Tar Spot Leaf Disease of Corn

What once was a cosmetic disease in Northern Indiana, Northern Illinois and Southern Wisconsin that did not contribute to yield loss has become an aggressive disease that is likely in many fields to contribute not only to yield loss but also to lower test weight, drydown, and stalk health.

Tar Spot has now been confirmed in Iowa, Illinois, Wisconsin, Indiana, and Michigan. I recently witnessed the effects of the disease first hand as I drove to northwest Illinois. Field after field looked as brown as if it were mid November instead of early September. I thought I would share with you a summary of information that I found about the disease. To get more detailed information, the best articles that I found were at these websites:
badgercropdoc.com/2018/08/29/holy-tar-spot-batman and extension.purdue.edu/extmedia/BP/BP-90-W.pdf.

Tar Spot is the result of a fungus that has been common in Latin America but was not identified in the U.S. until 2015 when it was confirmed to be present in cornfields in Indiana. It has since been confirmed in Illinois, Wisconsin, and Iowa in 2016, 2017, and now in 2018. Until this year, the disease has been considered a cosmetic disease. It came in late in the grain accumulation stages and limited its development to mere speckles on the topside of the corn leaves. Therefore it did not contribute to yield loss. This year, however, is a much different story. The disease is much more virulent and aggressive than in previous years. It seems that healthy, green plants turn into brown, dead plants in just a few days. The disease is initially expressed as “fish eye” spots on the upper side of the leaves. The dark spot in the middle of the eye then grows rapidly and takes on a “tar like” appearance and the texture of the leaf takes on a bumpy feel to the touch. These bumps are the fruiting bodies that erupt and spread the spores by wind to other leaves and plants. As the brown lesions around the dark spot enlarge, they rapidly destroy the leaf and the ability of the plant to produce its own food. If the plant has not reached physiological maturity, it will begin taking nutrients from the stalk. Not good!

So why is this year different?Fungus loves moist, relatively cool places to grow. The past few weeks have been an ideal environment for the fungi to develop and spread quickly. It is unknown yet as to whether there are two forms of the disease that are working together to create this rapid spread or whether it is just the fact that once the initial infection occurred, it weakened the plant and opened the doors and windows of the plant to all the other diseases that are in the air and loving the environment as well. Southern Rust is one that seems to be present on the leaves of many of the diseased plants that I have seen. Northern Leaf Blight and Grey Leaf Spot are common as well. More studies are being done to help get some answers to this question.

Other questions that remain unanswered include:

• Will these spores overwinter and return next year even earlier?

• Will an application of fungicide prevent the disease from starting and/or spreading next year?

• Are there differences among the corn hybrids as to their reaction to the disease?

• How much yield will be lost this year due to the disease?

• How will the disease affect drydown and test weight?

In the areas where the disease has flourished, we have had the “perfect storm” where all three components of the Disease Triangle (inoculum, host, and environment) were present.

1) The inoculum or spores blew in at the right time,

2) The spores had a susceptible host to land on and there is no known resistance to keep them from spreading, and

3) The weather was perfect for the spore germination and spread.

So what can be done proactively now?Scout your fields and look closely at the leaves. If the disease is just getting started, do not spray with fungicide. It will not help. If the leaves are already brown, do the pinch test on the lower part of the stalk. If it collapses easily between the thumb and forefinger, lodging is only a windstorm or two away. Our crop is 250 to 300 heat units ahead of a normal year and this disease will push the need for early harvest even further. Wet grain in the bin is better than dry corn on the ground.

I hope that you are reading this and find that you do not have Tar Spot in any of your corn. If you do have this disease in your fields, I hope this note is in time for you to be proactive in dealing with it.