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Release Date: 11/22/2023

Project leader: Maninder Singh

The effect of tar spot on corn yield has been well documented in Michigan since the pathogen was first detected in 2016. The effect of tar spot on metabolic responses within corn plants, however, is not well understood. This study will provide important information on photosynthetic losses caused by biotic stresses such as tar spot in Michigan corn. This information will help farmers in understanding disease thresholds and, ultimately, will help farmers make informed economic management decisions, alleviate losses, and improve farmer profitability.

This CMPM-funded tar spot research project has three objectives: to evaluate the effects of tar spot severity on leaf photosynthetic metabolism in corn hybrids with variable levels of resistance and fungicide applications; to understand the impact of tar spot and fungicide applications on metabolic defense responses, grain yield, and quality; and to quantify the effects of simulated leaf loss in corn plant on carbohydrate distribution, grain yield, and quality.

Corn was planted at two Michigan locations: East Lansing in Ingham County and the MSU Kellog Biological Station in Barry County on May 10, 2023, and May 17, 2023, respectively. Both locations are rainfed. Treatments at both locations included three hybrids differing in their resistant levels to tar spot (resistant, tolerant, and susceptible) and three fungicide treatments, (1) non-treated, (2) one fungicide application at the silking stage (R1), and (3) two applications of fungicide at label rate [one at R1 and one at dough stage (R3)] using Delaro 325 SC at 8 ounces per acre.

At both locations, stand counts were collected at the V6 growth stage. The Ingham County location was inoculated with tar spot at the V8 growth stage (and before a rain event) by spreading 150 grams of diseased corn leaves (collected in fall of 2022) per plot. A fungicide application at R1 was completed on July 25, 2023, at the Ingham County location, and on July 27, 2023, at the Barry County location for treatments 2 and 3. The second fungicide application at the R3 growth state for treatment 3 was completed in the third week of August.

Tar spot scouting is ongoing on a biweekly basis at both study locations but has not been detected in central or southwestern Michigan in 2023 as of August 14th, probably due to relatively dry and hot weather earlier in the growing season. However, rainfall in late August provided conditions favorable for tar spot development. Since tar spot generally develops later in season and has multiple infection cycles throughout the growing season, it is expected to eventually develop in field plots.

Photosynthetic measurements started in the first week of August (picture 1). Corn plants from the border rows of plots were destructively sampled and brought back to the lab for determining chlorophyll content, carbohydrate contents, hormone profiles, and other metabolites. Plots from both locations will be harvested at maturity and yield data will be obtained from the combine harvester. Grain samples will also be collected from the combine to determine yield components (kernel weight and kernel number per acre).

In addition to the two field sites, a greenhouse study is being conducted to understand any photosynthetic advantage provided by fungicide applications for disease-free plants. An experiment with two hybrids (one susceptible and one resistant) and two levels of fungicide (treated and non-treated) was set up in the greenhouse (picture 2). Fungicide application was done using Delaro 480 SC at 8 ounces per acre on August 8, 2023. Photosynthetic measurements similar to the field study will be taken for these plants.

To further understand how a decrease in photosynthesis (such as due to tar spot infection) can affect a corn plant, a field trial is being conducted to simulate leaf area loss due to tar spot and other stresses at the Ingham and Barry County locations, by removing leaves manually and using a defoliant (Drexel Defol 5). Defoliation occurred at three growth stages: silking, early dough (R4, ~silking+20 days), and dent (R5, ~silking+40 days). Defoliation levels include all leaves above ear leaf, all leaves below ear leaf (and including ear leaf), and a complete defoliation at R5 (Pictures 3 and 4). To simulate natural leaf area loss due to biotic stress, a chemical defoliant Defol 5 at 4.8 quarts per acre was also used for defoliating the lower canopy and whole plant at the R4 and R5 growth stages.

Next Steps: This is the second year of this research project. Upon completion, data from both years will be analyzed and the results will be published in the CMPM annual research report, which will be released in early 2024.


Picture 1: Photosynthesis measurement using Li-6400 on tagged leaves in the field.

Picture 2: Plants grown in the greenhouse to study the impact of fungicide on photosynthesis.


Pictures 3 and 4: Manual leaf defoliation (above ear leaf or below ear leaf) at silking stage.