Written by Ritika Lamichhane and John Nelson
UNL Plant Pathologist Dr. Tamra Jackson-Ziems and her team recently confirmed tar spot in Richardson County, just north of Falls City. The disease infestation is in very low levels, but it is important to begin scouting your corn fields. The Corn ipmPIPE helps track the appearance of tar spot. It is available online at corn.ipmpipe.org/tarspot. When you look at the map, keep in mind that the gray color signifies counties where tar spot was found in previous years and the highlighted yellow ones indicate that a sample from one or more fields in that county tested positive this year. Last year, tar spot was confirmed in 47 counties in eastern Nebraska, as far west as Adams, Hall, and Holt counties.
What does Tar Spot look like?
Tar spots produce small, raised black circular spots that look like tar. These spots cannot be scraped off the leaf and often have a tan to brown halo around them, giving them a “fisheye” appearance. These black spots can be mistaken for a few other things:
- Insect frass: This can be rubbed off with some moisture and does not show through both sides of the leaf like tar spot.
- Saprophytic fungi: These fungi grow on dead brown tissue and thrive in moist conditions. These saprophytic fungi are not raised above the surface like tar spot and may not have clearly defined margins as well.
- Rust spores: At the end of season, orange-red spores in common and southern rust turn black and look similar to tar spot. Rust spores can be rubbed off and often leave an orange to black mark on your finger.
Because of these similarities, it is important to be extra careful while scouting your corn field for tar spot.
What causes Tar Spot and where to scout?
Moderate temperatures (60°F-lower 70s°F) and frequent early and mid-season rainfall with average leaf wetness of 7 hours or more create ideal conditions for disease. With all the rain we have been getting this year we need to be especially cautious. The disease could be observed as early as V8 corn stage. The fungus overwinters in crop debris, so we should scout in those regions where we know the pathogen was present last year. Once the fungus is established and the weather conditions become favorable again, it can reproduce and cause infections.
In Nebraska, the most severe incidences of tar spot have been in sprinkler irrigated fields. This is because tar spot development is driven by higher relative humidity, longer periods of leaf wetness, and more consistent precipitation and/or irrigation. If you have a center pivot, inner spans may be more conducive for disease than the outer irrigation span and that is where you will need to scout first.
Some corn hybrids do have tar spot resistance ratings, so talk to your agronomist or seed dealer to see if there are any ratings for your hybrids. If you find you have a hybrid with a low tar spot tolerance rating, those fields would be another good place to begin scouting. Scouting should be done frequently, especially in the days and weeks approaching tasseling.
What should I do if I find Tar Spot?
Contact your local extension educator if you see anything that fits the description of tar spot. Samples with suspected tar spot can be sent to the UNL Plant and Pest Diagnostic Clinic (plantpathology.unl.edu/plant-pest-diagnostic-clinic) and they will diagnose the samples free of charge.
Foliar fungicides have been found to be effective in controlling tar spot. You can visit https://go.unl.edu/2023cornfungicideefficacy for specific product and their ratings for tar spot and other corn diseases. Any fungicide with a rating from good to very good (G-VG) should work for controlling tar spot. A recent study from Purdue University found that one well-timed fungicide application at VT/R1 – R3 growth stage reduced tar spot symptoms compared to corn with no fungicide. Even when two applications seem to enhance tar spot management, the return on investment is only slightly better than that of one well-timed application.
Another way to determine the severity of tar spot infection and the best time to apply fungicide is to use the Tar Spotter app. This mobile app, developed by the University of Wisconsin, helps you make management decisions for tar spot in corn by calculating the daily risk of disease infection. It uses GPS coordinates to assess if the weather has been favorable for the development of tar spot in a specific field.
