Stroma of Phyllachora maydis on a corn leaf.
- Tar spot is favored by moderate temperatures and persistent humid conditions.
- Scouting for tar spot prior to VT is essential, especially after recent rains that pushed through some areas.
- Fungicide applications should target the VT-R3 window, but do not lose sight of other more commonly occurring foliar diseases and forecast weather.
Background & Identification
Corn tar spot is a foliar disease that can occasionally be yield-limiting when significant levels of infection occur prior to R4. The disease is caused by a fungus that infects the leaves, stalks, and husks of corn. Approximately 14-20 days after infection black, slightly raised, often somewhat elliptical structures are produced on infected tissues. Frequently these structures, called stroma, will develop with the leaf veins resulting in an elongated, or whisker like appearance. If it is sufficiently humid, white or orange spores may be observed exuding from these spots with the aid of a hand lens. Stroma may push through upper or lower parts of the leaf surface (Fig 1).
There are several tar spot mimics that may be present in fields. Below are some tips for differentiating the two most common issues from tar spot.
Insect frass and other detritus (Fig 2): Often this is from flies, stinkbugs, rootworm beetles, and Japanese beetles, which increase in abundance in early July. Frass is frequently only found on the upper part of leaves, is flat, and can be easily rubbed upper with some moisture applied between the fingers. Remember: Just because it does not rub off does not make it tar spot. If you have not observed this disease before, ensure that you are sending samples or images off to experienced professionals such as state diagnostic clinics or extension professionals for additional confirmation.
Fig 2. Insect frass on a corn leaf (GROWMARK, Inc.)
Physoderma (Fig 3): Physoderma is a fungus that infects following driving rains that occur while the plant is still in the whorl. Foliar symptoms do not show up until later in the season, typically around VT. Look for light colored, yellow to green dots often oriented in strips on the leaf. Spots are often flat on the midrib. Frequently, “barber shop pole” strips may also be observed on stalks. Spots will not rub off but are not raised like stroma of the tar spot pathogen.
Fig. 3. Physoderma on a corn leaf (GROWMARK, Inc.).
Disease Development & Management
There is much that is unknown about the biology of this pathogen, which places a limit on our confidence in current management strategies and tools. We do know that tar spot is similar to other foliar diseases of corn in that it can go through many repeating lifecycles (spore- infection- symptom development – spore) as long as conditions are conducive for the pathogen. Persistent humid conditions that favor leaf wetness and moderate temperatures tend to favor disease development. Initial disease occurrence and development often is slow, but once approximately 3-5% severity is observed in a field, rapid symptom/sign progression can occur if conducive environmental conditions persist. Although it is believed that most of the pathogen inoculum originates from stroma contained in infested residue, the pathogen can move at least ¾ of a mile into nearby fields. This means that residue management and rotation may reduce onset and severity but will not prevent infection during an epidemic. Although we are still learning about this disease, we do know that once it is present it is there to stay and should be part of your regular IPM planning routine
The following can help minimize potential losses from this disease during the season.
- Scout your fields before tassel. If you confirm that tar spot is present at noticeable levels before VT and weather appears conducive for disease over the next 1-2 weeks, a VT-R1 fungicide application may be required. If the disease starts to build between VT and R4 and conditions are conducive, a fungicide application may be warranted. Applications made too early or late may result in less overall efficacy. Unlike with insects, thresholds for foliar diseases like tar spot are not very reliable due unpredictably in the environment and hybrid that impact disease development and impact. If stroma are hard to detect, it is unlikely that severe disease will occur in a short amount of time. However, if stroma are easy to find and it is humid or the forecast is calling for wet weather, disease may develop rapidly over the coming weeks.
- Consider fungicides if the disease is present. Although some fungicides have been shown to have somewhat better efficacy against tar spot than others, the most important decision you will make with fungicides is whether to apply one or not. If a fungicide is applied, select one with multiple modes of action. Applications made between VT and R1 provide the best chance for a return on investment and replicated university data indicate that earlier applications are unlikely to have utility for managing this disease. Lastly, do not lose sight of other diseases, such as grey leaf spot, northern corn leaf blight, and southern rust. A focus on tar spot without including the other diseases of concern could result in other diseases robbing yield.
- It is always raining under the pivot. If you are in a situation where you have a history of tar spot and you irrigate, your risk for disease is always elevated. Irrigate using more water per event with fewer events and avoid irrigation at night if possible. The goal is to minimize leaf wetness when temeratures are cooler at night.
References:
Kleczewski, N.M., et al. 2020. Documenting the Establishment, Spread, and Severity of Phyllachora maydis on Corn, in the United States, Journal of Integrated Pest Management, Volume 11, Issue 1, 14, https://doi.org/10.1093/jipm/pmaa012
Kleczewski, N. "Corn Tarspot in 2020." farmdoc daily (10):196, Department of Agricultural and Consumer Economics, University of Illinois at Urbana-Champaign, November 11, 2020.
Kleczewski, N. M., & Bowman, N. D. 2020. An Observation of Corn Tar Spot Dispersal from Agricultural Fields to an Isolated Urban Plot. Plant Health Progress.
Mueller, Daren S., et al. 2020. "Corn yield loss estimates due to diseases in the United States and Ontario, Canada, from 2016 to 2019." Plant Health Progress 21.4, 238-247.