Martin Chilvers<chilvers@msu.edu>, Michigan State University, Department of Plant, Soil and Microbial Sciences, and Kristin Poley, Michigan Corn Marketing Program -
Learn more about corn tar spot and its recent history in Michigan, the 2020 outlook and management options.
What is tar spot?
Tar spot is caused by the fungus Phyllachora maydis and can be identified by the raised, black spots that appear on corn leaves and husks. Surrounding some of the black spots may also be a tan halo, which is called a fish-eye lesion.
Each one of those black tar spots is the reproductive structure of the tar spot fungus, which can produce thousands of spores. Research has demonstrated that the tar spot fungus can overwinter in those black spots, releasing spores the next year.
Don’t confuse insect frass with tar spot. Frass will rub off the leaf with some water while tar spot structures will be embedded in the leaf and often protrude through the underside of the leaf.
Recent tar spot pressure in Michigan – 2018 and 2019
Tar spot pressure in 2018 was significant with losses of up to 50 bushels per acre. Frequent rainfall in 2018 and increasing disease inoculum load in the region led to those dramatic losses.
The 2019 season saw a much drier July and August, which put the brakes on tar spot and resulted in much lower loses. Although tar spot pressure was relatively light across Michigan in 2019, the disease did continue to spread and is now present across most of our corn acres.
Outlook for tar spot in 2020
What weather patterns could lead to greater tar spot pressure?
Cool, humid conditions with prolonged leaf wetness can lead to higher tar spot pressure in fields. Windy and rainy conditions may also spread spores to new fields, or new areas within a field. Irrigators should be careful that they do not promote tar spot with light frequent irrigation which may increase leaf wetness events.
At what point in the season should scouting start?
Tar spot has been found in early July in some years. As we approach silking, it’s a good time to scout for tar spot and other diseases such as northern leaf blight and gray leaf spot.
What control measures are available?
There are no hybrids currently available that are completely resistant to tar spot, but some varieties may have more resistance than others.
Testing of several fungicides is underway across the state and the Midwest for managing tar spot. One of the biggest factors for successful tar spot suppression is application timing. To date, early applications at the V8 growth stage are too early and provide little benefit. Research this season is going to focus on optimizing fungicide timing. It appears that scouting is critical to get fungicide applications at the beginning of an epidemic.
How can farmers get confirmation of tar sport in their fields?
“Symptoms of tar spot first appear as oval to irregular bleached to brown lesions on leaves in which raised, black spore-producing structures call stroma are formed... giving the symptomatic areas of the leaf a rough or bumpy feel to the touch… resembling pustules on leaves with rust.
Be ready to spray between the VT and R3 growth stages if you plan to use just one fungicide application and you are seeing tar spot increase. If you spray between the V8 and VT growth stages, be ready to monitor the smartphone apps and do more scouting as you might have to pull the trigger again later in the season.
Tar spot pathogens toxins that can hurt your cattle, etc.
-Many plant pathogens (e.g. bacteria, fungi) produce toxins that help the pathogen trick plant defense responses or kill plant tissues. This does not mean that the toxin has an effect on humans, cattle, swine, etc.
Due to the way maple tar spot disease is transmitted, complete control of maple tar spot is virtually impossible on mature trees. Prevention is the key with this disease, but if nearby trees are infected, you can't reasonably expect to totally destroy this fungus without community support.
Since tar spot does little long-term harm to the host, the leaves could be used as mulch. Leaves can be composted, but composting may not completely destroy spores, because home compost piles seldom heat up enough. Chemical management: Fungicides are available to treat tar spot proactively in spring.
Tar spot is a common, visually distinctive and primarily cosmetic fungal leaf spot disease. Tar spot can affect many species of maple, including (but not limited to) silver maple, sugar maple and Norway maple. Boxelder (also known as ash-leaved maple), willow, holly and tulip-tree can also be affected by tar spot.
The tar spot fungus appears to overwinter in infested crop debris, although the exact means of how the fungus overwinters, and the exact way it infects, are not known. Wind-blown or splashing rain likely moves fungal spores from crop debris onto the leaves of the new corn crop, which then becomes infected.
Tar spot is caused by the fungus known as Phyllachora maydis and can be identified by small, raised black and circular spots present on corn leaves, stalks, and husks (Figure 1).
Tar spot can infect as early as V3 growth stage. After rain or periods of high humidity the stromata will extrude spores, rain and wind will disperse these spores up to 80 yards and infect healthy corn plants. Once infected the new stromata will form on infected tissue in roughly 2 weeks.
And, to allay any fears of customers or clients, these Rhytisma fungi do not spread to other types of plants. The fungus overwinters in fallen leaves, so if there is a concern, raking up the leaves (followed by removal from the landscape or composting) can be effective to limit tar spot incidence.
Tar spot is a fungal disease complex of corn that has been destructive and yield limiting in Central and South America for nearly 50 years. Phyllachora maydis, the causal agent of tar spot, is an emerging corn pathogen in the United States, first reported in 2015 from major corn producing regions of the country.
Rotating to crops other than corn and using tillage to bury residue can help to reduce fungus inoculum levels in fields. Apply Fungicide When Needed. Early fungicide applications, at or before the first signs of development, have been effective against Tar Spot in trials.
Fungicide: There are a number of environmentally friendly organic products that do a good job at suppressing black spot, including sulfur, neem oil, and copper. Most products are available as foliar sprays or dusting powders and will control many types of fungal diseases.
This disease spreads by rain or overhead watering and can affect other nearby plants. Blackspot cannot survive in the soil, and cannot live longer than a month on infected tools.
Black Spot Identification/Symptoms: The first hint of black spot's arrival in your rose garden comes in the form of dark, round leaf spots with irregular, feathered margins. Unlike many fungal diseases, black spot only strikes the top side of leaves, never the undersides.
Introduction: My name is Stevie Stamm, I am a colorful, sparkling, splendid, vast, open, hilarious, tender person who loves writing and wants to share my knowledge and understanding with you.
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