By D. E. P. Telenko, M. I. Chilvers, N. Kleczewski, D. L. Smith, A. M. Byrne, P. Devillez, T. Diallo, R. Higgins, D. Joos, K. Kohn, J. Lauer, B. Mueller, M. P. Singh, W. D. Widdicombe, and L. A. Williams.
Tar spot of corn caused by Phyllachora maydis is a new disease to the United States that was first observed in 2015 in Indiana and Illinois (Ruhl et al., 2016). Since then, it has also been confirmed in Florida, Iowa, Michigan, Ohio, and Wisconsin (McCoy et al., 2018; Dalla Lana et al., 2019). In 2018, a tar spot epidemic occurred in the upper Midwest, which had a significant impact on corn grain and silage yield and quality (Figure 1) (Kleczewski et al, 2019).
Figure 1. The 2018 distribution map of tar spot infections confirmed in the US (Image source: Kleczewski et al. 2019. Corn Diseases: Tar Spot. CPN-2012)
Little field screening has been done in the US for corn germplasm resistance to tar spot. The 2018 epidemic created an opportunity to assess hybrids for resistance. Corn hybrid performance trials are conducted annually by most land grant universities in the Midwest across their respective states. Participating seed companies provide selected hybrids for testing in each states’ trial. Hybrid trial locations with significant natural tar spot pressure were selected for disease evaluation in northern Indiana and Illinois, and southern Michigan and Wisconsin, which allowed for collection in 2018 of initial data on hybrid grain yield impact.
For each state, trials were available through the state variety testing program. Plots were established in rain-fed fields and arranged in a randomized complete block design with 3-4 replications. Additional information for each location can be found in Table 1. Hybrids were rated at the R6 (physiological maturity) growth stage by estimating the amount of tar spot, either only on the ear leaves or on the ear leaves and above, on 3-6 plants or plot-wide estimates per plot. In some locations, percentage of canopy still green was evaluated at the same time. Values for each plot were averaged before analysis. In Michigan, gray leaf spot was also present at moderate levels, but this was not rated due to the difficulty distinguishing gray leaf spot from senescent tissue. Yield was determined by harvesting two rows of each plot and adjusting yield to 15.5% moisture. Mean disease severity and yield from all locations were combined to determine impact of tar spot on corn.
Table 1. Field site information where data was collected from state hybrid trials.
Variety Testing Program
Maturity range (days)
Disease rating date*
4 30-in rows, 23 ft long
29, 45, and 34
100-106, 107-110, and 111-113
4 30-in rows, 20 ft long
92-101 and 102-112
4 30-in rows, 22 ft long
62 and 64
98-106 and 104-113
2, 30-in rows,
31 Aug, 4 Sep
* No other foliar diseases were present in Wisconsin. Gray leaf spot was present in Michigan.
Hybrid reaction to tar spot
All hybrids rated in these trials were susceptible to infection by the tar spot pathogen. A range in symptomatology was noted from a minor infection with only a few stromata developing on the leaf (1-15% of the leaf surface with symptoms) to severe infection (40-50% leaf surface with infection) (see Figure 2). In addition, a necrotic halo formed around the stroma in some hybrids while there was no halo development in others. Less susceptible hybrids tended to have 15% or less leaf area where stroma develop, whereas highly susceptible hybrids had a range of 40-50% leaf area covered by stroma.
Figure 2. Tar spot leaf symptoms. Hybrid infection ranged from having both stroma with and without halo (A), large stroma (B), to only pin-point stroma at low 15% (C) to high 50% (D) severity.
A difference in the canopy greening was also noted – severe infections led to a rapid canopy senescence (Figure 3). While significant severity was noted, it was unusual to find ear leaves that were covered more than 60% by stromata. Once this level of coverage was reached, corn hybrids tended to senesce.
Figure 3. Hybrid reaction to tar spot infection. Both hybrids of the same maturity infected with tar spot in adjacent plots – A. Hybrid with >40% severity leading to rapid senescence. B. Hybrid with 25% disease severity, remaining green.
Impact of tar spot severity on hybrid yield
Illinois: The Illinois trial in DeKalb County, Illinois, had moderate to severe levels of tar spot ranging from 25-47% severity on Sept. 4 (Figure 4). For the early maturity group (102-106 day), yield ranged from 154-243 bushels per acre, and tar spot severity from 12-54%. The late maturity group (107-114 day) yield ranged from 127-266 bushels per acre, tar spot severity from 8-54%. In the early maturity group trial, yield loss was estimated at 0.49 bushels per acre for every 1% increase in tar spot severity, for the late maturity group trial, yield loss was estimated at 1.0 bushels per acre loss for every 1% increase in tar spot severity.
