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 (https://corn.ipmpipe.org/tarspot/). 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

www.Fieldcropnews.com or https://www.realagriculture.com/ontariodiagnosticdays

Crop Protection Network at www.cropprotectionnetwork.org/

Corn IPM Pipe – tar spot tracking and reporting at https://corn.ipmpipe.org/tarspot/

  1. If you suspect tar spot, please contact OMAFRA field crop plant pathologist Albert Tenuta at albert.tenuta@ontario.ca

Weather Data

Location Year Weekly September 6 – 12 Accumulated
Highest Temp (°C) Lowest Temp (°C) Rain (mm) Rain (mm) April 1st GDD 0C April 1st  GDD 5C April 1st CHU May 1st
Harrow 2021 30 10 4 456 2921 2129 3162
2020 25 9 25 380 2867 2073 3206
2019 29 11 38 487 2780 1983 3017
Ridgetown 2021 28 9 4 440 2760 1977 2978
2020 24 7 38 426 2722 1943 3014
2019 31 9 7 558 2626 1831 2825
London 2021 26 10 23 382 2745 1966 2948
2020 25 7 25 454 2646 1869 2900
2019 29 9 9 550 2508 1738 2716
Brantford 2021 28 9 36 361 2714 1935 2885
2020 24 6 19 241 2674 1903 2883
2019 29 8 274 2584 1802 2774
Welland 2021 26 9 38 369 2729 1946 2947
2020 25 6 8 303 2726 1949 3032
2019 26 10 1 443 2623 1835 2857
Elora 2021 25 8 52 340 2515 1745 2686
2020 24 4 14 343 2451 1699 2679
2019 26 6 26 451 2079 1371 2205
Mount Forest 2021 25 9 46 455 2517 1750 2710
2020 22 5 14 457 2426 1687 2696
2019 25 6 129 2328 1564 2516
Peterborough 2021 24 8 48 349 2526 1745 2671
2020 23 2 19 319 2490 1725 2687
2019 28 0 13 372 2300 1533 2398
Kingston 2021 24 12 16 268 2616 1835 2845
2020 24 5 3 338 2640 1857 2940
2019 27 5 13 403 2563 1769 2775
Kemptville 2021 25 10 22 313 2709 1922 2855
2020 25 3 3 355 2584 1811 2806
2019 27 2 203 2436 1664 2572
Earlton 2021 21 5 17 515 2339 1577 2432
2020 20 3 23 397 2159 1469 2371
2019 17 0 27 425 1917 1234 2039
Sudbury 2021 20 7 10 415 2378 1617 2518
2020 18 3 34 514 2224 1515 2452
2019 18 1 17 434 1977 1295 2125
Thunder Bay 2021 22 6 7 294 2231 1483 2334
2020 21 -2 47 292 2093 1394 2296
2019 20 1 36 383 1863 1177 1961
Fort Frances 2021 23 2 10 268 2309 1556 2447
2020 22 -3 8 334 2186 1464 2386
2019 21 4 65 498 1998 1278 2089
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 www.fieldcropnews.com.