• How and when tar spot arrived
  • The impacts it has on corn
  • How to manage tar spot
  • What to expect in the future

What is tar spot?
Tar spot is a fungal disease in corn caused by Phyllachora maydis. The disease causes black specks to form on the leaves. In Mexico and Central America, where the disease was discovered, it is referred to as the Tar Spot Complex because of the involvement of a second pathogen found on plants with the disease in that region. The additional pathogen, Monographella maydis, is thought to be responsible for the “fish eye” symptoms that coalesce and cause greater leaf tissue loss. However, the second pathogen has not been confirmed in the United States.

A map of current and previous tar spot infected areas as of May 2021.
Figure 1. A map of current and previous tar spot infected areas as of May 2021.
Source: https://corn.ipmpipe.org/tarspot/

When and how did tar spot get to North America?
One theory is that a tropical storm from the Gulf of Mexico in 2015 deposited wind-blown spores to northern Illinois and northern Indiana.

Where can it be found?
Tar spot has been confirmed across a widespread area of eastern Iowa, central and northern Illinois, southern Wisconsin, northern and central Indiana, southern Michigan, northwest Ohio, and southern Florida. It was also confirmed in Ontario in the fall of 2020 (Figure 1).

What conditions favour tar spot development?
Favourable environmental conditions that promote infection and disease development are cool 15° to 21°C (59 to 70°F), humid conditions (85% relative humidity) with long periods of leaf wetness (greater than seven hours). Wind-driven rain and storms can spread spores of the pathogen to new plants and fields. Additional research is needed to further understand conditions favourable for disease development in the United States and Canada.

What are the impacts of tar spot?
Like other foliar diseases, the impact of tar spot depends on how early infection occurs and the severity of the infection. Severely infected leaves at grain fill can limit sugar availability resulting in an inability to completely fill ears prior to black layer and an overall loss in kernel weight and yield. When photosynthesis is reduced because of a loss of leaf area, stalks may be cannibalized for sugars, which results in poor standability and lodging. Even in areas where tar spot is present, many fields may not have yield loss because the disease came in late or symptoms did not develop to levels that affect yield. The presence of tar spot does not necessarily indicate a reduction in yield. Yields in fields with tar spot may be reduced by many other stress factors, such as gray leaf spot, reduced fertility, or loss of stalk integrity. In some areas, tar spot can be seen progressing in the corn canopy on dead plant tissue following other disease infection or nutrient stress. Maintaining plant health throughout the season by reducing stress from lack of nutrients or from other pathogen infection may help reduce risk of yield loss.

Tar spot is a fungal disease
Figure 2. Tar spot is a fungal disease that appears as a series of black spots containing spores on corn leaves (A and B). Depending on the date of infection, the spores may proliferate on the plant's leaves and cause reduced photosynthesis (C), which can lead to stalk cannibalization and poor standability (D).

How does tar spot start and spread?
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.

Research from Mexico suggests that the incubation period (the time it takes from infection to visible lesions) may be as long as 40 days. Observations in the U.S. suggest that this period may be only 12 to 15 days under certain circumstances. Like other corn diseases, such as rust, gray leaf spot and northern leaf blight, tar spot is polycyclic, meaning that it can produce many generations of spores and infect the crop multiple times within the same season.

Do infected fields have the second pathogen?
Monographella maydis has not been detected in any submitted samples of tar spot in the U.S. However, observations in 2018 indicate that the secondary fungus is not required to cause damage to yield potential. P. maydis alone can cause yield loss under favorable environmental conditions. Farmers should monitor fields to help track this disease and determine if management tactics are warranted.

Does tar spot overwinter here?
Because symptoms have been observed in the same areas since 2015, it appears that the fungus is overwintering rather than being reintroduced from Mexico or Central America each year. However, more research is needed to learn how it overwinters, where it does so, if it survives on other alternative hosts, etc.

Are there differences in corn product resistance to tar spot?
While there are no corn products grown in the U.S. Corn Belt that are known to have high levels of resistance to tar spot, there appear to be slight differences among products in levels of susceptibility. Most North American corn products across all seed companies appear to be relatively susceptible to tar spot, but only a few products appear to have slightly less severe symptoms.

Comparing corn product reactions to tar spot, or any disease in isolation, can be tricky. Multiple side-by-side comparisons in fields with uniform levels of disease pressure are best suited for comparing product reactions. Comparing levels of infection between products in different fields may not be accurate. The severity of symptoms can depend on when the infection occurred, the quantity of tar spot fungus that overwintered in infested corn debris in a field, fungicide applications, infection in neighboring fields, and uneven levels of tar spot infection throughout a field can also complicate comparisons.

Does rotation help manage tar spot?
Rotation should not make tar spot worse, but it may not provide much control. Because the fungus appears to overwinter in infested debris, avoiding that inoculum early in the season should be beneficial depending on how much inoculum can move in from other sources (e.g., neighbouring fields) and how far the spores spread. Based on the widespread occurrence of tar spot in 2018, the fungus appears to spread rapidly over long distances when weather is favorable. In years with less favorable weather, rotation or management of infested debris may be of more value in limiting the development of tar spot.

Are fungicides available to help control the disease?
The overall assessment is that fields treated with a fungicide are better than those without. However, some fields sprayed with a fungicide still suffered some yield loss and standability concerns. It is unclear whether these applications were too late, too early, or simply not frequent enough to control the disease. It is also possible that because we are dealing with a new pathogen in combination with mostly susceptible germplasm, when experiencing perfect environmental conditions, one application could simply not be enough. Farmers, retailers and seed companies are used to working with products with respectable tolerance to many foliar diseases. With low tolerance to a disease, very susceptible crops historically have required multiple applications of fungicide to prevent yield loss. The combination of fungicides with more tolerant products will likely be the best means of help in the near term.

Delaro® Complete is a recommended foliar fungicide available in Eastern Canada for corn and soybean. Delaro Complete is labeled for suppression of tar spot on corn. For more information about Delaro Complete, please visit https://www.cropscience.bayer.ca/Products/Fungicides/Delaro-Complete and contact your retailer. Fungicide application should begin when disease first appears. A second application is permitted by the label and should occur within 7- to 14-days of the initial application if environmental conditions persist that favour continued disease development. Two fungicide applications may be needed in environments with high disease pressure.

Will we have tar spot again next year?
Tar spot has occurred in northern Illinois and Indiana each year since it was first discovered in 2015. Levels of severity have varied depending on weather and other factors. Infection of future corn crops by tar spot will largely depend on favorable weather conditions for the development of the disease. We are monitoring current and historical weather conditions to better understand risk and frequency of possible occurrence, particularly in the affected states of Illinois, Indiana, Iowa, Michigan, Ohio, and Wisconsin.