Tar spot, caused by the fungus Phyllachora maydis, was confirmed in Randolph County, NC in August 2026. The recent rains and lower temperatures likely helped to promote tar spot development. The development and spread of tar spot is generally favored by moderate temperatures between 60-80°F, humidity greater than 85%, and leaf wetness for more than seven hours per day.
First Report of Tar Spot of Corn Confirmed in North Carolina
(Updated: Sept. 1, 2026, 3:52 p.m.)
Scouting tips
Scouting will be especially important if wet weather continues. Phyllachora maydis is only known to affect corn.
The tar spot fungus produces small (1/16-3/4 inch), round to irregular, raised black structures called stromata (Figure 1) on both the upper and lower surfaces of corn leaves. Stromata may also develop on leaf sheaths, husks, and tassels. Occasionally, tan to brown ‘fisheye’ lesions with dark borders can develop around the stromata (Figure 2).
Physoderma brown spot, common and southern rust, and insect frass can be mistaken for tar spot. Table 1 summarizes the characteristics that can be used to distinguish among them.
Figure 1. Corn leaf with black stromata from tar spot
Table 1. Characteristics that differentiate tar spot from Physoderma brown spot, common and southern rust, and insect frass
|
Feature |
Tar Spot |
Physoderma Brown Spot |
Common and Southern Rust |
Insect Frass |
|
Appearance |
Small, round to irregular, raised black stromata; sometimes surrounded by tan-to-brown “fisheye” lesions |
Small gray to black spots embedded within leaf tissue |
Raised pustules; orange-red during much of the season but may become black when teliospores develop late in the season |
Dark material deposited on the leaf surface |
|
Location on plant |
Upper and lower leaf surfaces; may also occur on leaf sheaths, husks, and tassels |
Often concentrated near the base of the leaf and may occur on the midrib |
Primarily on leaf surfaces |
Anywhere insects have been feeding |
|
Distribution on leaf |
May occur along the entire leaf, often at greater density from the middle toward the tip; generally absent from the midrib |
Commonly clustered near the leaf base, including the midrib |
Pustules may be scattered across the leaf surface |
Irregularly distributed |
|
Texture |
Raised and bumpy |
Embedded in the tissue |
Raised and bumpy, especially when black telia develop |
Sits on the leaf surface |
|
Fingernail test |
Cannot be scraped off |
Cannot be scraped off because spots are embedded in the tissue |
Pustules cannot be scraped away completely; spores may rub off |
Can be scraped away |
|
Best distinguishing feature |
Raised black stromata that remain firmly attached and are generally absent from the midrib |
Flat, embedded Concentration near the leaf base and presence on the midrib |
Orange-red pustules earlier in the season or black, raised telia late in the season |
Easily removed with a fingernail |
Should I apply a fungicide?
When disease develops late, especially after R4-R5, yield impacts are often minimal. The optimal window for a single fungicide application is generally between tasseling (VT) and milk (R3) stages for suppressing disease, protecting yield and getting return of investment. Fungicides applied after R5 will not provide these benefits. So, this late in the season, no. But if you decide to make one or more fungicide applications, be sure to leave untreated check strips and calculate the associated costs and benefits so that you can accurately determine your return on investment. If you suspect tar spot in your field, contact your county Extension office and submit samples to the Plant Disease and Insect Clinic
Please feel free to contact me with any questions or concerns: daisy_ahumada@ncsu.edu
