- Tar spot was reported for the first time in the US in 2015 in IN then in IL and it has now been confirmed in Iowa, Wisconsin, Michigan, Ohio and Florida.
- Symptoms begin as oval to irregular bleached to brown lesions on leaves in which black spores are formed. Lesions may also be present on leaf sheaths and husks.
- Tar spot incidence and severity increased significantly in the Midwest in 2018 with new counties and new states having their first confirmed reports.
- Significant yield losses were observed in fields were tar spot developed early.
- Researchers in the Midwest are currently working on getting more information on Tar spot epidemiology, predictive tools and management.
- As of August 5th, 2019, Tar spot has been confirmed in a few counties of Illinois, Indiana, Michigan and Iowa. However, the severity of this disease has been low.
Biology and Disease Cycle
Tar spot is caused by the fungus Phyllachora maydis. This pathogenic fungus could be transported on fresh and dry corn leaves or husks. Spores released from these plants or their crop residues are also a source of inoculum.Ascospores of P. maydis are moved by wind or rain splash to corn. The disease is favored by cool temperatures (60-70F), high relative humidity (>75%) and long periods of leaf wetness (7h or more per night). Windborne spores are released in periods of high humidity. The ascomata of the fungus most likely overwinters on crop debris. Preliminary findings from University researchers (Wisconsin and Purdue) indicated that, regardless of fall tillage, 15-40% of Tar spot spores collected from infested residue survive the winter.
Symptoms
Tar spot symptoms include small, raised, black spots scattered across the leaf surface (Image 1 above) leaf sheaths and husks (Image 2 below). These black spots, called ascomata, are the fruiting structures of P. maydis. They protrude from the leaf surface giving it a rough or bumpy feel. They can be small flecks of about 1/64” up to about 5/64”. The lesions can merge together producing larger areas of blighted leaf tissue.
Symptoms of tar spot can be confused with symptoms of common and southern rust. Late in the growing season rust fungi switch from producing orange-red pustules to black teliospores. These black pustules can be easily mistaken for tar spot lesions. Additionally, we have seen pictures in fields, of what appears to be bird or insect feces on leaves, that resemble Tar spot symptoms (Image 3 below). One way to distinguish Tar Spot from these other spots is by scraping them away with your fingertips. Tar spot lesions will not be scraped off the leaf tissue while rust spores and poop will rub off.
Image 2. Tar spot lesions on husk (Source:GROWMARK, Inc.)
Image 3. Tar spot imposter symptoms
Management
Selection of least susceptible hybrids along with timely fungicide applications can help protect corn yields from Tar spot injury. Fungicide applications made during VT-R1 have proven effective to protect yields and may provide the highest return on investment. When making fungicide decisions, consider your risk factors. Late planting, continuous corn fields, especially where there was a history of Tar spot the previous year, may increase the risk of tar spot infection if the weather is favorable. However, the weather conditions in the last few weeks have not been conducive for Tar spot development. Keep in mind that corn is susceptible to yield losses or kernel abortion through R3 and so scouting for diseases should continue on through these early reproductive stages. If a fungicide application was made during V5-V7 growth stages but disease pressure is high and environment is favorable for disease development during the early reproductive stages, a second fungicide application may be needed.
As of early August 2019, a few fields in IN, IL, MI and IA have had positive confirmations of Tar spot this season. However, the severity levels of the disease has been low and the risk of Tar spot development has gone down significantly based on the current temperatures and humidity.