Tar spot of corn: identification, look-alikes and when a fungicide pays

Symptoms and where to look

Stromata are 1/16 to 1/4 inch (1.6–6 mm) across, round to irregular, slightly raised and firmly embedded in the leaf tissue, so a heavily infected leaf feels bumpy. They appear on both surfaces of green or dying leaves and, in severe cases, on leaf sheaths, husks and tassels. Some stromata develop a tan-to-brown halo with a darker border, called a fisheye lesion; the cause of the halo in the United States and Canada is not settled.

Early infections can be found as early as V5–V6 in fields with a history of the disease, but in most Corn Belt and Ontario fields the epidemic builds from July through August, after canopy closure. New stromata form 12 to 21 days after infection, so there are several cycles in a season. Purdue scouts fully expanded leaves at knee to hip height once corn reaches V8–V10, because that is where the first stromata are usually found. On susceptible hybrids, ear-leaf severity can exceed 50 percent by R5, and leaves begin dying prematurely at roughly 30 percent severity.

Look-alikes and how to tell them apart

The scratch test settles most cases: scrape the spot with a fingernail, or wet it and rub it (the "spit test" in Ontario). Tar spot stays. Rust pustules erupt through the epidermis and shed spores onto your finger. Late in the season, rust pustules mature into dark brown to black telia that feel bumpy and look very much like stromata; diagnosticians separate those by examining spores, so if the field decision depends on it, send a sample to your state or provincial diagnostic lab. Physoderma brown spot lesions are embedded rather than raised, sit on the midrib and sheath and in bands across the blade near the leaf base; tar spot stromata are not found on the midrib.

Conditions that favour it

Tar spot was first confirmed in the United States in 2015 and in Ontario on September 21, 2020, at Ridgetown; by the end of the 2021 season it had reached Wellington, Bruce and Grey counties. It is now established across much of the Midwest, the Northeast and southern Ontario.

The fungus overwinters in infested corn residue on the soil surface, and stromata can produce viable spores for a season. Ascospores and conidia are released under rain and high humidity and move by rain splash and short-distance wind, with longer-distance movement between fields also documented. Development is driven by moderate temperatures of about 60–74 °F (16–23 °C) and moisture events. Prolonged leaf wetness and high humidity favour infection, although Crop Protection Network (CPN) notes that constant rain or humidity is negatively associated with development, and that alternating wet and dry periods appear to matter most. Overhead irrigation lengthens leaf wetness and can create favourable conditions in an otherwise dry year. Corn after corn with heavy residue is a slightly higher risk, but rotation does not remove the risk because spores arrive from neighbouring fields.

Scouting and thresholds

Start walking fields at V8–V10 and check the lower canopy weekly through R3. Once corn reaches V10, run the CPN Crop Disease Forecasting Tool for your field; as of July 2025 it replaced the Tarspotter app, which is no longer supported, and it gives a daily risk value and a 7-day forecast for tar spot and gray leaf spot from local weather. Track confirmed county detections on the Corn ipmPIPE map.

There is no formal numeric threshold. Purdue's guidance is to hold off until the disease is active in the field and the crop is at or near VT–R2, and not to apply before V10. CPN reports that an application at dough (R4) can still pay when severity has reached 5 percent or more and grain fill is far from finished. Late in the season, use a push test on heavily diseased fields; if stalks are compromised, schedule those fields for early harvest.

Management

Hybrids. All commercial hybrids are susceptible to some degree, but they differ a lot. Use seed-company ratings and university performance trials, and put the most tolerant hybrids on fields with a history of the disease, on irrigated ground and in corn-on-corn.

Residue and rotation. Rotation and residue breakdown reduce local inoculum. Expect a limited benefit, because spores also arrive from neighbouring fields.

Fungicides. The products with activity on tar spot come from FRAC Group 3 (DMI triazoles, e.g., prothioconazole, propiconazole, mefentrifluconazole), Group 7 (SDHIs, e.g., fluxapyroxad, pydiflumetofen, benzovindiflupyr) and Group 11 (QoI strobilurins, e.g., azoxystrobin, pyraclostrobin, trifloxystrobin), most often as two- or three-way premixes. The Corn Disease Working Group publishes the CPN fungicide efficacy table every year, with ratings built from multi-year, multi-location field trials; use the current table rather than last year's memory, and follow Purdue's advice to pick a product with more than one mode of action. Most products protect for about 14 to 21 days, some longer depending on the active ingredient. Rates, intervals, preharvest intervals and grazing restrictions are on the label registered in your state or province. In Ontario, confirm the product carries a PMRA registration (PCP number) for tar spot; a US registration does not carry over to Canada.

When a spray pays and when it does not

A spray is most likely to pay when stromata are present in the lower canopy by VT–R2, the hybrid is rated susceptible, the forecast is cool and wet, and the field has high yield potential with most of grain fill still ahead. In a 2024 northern Indiana trial, a VT/R1 application protected 7–18 bu/ac (0.4–1.1 t/ha) against the untreated check. CPN puts yield loss in a moderate epidemic at 10–30 percent and in a severe one at 50 percent, which is the yield a timely single pass is protecting.

A second pass is a field-by-field call. It can pay when disease keeps increasing after a VT–R1 application, the weather stays favourable and the crop is still at R3 to early R4. In Michigan a late second application returned 20 bu/ac (1.3 t/ha) in 2019 and nothing in 2020, even with disease present.

A spray does not pay when there is no disease in the field at tassel and the forecast is dry; Iowa State found no yield effect from fungicide at any of six locations in 2022, and its pathologists note that corn yields are rarely higher after a fungicide when disease is absent. It also does not pay before V10, because the residual runs out before the infection period that matters, nor after R4–R5, because a fungicide cannot repair leaf area already lost. Leave two untreated check strips in every sprayed field so next year's decision rests on your own numbers.