Last year, we saw the disease tar spot in alarming number of corn fields for a variety of reasons, and it’s likely we’ll cross paths with it again this year because of the extra moisture we’ve received (and continue to receive) across the Corn Belt and South. Get to know the signs of tar spot and how you can help mitigate the disease so that you’re prepared for its impending arrival this summer.
What is tar spot?
Tar spot is a fungal disease that affects corn leaves and husks. According to Crop Protection Network, the disease was primarily located in parts of South America, Central America, the Caribbean and Mexico until 2015. The disease has since spread to the United States, affecting areas such as Illinois, Iowa, Indiana, Wisconsin, Ohio, Michigan and Florida. The spores survive on top of the soil during the winter and resurface the next growing season. The fungus, also known as Phyllachora maydis (or P. maydis), forms stromata on the leaves of the plant that can cause defoliation, which can directly affect grain fill around the development stage in corn. The spores are known to travel via wind or rain, putting your entire corn crop at risk of infection if detected.
How to detect tar spot
This foliar fungus produces small, irregular raised black spots (stromata) on the leaves of the corn plant, affecting both lower and upper surfaces. If wet conditions persist, some growers may even see a gel-like mass form on top of the spores. In some instances, brown spots that resemble fish eyes may appear. These are called fisheye lesions and occur because of a different fungus called Monographella maydis, also referred to as tar spot complex. Michigan State University Extension states that “growers are often describing the visual symptoms of the disease as leaves that have been speckled by black paint or motor oil.”
Tar spot can be mistaken for southern rust, so enlisting assistance from a local agronomist or university expert can help correctly classify the disease. One obvious distinction is tar spot stromata cannot be rubbed off the leaf, whereas corn rust pustules can.
Methods for management
Growers should consider fungicides if tar spot is detected in their fields. For example, Lucento is a newer fungicide that can help control tar spot and can be applied when the disease first appears through the R4 growth stage in up to two applications. According to FMC, the creator of Lucento, the fungicide “attacks disease from the inside out by delivering active ingredients through translaminar and acropetal movement that provides uniform leaf distribution, disease protection and keeps working long after application for control that lasts.”
Fungicide applications are difficult to time with this disease. Inoculation can occur without visible symptoms for 10–14 days. The best indicators of when to apply fungicides for tar spot are:
- Disease present in the area in previous year
- Warm, humid conditions followed by wet periods (cloudy days that allow leaf surfaces to stay moist into mid-morning)
- Tassel through R5 or dent. Fungicides protect photosynthetic tissue up to and through flowering and grain fill. These are critical times to protect with fungicides.
- Protection from secondary infections from fungicides can be beneficial (stalk rots and mycotoxins)
I also encourage growers to respect the rotation. Tar spot can thrive in corn, but it’s not known to survive on soybeans. If tar spot is a serious issue for you in 2019, make sure you give that field a break in 2020, or swap it for another crop to help reduce the risk of tar spot in coming years. Also consider tillage to help reduce the tar spot that may be overwintering in your soil.
If you think you’ve detected tar spot in your fields, don’t delay. Reach out to your local extension office with a sample to confirm the disease. The good news is there are ways to manage tar spot before it devastates your crop.