Why do you see sooty mold on small grains?

By Alyssa Collins and Paul Esker

As wheat, barley and other small grains mature and approach harvest, there is always the hope they will stay clean and bright until their journey to the elevator is completed. Unfortunately, this is not always the case. Some years growers may notice a dark or dirty appearance in their glumes. Upon closer inspection, this material is a loose growth of dry mold, which is usually black but other colors may also be observed and the condition is referred to as sooty mold.

What is causing this?

Sooty mold of grain heads can be caused by a variety of fungi that naturally occur in our environment. These fungi are not good at directly causing injury to the plant, but instead, they take advantage of tissue that is already mature and therefore no longer alive. Once the small grain plant reaches maturity, the surface tissues on the stem and around the kernels no longer produce the same chemical plant defenses that they did when they were alive and so these saprophytic fungi can now start to grow on that tissue and use those nutrients. Normally, as farmers, we would consider this a useful thing because we want fungi to break down crop residue in our cropping systems. But in this case the process is taking place a little too early for us.

Why don't we see this every year?

Sooty mold is likely to be an issue whenever crop harvest is delayed. The longer the crop stands in the field following maturity, the greater the chance for an opportunistic fungus to become established on the senescing tissue. This is exacerbated and accelerated in wet or even humid conditions. Sooty mold is also more likely to be an issue in situations where the crop was previously stressed, and any injury or death occurred. Tissues injured by freeze damage or prolonged drought during the growing and ripening season are more likely to be affected by sooty mold, especially given humid conditions at maturity.

Will this hurt my crop or animals?

Sooty mold does not impact yield, but since it preferentially affects already damaged tissue it may appear that it does. If moist conditions persist ahead of harvest, this may progress into black point which is a discoloration of the tip of the kernel inside the glume. In this case, quality may be affected. There are no additional mycotoxins associated with sooty mold, but its presence may make it difficult to determine if the grain is also affected by Fusarium head blight or scab. If there is any question, farmers should conduct a test ahead of feeding.

What can we do about it?

You may already be taking steps that will help. Whatever management steps can be taken during the growing season to limit stresses to the crop will aid in the prevention of sooty mold. Preventing chemical burn, insect injury, and other diseases keeps tissues healthy. Those events such as freeze, hail, drought which are beyond the control of the farmer will also contribute to susceptibility.

Most fungicides applied for Fusarium head scab will also confer some protection against these weakly pathogenic fungi. The longer harvest is delayed by weather or other issues, the more opportunity sooty mold has to progress to a quality-limiting issue.

The key to avoiding trouble with sooty mold with timely harvesting. Typically, the quality preserved by an early harvest sufficiently offsets the costs of drying the grain by a few percentage points. If there is no avoiding harvesting dirty grain heads, consider protecting your lungs by wearing a dust mask.