Reducing Tar Spot Disease


Most farmers had a good corn harvest but corn tar spot (Phyllachora maydis) was an issue. Tar spot is a corn disease that came from Central America and seems to be spreading by wind into corn growing states. 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. Unfortunately, not much is known about this disease. 

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 recommend tillage to bury the residue, but that is not really a long-term solution. About 90-95% of all cropland being planted to corn is tilled, so is tillage really that effective? 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.

All diseases attack weak growing plants and organisms. Healthy plants growing on healthy soils have less pests (weeds, insects, diseases). When plants are not healthy, they have incomplete photosynthesis which means they do not produce full proteins. Insects can sense this and they attack the plant and this can open plant leaves, stalks, or roots up to increased disease infection. While healthy plants can get infected, usually the yields losses are much smaller or much less severe. In today’s COVID pandemic, the same principle applies where weak or immune compromised individuals have the highest disease severity.

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 and animals can function at their highest potential. Why was the tar spot disease so prevalent this year? Could it also be exasperated by a micro-nutrient deficiency? That is unknown presently, but I will share some observations.

Almost all internet tar spot images showed 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 be dark green. Zinc activates nearly 300 different enzymes in plants (luckily, many are somewhat redundant). Manganese deficiency shows up as a yellow midrib in corn. Zinc deficiency was rampant this year, I found it throughout the Midwest. The glyphosate herbicide (Roundup) also ties up or chelates many micro-nutrients. The 2020 dry weather in the fall, winter, and spring may have contributed to micro-nutrient deficiency this year and may be a contributing factor to increased disease severity.

When I visited a no-till, cover crop Indiana farmer near Cincinnati this summer, he had some great looking corn. He had a soil type similar to Paulding Clay which would typically yield 150- 175 bushels of corn. This farmer is regularly getting 240-280 bushels (80-100 bushel more) by supplementing with $60 total of a micro-nutrient package at planting, side-dressing, and sometimes with Y drops later in the growing season. This farmer had just a small amount of zinc deficiency and only a some tar spot disease lesions, on a few plants. The neighbor’s corn (150- 170 bushel corn maximum) had zinc, manganese, sulfur, copper, calcium, and boron deficiencies with at least five different corn diseases present. The yield ield difference could be due to corn variety, but soil and fertility management could also be a factor.

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 micro-nutrient element in Maneb is manganese with zinc in Zineb (plus some sulfur). Most fungicides are composed of micro-nutrients needed to activate the fungicide. Do not have the answer but is it possible that micro-nutrient deficiencies played a role in tar spot disease severity this year? Healthy soils equal healthy plants and healthy people.