Tar Spot Outlook

When should farmers begin scouting for tar spot this season?

The best time to start scouting was a week ago, but the next best time is now! Our first reports of tar spot in Pennsylvania came from Berks and Snyder Counties during the last week of June, with additional sightings in Lancaster and York Counties as of July 1. The disease was present, albeit limited, in these locations. This pathogen is favored by periods of wetness and relatively cool temperatures, which means the first half of June presented great conditions for infection. The blazing heat that descended at the end of the month will likely slow down new infections.

What are the best management recommendations?

• Know your hybrid and its purported level of resistance. Understanding how tolerant your corn is to both tar spot and grey leaf spot can help you determine the benefits you may or may not receive from a fungicide treatment.  This can also allow you to properly identify less-typical symptoms in resistant hybrids.
• Many areas of the state were affected by weather this spring, which delayed corn planting over a wide time period.  Because of this, we will need to monitor the onset and development of tar spot in our crops over a wider window. This will make it difficult to time a fungicide at the optimal period for the entirety of our corn, but it will also provide the benefit of spreading our risk, as at least some of our corn's most vulnerable growth stages are less likely to coincide with times of high disease pressure.
• Don't jump the gun on spraying. While we may see some symptoms on very young corn this year, our best results come from a fungicide application between V10 and R3. Remember that the priority for protection is from the ear leaf to the top of the canopy. These are the parts that contribute the most to grain fill and final quality. Scout to stay aware of disease progress in your fields, and if severity approaches 5% on the ear leaf or higher, yield impact is likely. Use this tool to train your eye for scouting.  

What is new, based on our experiences over the last few years?

• One spray for two diseases. Ongoing research at Penn State suggests that a single, well-timed fungicide application can effectively manage both gray leaf spot and tar spot. Visit the corn fungicide efficacy guide to view the best products for the management of both diseases. 
• Try out the new Crop Disease Forecasting Tool. This is a web-based tool that utilizes your local weather conditions to provide risk assessments for several field crop diseases, including tar spot. This tool anticipated the increased risk of early tar spot onset that we are seeing in the Mid-Atlantic. It currently predicts a lower risk of tar spot, but a high risk of grey leaf spot. Check your risk levels when your corn approaches tasseling to help you make fungicide decisions.