Figure 4. Tar spot severity of corn hybrids evaluated in Illinois in 2018.
Indiana: The 2018 Purdue Performance Trial in Wanatah, Indiana, had moderate to severe levels of tar spot (Fig. 5). All hybrids demonstrated some level of tar spot ear leaf infection on Sept. 18. For the early maturity group (101-106 day), average yield ranged from 222-239 bushels per acre, and tar spot ranged from 28-62%; the middle maturity group (106-111 day) yield ranged from 222-248 bushels per acre, and tar spot severity 23-57%; and the late maturity group (111-113 day) yield ranged from 223-245 bushels per acre, with a tar spot severity range of 13-50.0%. It was estimated that a 0.32-0.41 bushels per acre loss occurred per 1% increase in tar spot severity when averaged across these trials. Thus, when averaged across these trials, yield loss due to tar spot was estimated at 23-41 bushels per acre.
Figure 5. Tar spot severity of corn hybrids evaluated in Indiana in 2018.
Michigan: The 2018 Allegan County corn performance trial had low to moderate levels of tar spot (Figure 6). All hybrids demonstrated some level of tar spot. For the early maturity group (92-101 day), yield ranged from 206-258 bushels per acre, tar spot severity from 18-49% and percent canopy green from 8-73%. For the late maturity group (102-112 day), yield ranged from 209-264 bushels per acre, tar spot severity from 1-29%, and percent canopy green from 6-88%. The level of tar spot severity was inversely correlated with canopy greenness, supporting the observation that tar spot disease was causing premature leaf death or senescence. In the early maturity group trial, yield loss was estimated at 0.48 bushels per acre for every 1% increase in tar spot severity. For the late maturity group trial, yield loss was estimated at 0.61 bushels per acre for every 1% increase in tar spot severity.
Figure 6. Tar spot severity of corn hybrids evaluated in Michigan in 2018.
Wisconsin: In Wisconsin, severity on ear leaves ranged from 9.0–50% (Figure 7). In general, as severity increased, there was a corresponding decrease in green canopy, indicating that high tar spot severity led to rapid drydown. For corn that ranged from 98-106 days relative maturity, yield potential was estimated at 254 bushels per acre. For every 1% increase in tar spot severity on the ear leaves, a corresponding decrease in yield of 0.79 bushels per acre was measured for these hybrids. In the later relative maturity hybrids (104-113 days), yield potential was estimated at 262 bushels per acre with yield loss at 1.36 bushels per acre for every 1% increase in tar spot severity on ear leaves. Thus, averaging across both trials for hybrids with high tar spot severity, yield loss was estimated to range between 40-60 bushels per acre.
Figure 7. Tar spot severity of corn hybrids evaluated in Wisconsin in 2018.
The 2018 tar spot epidemic was the first time yield losses were documented in the US. Data from university hybrid trials in Illinois, Indiana, Michigan, and Wisconsin demonstrated a range in hybrid susceptibility and reaction to tar spot, and it is estimated that a 0.32–1.36 bushel per acre loss occurred per 1% increase in tar spot. Field observations also indicated that maturity may have influenced susceptibility and yield impact during the 2018 epidemic. To assess this, hybrids were grouped by relative maturity, 92-106 and 107-114 days, to evaluate the impact of tar spot severity on the ear leaf, rated near maturity (R5/R6), on yield. This preliminary data shows that for every 1% increase in tar spot severity on the ear leaf, there was a 0.48 bushel per acre yield loss for hybrids with 92-106 day relative maturity and a 0.83 bushel per acre yield loss for hybrids with 107-114 day relative maturity (Figure 8). These figures suggest that those fields that experience 40-50% tar spot severity on the ear leaf by R5/R6 saw a loss of 16.8 to 38.7 bu/A (1130 to 2605 kg/ha), which corroborates the growers’ reports from regions with severe infections reporting losses of 20-60 bushels per acre (1,345-4,035 kg/ha).
Figure 8. Impact of tar spot severity on the ear leaf, rated near maturity (R5), on corn hybrid yield in Illinois, Indiana, Michigan and Wisconsin performance trials. Corn hybrids were grouped by relative maturity (92-106 day and 107-114 day).
Discussion of hybrid maturity and tar spot severity.
Farmers in these states with a history of tar spot and adjacent states are urged to scout their fields during the growing season to ascertain if the disease is present. Although fungicides are available to aid in managing the disease, the use of partially resistant hybrids is an essential component of tar spot management, along with crop rotation and tillage. Understanding the underlying factors impacting disease resistance will provide producers and the agricultural industry with a cost-effective means to manage this disease.
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