Tar spot was hardly a blip on the radar until a decade ago when it first arrived in Indiana and Illinois cornfields. Now, the foliar disease causes major grain losses across the region each year, and it poses a threat to corn silage yield and quality as well.

Tar spot thrives in cool and humid environments that allow for long periods of leaf wetness. It prefers temperatures between 60°F and 73°F with a relative humidity above 75%, so it should be no surprise to Midwestern farmers when the fungal disease develops when these conditions are present during the first half of the growing season. In fact, many states reported tar frequenting cornfields during the month of June.

The foliar disease appears as small, yellow lesions that turn brown or black with a chlorotic border, and infected corn experiences early senescence and drying. Most tar spot research has been centered around its impact on grain yield. But considering the trend of higher corn silage inclusion rates in dairy rations, a team of researchers at Michigan State University questioned how tar spot affects the forage yield and nutritive value of corn silage as well.

In their study, which is published in Crop, Forage & Turfgrass Management, Harkirat Kaur, Martin Chilvers, Kim Cassida, and Maninder Pal Singh found that corn silage that was affected by tar spot had lower neutral detergent fiber (NDF) digestibility and predictive milk yield. They also found that the foliar disease negatively affects corn silage crude protein (CP) and starch content. And increased tar spot severity caused up to 27% dry yield loss in corn grown for silage.

Collecting data

From 2021 to 2023, the researchers conducted field trials across six locations in central and western Michigan to quantify tar spot’s impact on forage yield, nutritive value, and predictive milk yield. The trials included two corn hybrids — one with partial resistance to tar spot and one with no resistance to tar spot. They also included three fungicide treatments: one application at silking, two applications at R1 and R3 stages, or no application. The researchers note that all other management was based on general farming practices of the region.

Fields were scouted every week to monitor plant growth stages for timely fungicide applications. Tar spot evaluations began at first incidence or R1 stage and then were performed on a weekly to 10-day basis. The researchers rated 20 ear leaves for tar spot in each plot and estimated disease level throughout the field before making final disease evaluations at harvest. After harvest, representative samples were collected from each plot and tested for NDF, acid detergent fiber (ADF), in vitro true digestibility, NDF digestibility, CP, and starch.

When scouting for disease, the researchers divided tar spot severity into three categories: low (1% to 5%), medium (6% to 10%), and high (11% to 20%). Then, they estimated mean differences between tar spot severity and forage yield.

Variable interactions

Tar spot symptoms showed up in three plots throughout the experiment — the Branch location in 2021 and the Ottawa location in 2021 and 2022. The partially resistant hybrid had 50% lower tar spot severity than the susceptible hybrid at both locations.

Fungicide applications did reduce tar spot severity compared to nontreated corn; however, the researchers note that there was no observed difference in tar spot severity between the plots that received one application versus two applications.

Partially resistant hybrids had greater yields than susceptible hybrids at the affected plots, which was likely related to lower tar spot severity during these site-years. With that said, fungicide applications were not shown to improve or protect dry yield in any plot. Since overall disease severity was low, the researchers suggest that applying fungicide to protect yield will likely only be economical with higher tar spot severity.

Although there were no observed interactions between forage nutritive value and hybrid resistance or fungicide applications, the researchers note NDF digestibility and predictive milk yield was lower in plants affected by tar spot. With that said, fungicide applications did not appear to affect forage nutritive value in any plots.

An early harvest advantage

Since tar spot infections were not severe in this study, the researchers note it will be critical to conduct further research under higher disease pressure. They will also explore tar spot severity in more hybrids and fungicide treatments. With that said, they conclude that tar spot does reduce forage yield and nutritive value of corn silage and requires an integrated approach to disease management.

Another important factor of the results was the weather. Even though observed conditions during the experiment were consistent with 30-year averages on the Michigan farms, two locations — Ingham and Barry — had the wettest July on record in 2023, followed by a dry September.

The timing of corn silage harvest can also influence disease incidence. “Our field trials did not show high disease severity, probably because infections began in late August and silage was harvested between early to mid-September, thus providing a very little window for disease progression,” the researchers write.

For this reason, they suggest one solution for grain producers could be to harvest their crop for silage instead of letting it reach physiological maturity if tar spot severity is concerning by early dent stage. At this point, disease severity and yield loss will likely only go up.

“Overall, the most practical approach to combat tar spot in silage corn is incorporating disease resistance, field scouting, and timely fungicide applications as a part of integrated management,” they write.