Tar spot wasn’t a concern for most corn growers until recently. In the last few years, this fungal pathogen has made its presence known and is forcing growers to shift their strategies, along with the hybrids they grow.
Tar spot is a foliar disease from the fungal pathogen Phyllachora maydis. It’s endemic mostly to Central and South America, as well as parts of Mexico. However, it was first identified in the U.S. in 2015, and has been spreading ever since. Tar spot hasn’t been identified in every state – it’s mostly concentrated in the Midwest. It’s currently confirmed in Illinois, Indiana, Florida, Iowa, Michigan, Minnesota, Missouri, Ohio, Wisconsin and Nebraska.
The effects can be devastating: According to the American Phytopathological Society, tar spot has led to about 4.5 million tons of corn lost in the U.S.
So, if you’re a corn grower in those regions (or even elsewhere), it’s critical to protect yourself against tar spot – for the sake of your future yields.
What causes tar spot
Cool, wet weather. Temperatures between 60 and 70°F are ripe for tar spot development, along with humidity higher than 75%. Fields enter the danger zone after they’ve been exposed to weather that leaves moisture on leaves for seven hours or greater (this can be a long rainstorm, but also long-term fog or dew). Wind can also make a bad situation worse by dispersing the pathogen to other fields.
How to recognize it
You’ll likely see small, raised black spots on both upper and lower leaf surfaces—spots that can’t be scraped off. A tan or brown halo can also surround the black spots, creating a “fisheye” lesion.
Luckily, agronomic advancements have given us several mitigation strategies against tar spot. These range from minor reduction tactics to more significant control measures:
Rotate to other crops
Since residue can decompose and reduce primary inoculum, it can be helpful to plant fields with other crops before and after your corn season. Currently, it’s unknown how long these rotations should be to reduce inoculum. And, crop rotation won’t necessarily reduce your risk from locally dispersed inoculum (remember: the pathogen can be carried by wind).
Consider tillage: it buries disease residue and can increase the decomposition rate. However, this also won’t protect fields from locally dispersed inoculum.
If tar spot has already made its way to your fields, determine the level and dispersal of infection so you can time your fungicide application accordingly. In general, you should apply fungicide at the tasseling or silking stage. However, if there has been early disease establishment, this may warrant application at the vegetative stage. Another helpful tip is to use fungicides with multiple modes of action, which can help vary the plants’ uptake.
Avoid highly susceptible hybrids and choose one that minimizes your risk
Hybrids can range anywhere from fully susceptible (will show disease development the earliest) to partial tolerance (can hold off the disease for a week or two but won’t stop development).
By studying tar spot early and identifying less susceptible hybrids, AgriGold was one of the first seed brands to publish tar spot ratings for its hybrids – identifying several options with exceptional tar spot tolerance.
While it’s impossible to completely inoculate your fields against tar spot, you can still choose hybrids that give you the best chance at maintaining your yield.
For the best tolerance, try:
- Under 110 RM — A631-90, A636-16 (Conv, VT2), A637-55 (5222), A6424 (3111), A638-63WX, A639-40VT2
- Over 110 RM — A643-41, A645-80 (3110, GT, Conv), A647-46VT2
For top hybrids with high tolerance, try:
- Under 110 RM — A626-08 (VT2, STX), A628-16VT2, A629-22 (VT2, STX, Conv), A630-10STX, A636-11 (VT2, STX), A637-56 (VT2, Conv), A638-74VT2
- Over 110 RM — A642-99WX, A6499 (VT2, STX, Conv), A643-52 (VT2, STX), A6544VT2, A644-32TRC, A646-12 (VT2, STX), A647-90VT2
Ready to talk seed? Contact your AgriGold agronomist to find the tar spot-tolerant hybrid with the right fit for your unique fields.