Scout for Tar Spot a.k.a. “Fisheyes”

Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu, Jill P., Extension Plant Diagnostician; jillp@udel.edu and Dan Severson, New Castle Co. Ag Agent; severson@udel.edu

There is a foliar corn disease commonly found in Mexico, Central America, and South America that has migrated to the United States. The disease is termed Tar Spot and is a caused by the fungus Phyllachora maydis. In the countries where the disease was discovered it is termed the Tar Spot Complex due to the involvement of a second fungal species Monographella maydis. However, this second fungal pathogen has not been detected in the United States.

Tar Spot was first confirmed in northern Illinois and Indiana in 2015. One theory on how the disease reached the United States was that spores were wind-blown by a tropical storm that originated in the Gulf of Mexico. In recent years the disease has been confirmed in other parts of the United States from New York to Florida including Delaware’s neighboring state Pennsylvania.

 

 

The fungus produces small, raised, black, bumpy lesions that resemble splattered specks of tar on the corn leaves (Figures 1 & 2). The small black bumpy structures are termed stromata and can be observed on both the upper and lower surfaces of the leaves. The growing conditions that favor Tar Spot also favor other fungal diseases such as Common and Southern Rust, Physoderma Brown Spot and Sooty Mold which can easily be mistaken as Tar Spot. To properly confirm the pathogen in question it is best to send a sample to a plant diagnostic lab.

 

 

A unique sign of Tar Spot that may develop is that a tan halo of dead tissue can form around the stromata producing a lesion that resemble “fisheyes” (Figure 3). Another distinguishing characteristic of tar spot is that the stromata does not wipe off the corn leaf when you rub it with your fingers.

 

 

According to Crop Protection Network (CPN), Tar Spot has been observed most often during or after silking through grain fill (R1-R6) however, the disease may appear earlier (Kleczewski et al. 2019). Lesions can develop on both the upper and lower leaves and in severe cases lesions may appear on leaf sheaths, husks and tassels.

Tar Spot favors an environment that is similar to the Mid-Atlantic region:

• cool temperatures 59-70 °F,
• high relative humidity, greater than 85%
• long periods of leaf wetness > 7 hours natural or irrigated

When conditions are favorable Tar Spot produces spores continuously during the growing season. Spore release from the stromata is initiated by rain and humidity and in turn the wind and rain distribute the spores. The spores then attach onto the leaves of a corn plant. Corn is susceptible to Tar Spot at any growth stage leaving the crop vulnerable throughout the season. Tar Spot has shown it can survive and overwinter on infected corn debris in the United States including in Pennsylvania.

Although multiple stress factors may contribute to yield loss like weather, fertility and time of infection, Tar Spot has been shown to cause economic damage. In 2018, 20-60 bushel/acre yield loss in severely affected regions of Indiana were reported (Telenko & Creswell, 2019).

The reduced leaf area caused by the stomata interferes with photosynthesis, limiting sugar production and signaling the plant to shut down. If the availability of sugar at grain fill is limited, yield loss is usually associated with reduced ear size, poor kernel fill and possible lodging. In addition, if the crop is to be used for corn silage the quick dry down may lead to silage that is chopped at a lower than optimum moisture level resulting in fermentation challenges. The dryer the silage is at harvest the harder it is to achieve proper packing density. Improper packing density leads to reduce forage quality, loss of nutrients, growth of other fungi and mycotoxins. One management strategy is to prioritize crop harvest based on disease severity and harvest fields that are drying down faster than usual first.

Given that this is a new disease to our area – our knowledge is limited. However, CPN offers six management strategies have the potential to reduce the negative effects of Tar Spot.

1. Hybrids –All hybrids have some level of susceptibility; some are more tolerant and show less disease than others. Since this disease is new to the area look at genetics from the Mid Central States. Talk to a sales representative or an Extension Agent about availability in our area. Check rating scales – Compare apples to apples!
2. Fungicides – Fields where fungicides were applied experienced less tar spot development than those without. However, proper timing and frequent applications are required to reduce Tar Spot. Multiple applications during the VT-R3 growth stage seem to be better than a single application. There are fungicides with 2ee labels that can be used to manage the disease, furthermore fungicides with 2 or 3 modes of actions appear to work best. Fungicide efficacy trials are underway, but are in the early stages.
3. Irrigation – Excessive irrigation or frequent light irrigation may increase Tar Spot. Long periods of leaf wetness, greater than 7 hours, are conducive to the disease (DEKALB, 2021).
4. Crop rotation – Crop rotation can help; however, it is unknown how many years of rotation away are needed. This practice allows residue to decompose therefore reduce the risk of disease.
5. Tillage – Tillage of fields can help reduce the risk of disease by burying spores which can increase rate of decomposition infected corn residue and reduce spore counts.
6. Scout – Scout often and early. Apply fungicides as needed. Harvest early if lodging is going to be an issue. Of course, there is an app for that – The University of Wisconsin has a Phone app: Tarspotter. The purpose of the app is to assist farmers in making management decisions for Tar Spot (Wilde, 2022).

 

Summary

Tar Spot has not been detected in Delaware, but it has been confirmed in Lancaster County, Pennsylvania. The disease has been spreading in the United States and the Mid-Atlantic region has the environment that is favorable to Tar Spot. There is a mapping systems that shows the areas that have been affected by Tar Spot 2015-2021. This disease is new to the United States therefore our knowledge about how to manage it is limited. Scouting is paramount – scout fields regularly during the growing season for potential issues. If you suspect Tar Spot in any fields, please contact your local Extension office. Be vigilant and keep a “FishEye” out.

References

DEKALB, ASGROW, DELTAPINE. (2021, July). Tar Spot Q and A. https://www.dekalbasgrowdeltapine.com/en-us/agronomy/tar-spot.html. Retrieved from https://www.dekalbasgrowdeltapine.com/en-us/dekalb.html

Kleczewski, N.M., Chilvers, M., D.S., Plewa, D., Roberson, A.E., Smith, D.L., and Telenko, D.E (2019). Corn Disease Management: Tar Spot. Crop Protection Network CPN 2012-W.

Telenko, D., Creswell, T. (2019) Diseases of Corn: Tar Spot. BP-90-W

Wilde, M. (2022, February). Watch for Tar Spot in 2022 Six Management Tactics to Tackle Tar Spot [web log]. Retrieved April 28, 2022, from https://www.dtnpf.com/agriculture/web/ag/crops/article/2022/02/03/six-management-tactics-tackle-tar.