Tar Spot and Fungicides on Corn: Will it Pay?

One of the hottest topics this past winter revolved around how widespread tar spot would be in the summer/fall of 2025 and how to manage it. The main management decision is whether and when to apply a fungicide. Many of us didn’t make a definite decision, preferring to proverbially kick the can down the road until we had more information. Fortunately, or unfortunately, the time to decide whether to apply fungicides on corn for northern corn leaf blight (NCLB), gray leafspot, and/or tar spot is here.

If you haven’t made the decision yet, here is the pertinent information that plant pathologist Darcy Telenko presented at our annual meeting this past February.

Presence of tar spot:

● Past history- Tar spot was first identified in western NY in the fall of 2021. It could be easily found in Chautauqua, Erie and western Cattaraugus counties in the late fall of 2024. It was found, but was less prevalent in counties to the east. This is important. In previous years, the spores would have blown in from infected residue on corn to the west. Now it will be starting locally.

Weather matters for tar spot:

Scouting- There are no defined scouting procedures for tar spot, but it is important to keep an eye out for early development as it may help to determine when a fungicide should be applied. Although we will not be systematically scouting for tar spot, the staff of WNYCMA will be on the lookout as we visit your fields throughout the summer and early fall. It has been reported that some tar spot was found in Niagara County last week.

If the risk of tar spot in an area is high, there are other considerations to take into account before spraying.

Susceptibility of hybrids planted

● Resistance- Many companies have a rating of their hybrids for resistance to tar spot. There are no hybrids that are 100% resistant, however, a quick look at your seed catalog can help identify if you have any highly susceptible hybrids that may need more protection.

● Research studies of fungicidal treatments, either at planting or with an herbicide application are unlikely to give any significant protection against tar spot.

Value of the crop we are protecting

● The value of the crop to be protected should factor into whether a fungicide is likely to pay. Obviously, corn at $4 per bushel, will take a larger yield response to pay for the fungicide versus $7 corn. A calculator on the Crop Protection Network site can be used to help make that decision. The calculation is similar whether the crop is intended for silage or grain.

● Yield potential of stand. Surprisingly, the health of the crop has little effect on the potential for infection or the % yield loss. In other words, droughty or nutrient deprived corn is no more susceptible than healthy corn. Fungicide applications on stands with poor population due to weather, compaction, etc. are less likely to pay because the yield potential is lower.

How much yield do we stand to lose

● If fields are infested early with tar spot and the weather is cool and dry during pollination, losses can be substantial. In our area, late developing infestations have not resulted in significant yield losses.

How much does it cost to make an application? There are a number of products out there that reduce tar spot symptoms. Costs can range from $30 to $60 per acre depending on whether a farm does one or two passes.

Are fungicides effective? One past research study showed an average $29.2 to $48.5/acre net return under high tar spot disease pressure relative to no fungicide treatment. However, under low disease pressure the average was -$25.8 loss to $1.6/acre net return.

If you do decide to apply a fungicide, what should the timing be?

● Michigan State University Extension recommends fungicide applications from VT/R1 to R3. (Tassel to milk stage) Most pathologists do not recommend spraying before V10. The goal is to protect the ear leaf and leaves above it, which are most important for grain production.

Conclusion:

This year has been incredibly challenging. Fortunately, the current weather- hot and humid with less fog than some years- is less than ideal for tar spot. So far, very little has been found in western NY. If you have historically sprayed for northern corn leaf blight and/or gray leaf spot, an application at tasseling to silking is coming right up. We will continue to monitor corn fields.