greater than 90% or periods of prolonged (12+ hours) leaf wetness. Scout the lower corn leaves about two to three weeks before tasseling. Corn leaf lesions are long (up to 2 inches), narrow, rectangular, and light tan colored turning grey. Northern corn leaf blight symptoms are canoe-shaped lesions 1 to 6 inches long. The lesions are initially bordered by gray-green margins that turn tan colored and may contain darker areas where the fungal spores form. Northern corn blight starts on the lower leaves when temperatures are 64 to 80 degrees with high moisture. Grey leaf spot and northern corn blight generally have to be sprayed preventively before the disease shows up. Pick corn varieties that are resistant to both these two diseases if possible.

Corn anthracnose symptoms start on the lower leaves is often the first corn disease that shows up. Small, round to irregular, water-soaked spots first appear on lower leaves. Spots later turn yellow and then brown with reddish-brown borders. Anthracnose thrives under cooler temperatures and moisture and tends to slow down when summer temperatures are hot and dry. Most crops can resist fungal disease better when they have adequate nutrition and good soil health.