Crop Report – September 15, 2021

There has been considerable discussion around tar spot disease in corn this season and of course many questions as well. Let’s go over some of these questions!

  1. Where did Tar Spot common from? In 2015, Tar Spot was confirmed in the Midwest US (Indiana/Illinois) where it was believed to have been blown in from Mexico on storm systems. Since 2015, tar spot has spread from Iowa to Pennsylvania. The development of tar spot in Indiana, Ohio and particularly Michigan with confirmed tar spot counties next to the Ontario border was of concern going into 2020 growing season and was not unexpected when first confirmed in Ontario on September 21, 2020 in Ridgetown. By the end of 2020, tar spot was confirmed in Essex, Chatham-Kent, Lambton, Elgin and Middlesex counties (Figure 1).
Figure 1. Tar Spot Distribution August 18, 2021
Figure 1. Tar Spot Distribution August 18, 2021
  1. Is Tar Spot here to stay? Unfortunately, the answer is YES! Going into the 2021 growing season, the big question was would the pathogen overwinter and on July 2, 2021, tar spot was found in West Elgin. This wasn’t a surprise considering what has been occurring in Michigan and other Great Lake states since 2015.
  2. Where is Tar Spot now in Ontario? Ontario along with US extension specialists are monitoring the real-time movement of tar spot through the Corn ipmPIPE initiative ( Tar spot in 2021 has expanded from the original lower 5 counties in southwestern Ontario to as far east as Wellington county and north into Bruce and Grey counties over the past month (Figure 2).
Figure 2. Tar Spot Distribution September 15, 2021
Figure 2. Tar Spot Distribution September 15, 2021
  1. What does Tar Spot look-like? Tar spot appears as small, raised, black tar-like spots scattered across the upper and lower leaf surfaces. These spots are stroma (fungal fruiting structures). If viewed under the microscope, hundreds of sausage-shaped asci (spore cases) filled with spores are visible. When severe, stroma can even appear on husks and leaf sheaths. Tan to brown lesions with dark borders surrounding stroma can also develop. These are known as “fisheye” lesions. (Figure 3.)
Figure 3. Scouting for Tar Spot
Figure 3. Scouting for Tar Spot
  1. Are all black tar-like spots on a leaf Tar Spot? NO! Tar spot is most often confused for insect frass (poop)! It is easy to distinguish insect frass from tar spot lesions, all you need is some water or “spit”. Wet the leaf spot and rub the area between your finger, hence the scientific procedure name of the “spit-test”! If the spot rubs off the leaf, it is not tar spot which produces as mentioned earlier raised black lesions which are embedded in the leaf and don’t rub off.  Keep in mind, other diseases such as rust and physoderma brown spot can also be confused with tar spot. Rust forms orange/red lesions which erupt through the leaf surface (volcano-like) and as they get older can turn black/dark brown but when you rub the lesions the spores do rub off leaving a smudge on your finger. Physoderma form flat brown lesions primarily on the leaf mid-rib or near leaf base unlike tar spot which usually occurs from the middle toward the tip of the leaf.
  1. What causes Tar Spot? Phyllachora maydis is the pathogen which causes tar spot in Ontario and the US. In Latin America, where tar spot has a long history, another fungus, Monographella maydis occurs forms a disease complex with maydis known as the tar spot complex which in that region is associated with the tar spot fisheye lesions mentioned above. Although we have fisheye lesions, M. maydis has not been detected in Ontario or the United States to date.
  1. Is Tar Spot on my maple trees or garden plants the same? NO! Tar spot on your maple trees is caused by another pathogen called Rhytisma acerinum and not the corn tar spot pathogen Phyllachora maydis. Sadly, plant pathologists are not the most creative when it comes to naming plant diseases an if it looks like a tar spot, it must be a tar spot!
  1. Does Tar Spot produce a toxin? There is no research or studies which indicate that tar spot produces any mycotoxins on the plant whether in the grain (ear) or stalk. Although this is good news, early senescence, or death along with increased stalk lodging potential due to tar spot infection could result in increased contact with soil borne mycotoxin producing fungi or ear rots. Grain or silage corn to be used for feed should be tested for mycotoxins as a common practice.
