Agronomy •  2023-05-11

Agronomy Advice: How to deal with tar spot

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tar spot on corn

Hot, dry growing conditions in Eastern Canada last year meant 2022 wasn’t a year for tar spot. It’s hard to predict if weather conditions will be conducive to the disease year over year, but one thing’s for certain, you can’t let your guard down because tar spot is here to stay in Ontario, and it can wreak havoc on yields.

A relatively new corn disease, tar spot has been confirmed across southern Ontario corn fields, and even overwinters in corn residue. In fact, tar spot thrives in cooler, moist weather growing conditions, making it more prevalent in the Great Lakes region. This map  shows where tar spot has been confirmed across North America. If disease development conditions are right (cool temperatures (60-70oF, 16-20 0C), high relative humidity (>75%), frequent cloudy days, and 7+ hours of dew at night), the disease replicates quickly within a 21-day cycle. Tar spot’s ability to sporulate ballistically, or shoot infected spores far and wide, accounts for its rapid transmission.

A yield-robbing disease, tar spot kills the plant’s leaves, effectively killing the plant. Under severe infestation, where most of a field shows 50% severity on the ear leaf during grain fill, significant yield losses of 20-60 bu/ac have been reported.1 If infection is not identified soon enough, or the disease is left untreated farmers face a significant yield loss.

Start with hybrid selection

Dealing with tar spot requires a combination management strategy of in-plant tolerance and fungicide application. Selecting corn hybrids that offer tar spot tolerance is your first level of defense against the disease. Genetic resistance to tar spot should be the number one consideration when it comes to managing the disease, especially if tar spot has been confirmed in your area. Research has shown that corn hybrid genetics have a greater impact on symptoms and yield loss than either cultural or chemical management practices.2

In-season control

June marks the start of tar spot scouting season. This disease is very weather dependent, so if the forecast is calling for humid conditions, start scouting, especially if tar spot has been confirmed in the area or field in previous years. Initial symptoms are small brown lesions that darken with age. For more scouting and disease identification tips, click here.

In-season fungicide applications can help control the disease. Be sure to consult your trusted agronomist, retailer, or local Corteva Agriscience™ Territory Manager for advice on products, rates and application timing to manage the disease.

What’s next?

Corteva scientists have been studying tar spot and screening hybrids to develop a rating scale to communicate a hybrid’s level of tolerance. A large network of research plots in Eastern Canada have been established and are already helping to characterize more products to manage tar spot, including genetic responses to support disease tolerance. Corteva is also collaborating with industry partners and extension agencies to research and monitor this significant disease.