Crop Report – August 18, 2021

Figure 3. SDS leaf symptoms (left) and stem (right)

Tar spot in corn was first identified last year in Ontario on September 21, 2020 and confirmed to overwinter in Ontario on July 2, 2021 in West Elgin. Since then, tar spot has been also detected in Essex, Chatham-Kent, Lambton, Middlesex and Norfolk counties as of July 18, 2021 (Figure 1). This was anticipated since tar spot has continued to spread in the Midwest US since 2015 and can overwinter in the region. The frequent rains (from the midwest US especially Michigan/Indiana) and high humidity along with warm temperatures in southwestern Ontario have favoured tar spot development in 2021 (Figure 2). In most cases, incidence and severity is very low and will have minimal impact on yield as corn approaches black layer but in fields which had early infection and no fungicide there will likely be an impact on yield if favourable environmental conditions continue (Figures 2b and 2c). In the US, yield losses from 20-60 bu/acre have been observed under similar conditions.

Figure 1. Tar Spot distribution August 18, 2021

Figure 1. Tar Spot distribution August 18, 2021

Figure 2. Scouting for Tar Spot
Figure 2. Scouting for Tar Spot
Figure 2b. Tar spot untreated (lesions and yellowing of leaves up the plant.
Figure 2b. Tar spot untreated (lesions and yellowing of leaves up the plant.
Figure 2c. Tar Spot fungicide treatment (tar spot lesion on lower leaves and progression up plant compared to the untreated treatment in Figure 2b.
Figure 2c. Tar Spot fungicide treatment (tar spot lesion on lower leaves and progression up plant compared to the untreated treatment in Figure 2b.

Currently there is limited long-term research-based information for disease management of tar spot in the US Midwest and Ontario but OMAFRA in conjunction with the Tar Spot Working Group (TSWG) have a number of on-going trials to develop long-term integrated pest management (IPM) strategies for tar spot management. There are no known tar spot resistant hybrids but in an OMAFRA/ screening trial in Rodney, Ontario, there appears to be varying degree of tolerance in the 64 hybrids being evaluated. Fungicide evaluations and timing trials are also being conducted in Ontario and the US which have shown many of the commercially available fungicides are effective against tar spot as well as other corn diseases such as northern corn leaf blight and Gibberella ear rot at the traditional tassel or silking (VT/R1) timing.

What can you do now?

1) Tar spot is obviously a concern and be prepared but we don’t have to fear the disease, therefore, keep updated on tar spot movement and information. TSWG has developed a real-time tar spot tracking system which includes Ontario, so please visit the Corn ipmPIPE website at http://corn.ipmpipe.org/tarspot for updates and target scouting (Figure 1).

2) Scout for tar spot and other diseases in your fields regularly. Tar spot appears as small, raised, black spots scattered across the upper and lower leaf surfaces (Figure 2). 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 which are known as “fisheye” lesions.

3) Keep in mind, other diseases such as rust and physoderma brown spot as well as insect frass (poop) can 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.

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

For more resources and information please visit:

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/

Sudden Death Syndrome

Sudden death syndrome developed early this year in the southwest and symptoms have increased the past few weeks due to the crop stage (post flowering) and weather conditions.  The SDS fungus produces toxins in soybean roots that are transported to leaves and foliar symptoms are more severe after frequent or heavy midseason rains such as what we have had.  SDS infected plants will begin to show distinct yellow speckling or spotting between the veins (interveinal chlorosis). As the disease progresses, these yellow areas become larger and in severe cases the entire area between the veins becomes brown (necrotic) and dry (Figure 3). The leaves curl from the outer margin. These brown areas eventually disintegrate (fall out) leaving only the veins on the leaf. All through this process the veins remain green. The next symptom is often premature defoliation and death of the plant. In most cases, the petioles (leaf stalks) on these dead plants remain attached to the stem.

Figure 3. SDS leaf symptoms (left) and stem (right)
Figure 3. SDS leaf symptoms (left) and stem (right)

Cutting the root and the stem will expose a light brown or gray discolourization of the cortex tissue in the tap root and the stem (Figure 3). This discolourization will extend from the roots up through the stem. Although the leaf symptoms may resemble brown stem rot, in sudden death infected plants the pith (center) of the stem remains white whereas brown stem rot infected plants have a brown and disintegrated pith.

There isn’t anything that can be done at this stage for SDS, but growers should compare the various varieties they have on the farm or neighbouring fields to help in selection of SDS tolerant varieties for 2022. Consider using a SDS seed treatment such as ILEVO or Saltro which when combined with SDS resistant varieties have shown in combined regional field tests in Ontario and the Midwest US to reduce risk and disease symptoms but most importantly increase profits with a 9.5 bu/ac yield increase compared to these same seed treatments on a SDS susceptible variety (Figure 4).

Figure 4. SDS see treatment trials from 2013-2019
Figure 4. SDS see treatment trials from 2013-2019

Weather Data

Location Year Weekly August 9 – August 15 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 31 12 53 428 2373 1706 2490
2020 31 13 0 296 2289 1635 2497
2019 28 13 2 370 2219 1562 2316
Ridgetown 2021 30 9 34 397 2235 1577 2341
2020 30 12 0 298 2182 1543 2361
2019 28 11 9 514 2100 1446 2183
London 2021 30 9 24 329 2217 1565 2312
2020 30 12 19 330 2127 1490 2275
2019 31 11 25 452 2004 1374 2106
Brantford 2021 31 9 7 289 2188 1533 2265
2020 32 12 3 210 2147 1516 2261
2019 30 10   274 2075 1433 2166
Welland 2021 31 16 11 303 2202 1544 2306
2020 31 13 0 269 2179 1541 2363
2019 28 10 5 362 2137 1479 2260
Elora 2021 30 10 5 265 2027 1381 2103
2020 30 10 0 280 1971 1359 2116
2019 29 8 2 359 1641 1068 1690
Mount Forest 2021 30 8 8 328 2032 1390 2122
2020 30 11 0 387 1948 1349 2129
2019 28 9   129 1869 1245 1978
Peterborough 2021 31 7 7 300 2028 1372 2081
2020 31 9 8 201 1958 1348 2074
2019 29 6 2 334 1851 1223 1879
Kingston 2021 28 11 27 238 2089 1433 2182
2020 29 15 2 268 2131 1487 2320
2019 27 12 4 354 2057 1403 2163
Kemptville 2021 31 9 24 264 2185 1523 2227
2020 31 12 14 235 2109 1477 2245
2019 29 8   203 1958 1326 2013
Earlton 2021 29 8 30 452 1888 1251 1905
2020 32 10 3 312 1773 1223 1956
2019 27 6 1 349 1543 995 1631
Sudbury 2021 29 9 49 366 1940 1303 1996
2020 29 14 19 380 1842 1268 2034
2019 27 9 1 351 1573 1025 1669
Thunder Bay 2021 34 5 8 247 1784 1162 1817
2020 29 9 11 181 1645 1102 1792
2019 27 6 1 265 1486 935 1553
Fort Frances 2021 33 1 16 202 1881 1252 1956
2020 28 7 82 289 1785 1203 1929
2019 27 6 0 292 1612 1032 1681
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.