While corn is one of the most resilient crops to grow, we also know it’s prone to disease and fungus that threatens its ability to thrive in Iowa. In 2015, Illinois and Indiana had an infestation of a fungus called tar spot. As of 2022, Iowa State University has identified Tar Spot in all of Iowa’s 99 counties. Even though advances in fungicide and knowledge of how to prevent tar Spot has expanded, Tar Spot still poses a serious hurdle that Iowa farmers are going to have to overcome in their future harvests.

Tar Spot can be identified as early as the first week of August to as early as late June or early July — depending on weather conditions. Its unique properties allow it to rapidly spread across entire leaves in as little as two weeks. However, Tar Spot can be difficult to scout. In fact, the disease can be dormant in the corn’s system for up to 21 days before it can even be identified on the lower leaves. If tar spot is identified in early August, total yields can be impacted due to plants prematurely shutting down.

Ideal Climate Conditions

Tar spot's ideal climate conditions are:

Cooler temperatures (60-70 degrees)
High relative humidity (>75%)
Prolonged leaf wetness (~ ~ 7 hours)
Low sunlight and foggy weather

It’s important to remember that these conditions can all occur during nighttime hours even if daytime temperatures rise above its desired range. It typically infects lower leaves first before spreading up the entire canopy.

Suppression is Crucial

Most farmers are going to spray their respective fungicides at VT (tassel time). All hybrids are going to have a different tolerance to Tar Spot and how the applied fungicide responds. Spraying fungicide just once per season sometimes won’t be enough to keep the disease from resurfacing later in the season. Our general recommendations for 2023 are:

Start with a tolerant hybrid with good late season plant health
Observe the weather for favorable tar spot conditions (a wet June and July) and be ready to spray fungicide at VT with at least 14% group 11 strobilurin.
Every acre is different, and not one hybrid will react the same way to tar spot.

Suppressing Tar Spot is going to be crucial to maintaining your target yields for the 2023 season. Landus account leads and our GROW Solutions Center agronomists are ready to guide you through processes and practices we recommend.