Everything you need to know about tackling tar spot

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Tar spot (Phyllachora maydis) is a relatively new corn disease in Canada, having spread across southwestern Ontario and is progressing into central and eastern areas of the province.

Tar spot gets its name from the fungal fruiting bodies it produces on corn leaves that look like spots of tar, developing black oval or circular lesions on the corn leaf.

Now considered an endemic disease in Ontario, tar spot inoculum can easily spread when environmental conditions are conducive to infection. Here’s a short summary of everything farmers need to know about how to prevent, identify and manage tar spot in Eastern Canada:

Conditions to watch for

Favourable environmental conditions that promote tar spot infection and disease development are cool 16° to 20°C, humid conditions (>75% relative humidity) with long periods of leaf wetness (greater than seven hours). Frequent cloudy days and nights with more than seven hours of dew also signal favourable disease conditions.

Tar spot is polycyclic, and capable of completing its life cycle several times within a growing season. Under conditions with prolonged leaf wetness, the disease moves up the plant infecting new leaf tissue and slowly diminishing the plant’s ability to capture sunlight and photosynthesize.

Identifying symptoms

Tar spot can rapidly spread through the corn canopy under favourable conditions, causing premature leaf senescence.

Scouting for this disease starts in late June or early July (ahead of flowering) to catch the earliest infections and requires walking through fields – drive by scouting isn’t an effective strategy to identify tar spot.

Initial symptoms are small brown lesions that darken with age and the texture of the leaf becomes bumpy and uneven. The fruiting bodies that develop when a plant is infected appear as small black spots, like what tar might look like when it is splattered onto a surface. These are called stroma and can appear in the lower canopy or upper canopy, depending on environmental conditions and winds that cause disease transmission.

Under favourable conditions, tar spot spreads from the lowest leaves to the upper leaves, leaf sheathes, and eventually the husks of developing ears.

Disease impact

Tar spot reduces yield by reducing the photosynthetic capacity of leaves and causing rapid premature leaf senescence.

This disease can significantly reduce crop yield and quality, and even cause plants to prematurely die. In situations where corn plants are not killed by the tar spot, but are still heavily infected, the reduced efficiency of photosynthesis resulting from leaf senescence can impair the plant’s ability to maximize grain fill and reach its full potential in kernel weight, a significant component of crop yield.

Affected ears can have reduced weight and loose kernels, and kernels at the ear top may germinate prematurely. Severe infections can cause leaf necrosis. Severe tar spot infestations have also been associated with reduced stalk quality.

Crop management

Tar spot is manageable, but once a field has become infected, the risk of disease will always be present in future growing seasons.

The first step to manage tar spot is to select corn hybrids that are genetically tolerant to the disease. Farmers should check hybrid scoring for tar spot tolerance when making seed decisions that meet the agronomic and disease management needs of their farm. Next, farmers should be prepared to deploy a fungicide at the VT-R1 stage of crop growth. The optimal control timing of tar spot is when the risk of infection is high.

Forecasting tools are available to help farmers make tar spot management decisions. Tarspotter, a corn tar spot disease forecaster app was developed by the University of Wisconsin-Madison and uses research-based models to forecast the risk of tar spot fungus being present in a corn field. Using the app, farmers can input site-specific information about their corn field, which combines the information with models to predict the best timing for tar spot treatment.

Farmers can protect corn from tar spot with Acapela™ fungicide that provides four movement properties that quickly and efficiently surround, penetrate and protect corn leaves and stems to prevent infection on the entire plant, leading to more consistent performance and higher yield potential.

How Corteva Agriscience is helping farmers manage tar spot

We’re here to help – from disease prevention through to crop management.

Corteva scientists have been closely studying tar spot and screening hybrids to develop a rating scale to communicate a hybrid’s level of tolerance. The good news is, significant genetic response to tar spot has been observed and quantified by our team. Our robust research and development process contributes to our genetics plant health expertise, producing products that are built to withstand a range of diseases.

With one of the largest global corn genetics libraries in the world, Corteva has access to a diverse set of germplasm to develop new products that deliver superior plant health. Our company is dedicated to supporting farmers as they navigate this new disease, investing in ongoing research, genetic and seed development, effective crop protection products and on-the-ground agronomic support to control this new disease.

Just like the U.S., where tar spot has spread throughout corn growing regions, tar spot is expected to continue infecting fields throughout Ontario. Farmers need to be on the lookout for infection, scout diligently and include tar spot management practices into their crop plans each year to reduce the impact of this disease.

For more information or product recommendations for corn or tar spot management contact your trusted agronomic advisor, retailer, or local Corteva Agriscience Territory Manager.