Monitoring Corn Diseases

Two corn diseases farmers need to be monitoring again this year are vomitoxin and tar spot. Cooler temperatures especially at night and August rains tend to promote Gibberella (GIB) erot or vomitoxin. GIB is caused by a fungus named Fusarium graminearum, also called Gibberella zeae, causing ear rot, stalk rot, and head scab in both corn and wheat. Corn symptoms include a reddish or pinkish-white mold on the ear tips which is a toxin. The pathogen over- winters on plant residue, usually corn stalks/leaves and wheat residue (straw and chaff).

GIB ear rot is most prevalent when cool wet weather occurs for about 21 days after silking. Fields most susceptible are corn after corn or corn after wheat, especially if the wheat was infected with Fusarium head scab. Corn that is stressed from lack of nutrients, by insects, or other types of plant stress (soil compaction, poor soil health) tend to have higher levels of GIB ear rot. Some corn varieties, when they start to dry, keep their husk upright which may allow moisture to accumulate at the ear tip. Corn varieties where the ear hangs down may be better for getting moisture out, as long as the ear does not fall out.

Adequate nutrient availability may play a role. Nutrient stresses include a lack of nitrogen (N), phosphorus (P), and potassium (K) imbalances. High N with low K is often associated with higher levels of GIB ear rot. Nitrogen fertilization in the form of ammonia and adequate levels of P may decrease GIB ear rot. In corn, about 50% of P is absorbed by plant roots after plants start the reproductive phase. Another element that helps reduce GIB ear rot is chlorine (Cl). A good source of fertilizer is potassium chloride (KCL) to help fight this pathogen. The Cl also makes manganese more plant available to help fight the pathogen.

To reduce GIB ear rot before it occurs, follow these guidelines. Good crop rotation is the first step. Avoid planting corn or wheat after fields that show signs of this infection. Good soil health and fast residue decay are needed to reduce level of the pathogen. Ideally, at least 80% of your residue (wheat or corn) should be decayed by planting time. Crop residue decay is dependent upon adequate soil moisture, soil temperature (above 500F), adequate microbial activity (mycorrhizae fungus for lignin breakdown), and good macrofauna (springtails, mites, earthworms, etc.) to decay residue quickly. Fungicides may help manage GIB but results are variable and inconsistent.

Farmers can also select crop varieties that are somewhat resistant to GIB ear rot. Earlier maturing corn varieties have more time for crop residue to break down in the fall. Also, planting a cover crop, not only improves soil health and increases beneficial soil microbes, it also creates a micro-environment (warmer, higher moisture) where crop residues may decay faster.

Corn Tar spot (Phyllachora maydis) is another corn disease . Tar spot may be spread by wind into growing corn. Tar spot is a fungus that grows rapidly when temperatures are 60-700F and humidity is 75% or higher with rainy, foggy, cloudy summer weather spreading this disease. Corn will mature early with reduced ear weight, poor kernel fill, stalk rot, and possibly lodging with yield losses ranging from 0-60 bushel per acre, depending upon disease severity.

Initially, tar spot begins as mild yellow streaks flecks of yellow on leaves, stalks and husks. Then the yellowing areas turn black and raised fungal stromata form with tan halos, causing bumps on both sides of leaves but also in tassels. Infection can start early in corn (V3 or three true leaves) especially after a rain or with high humidity. Spores may grow and spread about 80 yards by wind. Several generations per year can be generated. Under the right conditions, the disease can spread quite rapidly within days.

No corn hybrid is totally resistant to tar spot but some varieties (especially early maturing) are more tolerant than others. Fungicides may help but time of application is critical for optimal success and usually have to be applied early. The tar spot inoculum can survive the winter, so again getting corn leaves to decompose quickly helps reduce the spread. Crop rotation and reducing plant stress with optimal fertility may help reduce tar spot severity. Some universities recommend tillage to bury the residue, but that is not really a long-term solution. A parasitic fungus (Coniothyrium phyllachorae) is associated with the tar spot and may be a natural predator. Nutrient deficiencies of zinc, manganese, copper, and possibly iron are often associated with higher levels of tar spot. Keeping plants nutritionally healthy helps corn plant resist many diseases.