Fighting Corn Tar Spot

 

Corn Tar Spot (Phyllachora maydis) is showing up in Southwest/Central Ohio in Preble and Madison counties. Corn that is tasseling or reaching silking stages with warm, humid, foggy, cloudy and wet weather create favorable conditions for corn tar spot to develop. Tar spot looks like small raised black spots on corn leaves when air temperatures are 64°F - 73°F, high humidity (>75%), and long periods of leaf wetness. Once the tar spot reaches the upper leaves during grain fill, photosynthesis is reduced and it decreases kernel development and grain yield.

Tar spot is a corn disease originated in Central America and is spreading rapidly by wind into corn growing states. 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.

There are several ways to combat tar spot. No corn hybrid is totally resistant but some varieties (especially early maturing) are more tolerant than others. Fungicides may help but time of application is critical for optimal success. The tar spot inoculum can survive the winter, so getting corn leaves to decompose quickly helps reduce the spread. Crop rotation helps but it appears that the inoculum is fairly widespread. Reducing plant stress with optimal fertility helps reduce tar spot severity.

Some universities have recommended tillage to bury the residue, but that is not really a long-term solution. Recent research shows that tillage is not the answer to solving tar spot. While the fungus overwinters in corn residue, the spores are easily carried by the wind across vast regions, making residue management alone ineffective for preventing outbreaks. Even if you bury all your infected residue, your crop can still be infected by fungal spores blowing in from neighboring fields, counties or other states. Abandoning your no-till or strip-till systems for aggressive tillage to control tar spot will only have a marginal effect at best while increasing your risk of soil erosion.

Long-term no-tillers who use cover crops can decompose corn residue quickly by changing the micro-environment and getting more moisture, microbes, and earthworms to incorporate the leaf residue into the soil just effectively as a tillage tool. In addition, a parasitic fungus (Coniothyrium phyllachorae) associates with the tar spot and may be a natural predator to tar spot. Biologicals (saprophytic microbes) applied in early fall can be used to speed up crop residue decomposition and decrease the spread of tar spot.

All diseases attack weak growing plants and organisms. Healthy plants growing on healthy soils have less pests (weeds, insects, diseases). While healthy plants can still get infected, usually the yields losses are much smaller or much less severe. Since you cannot till your way out of this disease, your most effective strategy involves a combination of genetic resistance and either using fungicides or creating a healthier plant through foliar applications to decrease micro-nutrient deficiencies.

Micro-nutrients are critical for healthy plants. Most plant proteins that are converted to enzymes need a certain micro-nutrient to activate the enzyme. Enzymes speed up biological processes so that plants can function at their highest potential, including increasing photosynthesis and also fighting off diseases.

Almost all tar spot corn shows signs of zinc deficiency. Corn zinc deficiency shows up as a white midrib in the center of the corn ear leaf while healthy corn leaf midribs should usually be dark green. Zinc activates nearly 300 different enzymes in plants. Manganese deficiency shows up as a yellow midrib in corn. Zinc, manganese, iron, and copper are several micronutrients deficiencies that may be associated with increased corn tar spot diseases. Foliar applications of these nutrients may help reduce tar spot incidence.

Fungicides are known to help but timing is critical with most fungicides needing to be applied early, before the disease is severe. Mancozeb is a combination of two fungicides, Maneb and Zineb. The central micronutrient element in Maneb is manganese with zinc in Zineb (plus some sulfur). Most fungicides are composed of micro-nutrients needed to activate the fungicide. All these fungicides need to be applied early or they do not work effectively. Perhaps the fungicides are just another way of getting micronutrients into the plant?

Researchers are developing and researching corn hybrids that are resistant to tar spot. The problem is that it may take several years. They have developed new methods for studying tar spot infection, so help may be coming. Fungicides with Group 3, 7, 11 have the longest residual time and highest corn tar spot efficacy. These fungicides include Veltyma (3,11), Delaro Complete (All), Miravis Neo (All), and Trivapro (All) applied from VT tassel to R3 milk stage for best efficacy.