  1. Does tillage and rotation impact Tar Spot? This is an area which has not had a lot of research and plans are in place to establish Ontario trials. Although the pathogen overwinters in residue this might be minor since spores can travel long distances.  Based on field observations, corn on corn with high residue levels will be slighter higher risk but the weather will be the key to how severe and rapid the disease can develop. Once tar spot becomes more established in Ontario, it is expected to be similar to other endemic diseases such as fusarium head blight, northern corn leaf blight, white mould and sudden death syndrome to name a few.
  1. Do we have any tar spot resistant hybrids? Based in what we have seen and observed in hybrid trials, there does not appear to be any known resistance to tar spot in commercial corn hybrids in Ontario or for that matter in the US. There is, however, some degree of tolerance or partial resistance and a 64 hybrid OMAFRA trial in Rodney, ON does show some good candidate hybrids (Figure 4). The Ontario corn performance trials in Ontario are being evaluated now to determine if tar spot ratings could be obtained similar to what was done in 2018 for Gibberella ear rot and DON mycotoxin.
Figure 4. 2020 Hybrid Screening Trial in Rodney, Ontario
Figure 4. 2020 Hybrid Screening Trial in Rodney, Ontario
  1. I applied a fungicide at VT/R1 but I have Tar Spot? All fungicides have specific window of activity and for most products that is 14 to 21 days. Many factors impact the residual activity of a fungicide active ingredient including hybrid and degradation of the fungicide by the plant, ultraviolet rays and dilution as the plant grows are some of these factors. If tar spot spores land during these 21 days for example, the fungicide will be best able to slow or inhibit spore germination and penetration of the plant tissue especially if combined with hybrid resistance or tolerance (Figures 5 and 6). If the spore germinated and the fungus (mycelium) has already infected the plant some 3 or 4 days earlier, the fungicide application will likely have limited activity. How well various fungicides work against tar spot depends on the product efficacy against tar spot, timing, weather, hybrid susceptibility, disease levels at application timing and spore load in region.
Figures 5 & 6. Difference between susceptible and tolerant hybrid tar spot control with fungicide
Figures 5 & 6. Difference between susceptible and tolerant hybrid tar spot control with fungicide
  1. Will I need a second or late fungicide application for tar spot management in the future?   It is important to realize that fungicides should be applied to reduce disease development. Remember that many fungicides only effectively manage disease for about 14 to 21 days after application. Therefore, if a disease continues to increase, a second application might be beneficial, but there are a few things to consider first.  Applying a fungicide late in the season will not ‘cure’ an already diseased plant. Dry matter accumulation in the kernel continues until physiological maturity which is close to black layer (R6). However, the rate of dry matter accumulation decreases quickly, starting at the beginning of R5 (dent). The degree of yield protection will depend on the level of disease currently in the crop, susceptibility of that hybrid to tar spot (and other diseases), and upcoming weather conditions (Figure 7.).

  1. What are some additional tar spot resources?

Ontario Diagnostic Days Episodes – #2 (2020) #8 (2020) and #3 (2021) at or

Crop Protection Network at

Corn IPM Pipe – tar spot tracking and reporting at

  1. If you suspect tar spot, please contact OMAFRA field crop plant pathologist Albert Tenuta at

Weather Data

LocationYearWeekly September 6 – 12Accumulated
Highest Temp (°C)Lowest Temp (°C)Rain (mm)Rain (mm) April 1stGDD 0C April 1st GDD 5C April 1stCHU May 1st
Mount Forest202125946455251717502710
Thunder Bay20212267294223114832334
Fort Frances202123210268230915562447
Report compiled by OMAFRA using Environment Canada data. Data quality is verified but accuracy is not guaranteed. Report supplied for general information purposes only. An expanded report is available at