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Release Date: 10/13/2022

By: Dr. Manni Singh, MSU

Tar spot is a fungal disease of corn that has recently become a threat in Michigan and the North Central Region. Tar spot gets its name from the small, raised, black and circular spots that form on leaf surfaces called stromata. As disease severity increases, stromata are surrounded by halos of dead tissue, which, over time, can cause leaf blight and death of plants. The infection results in early and rapid tissue death, reduced ear size, poor kernel fill, and potential for lodging. Major knowledge gaps exist within the plant-pathogen interactions of this disease. Tar spot requires plant metabolites (such as carbohydrates produced in photosynthesis) as a food source to grow and reproduce. This, coupled with the leaf lesions reduce healthy leaf surface area for photosynthesis and carbohydrate production, will eventually reduce yield. A better understanding of how different corn hybrids respond to this pathogen and when can fungicides be applied to reduce the negative effects on corn growth and productivity is critical .

Project objectives: 

1. To evaluate the effects of tar spot severity on photosynthetic metabolism and metabolic defense responses in corn hybrids with variable levels of resistance to tar spot and at varying levels of fungicide treatment

2. To understand the impact of tar spot and fungicide treatments on whole plant carbohydrate relations, dry yield, and quality of corn grain.

3. To associate changes in photosynthetic capacity to visual leaf lesion area using a non-linear model to determine predictiveness of visual assessment on disease severity.

To date, three hybrids differing in their resistance levels to tar spot (resistant, tolerant, and susceptible) and three fungicide treatments, 1) non-treated 2) one fungicide application at the silking stage and 3) two applications of fungicide (one at silking and one at dough stage) using Delaro 325 SC @ 8oz acre-1 were planted at two locations (Ingham and Ottawa) in Michigan. Plots at Ingham were exposed to disease spores at V8 by spreading 150 grams of diseased corn leaves (collected in fall of 2021) per plot.

Additionally, a small plot study was initiated to simulate leaf loss that could occur due to various biotic or abiotic stressors during multiple reproductive growth stages (Figure 1). This could help improve understanding of source-sink relations and impacts on corn yield and quality. Information generated can be used to identify stages critical for maintaining healthy leaf area and preserve yield by making timely management decisions.

When asked about the value of this research to farmers, Dr. Manni Singh said, “At early to middle reproductive growth stages for corn, there is still time for tar spot to have an impact on the ability of the corn plant to fix carbon – which ultimately impacts yield potential. There are many things we still don’t know about the effects of this disease on corn physiology and yield potential. This study will hopefully get us closer.”

Next steps: Tar spot scouting is ongoing on a biweekly basis and photosynthetic measurements of corn plots began the first week of August (Figure 2). Plants from the border rows of research plots will be destructively sampled and brought back to the lab for quantifying various stress indicators. The data gathered from this research will help in understanding the response of corn plant to biotic stresses. Upon completion of this research cycle, data from both years will be analyzed for trends and the results will be published in the CMPM annual research report, which will be released in early 2023.

Figure 1: Corn plants are defoliated to simulate leaf loss due to tar spot and other stresses. Leaf area below the ear leaf was defoliated early in the reproductive stages (silking) in this picture to simulate yield losses due to various biotic or abiotic stressors.

Figure 2: In-field measurement of leaf photosynthetic rate using LICOR 6400XT by graduate student (Harkirat Kaur). Leaf chamber when clipped to a leaf creates a closed loop with gas analyzer and measure amount of carbon fixed among multiple other parameters.