Grass Diseases Underfoot Now

Leaf spot causes brown spots on leaf blades.
Leaf spot causes brown spots on leaf blades.

By Christine Engelbrecht
Plant Pathologist
Iowa State University Extension

The cooler fall temperatures bring welcome relief from the summer heat. As we trade bare feet for shoes, the grass underfoot in our lawns is relieved to see some cooler, wetter weather. But the autumn weather also makes many fungi happy, and several fungal diseases of lawns may show up this time of year.

Perhaps the most noticeable disease of turfgrass is rust, a fungal disease that causes orange spots on the grass blades. The overall lawn can have an orange or yellow cast. A closer look reveals that the orange spots are actually raised pustules, full of bright orange, powdery spores that are easily picked up on shoes, pant legs or mowers that traverse the lawn. 

Rust typically first appears in midsummer, but often persists until fall. Rust does not kill the grass but can weaken the grass plants, making them more sensitive to other stresses, such as drought stress or winter injury.

Powdery mildew 
Powdery mildew is another fungal disease with distinctive symptoms. Affected grass blades appear to be covered with powdery white residue, and a lawn may have a whitish cast. Powdery mildew is especially common in the shade and in wet areas with little airflow. In some lawns, powdery mildew reappears in the same shady spots every year.  

Like rust, powdery mildew does not kill the grass, but can weaken it and make it more sensitive to other stresses.

Leaf spot
One of the most common diseases of turfgrass is leaf spot. This disease is especially common on Kentucky bluegrass, the main component of most Iowa lawns. Leaf spot causes brown spots to appear on leaf blades. Affected leaf blades may die and turn straw-colored, resulting in a diffuse thinning of the grass, sometimes called “melting out.” Leaf spot occurs in most Iowa lawns and usually does not cause major problems, but in severe cases the disease can cause death of large portions of the lawn. 

Managing diseases in the lawn
Management of all three of these diseases relies on keeping turf in good vigor through proper care:   

  • Overseed severely affected areas with a blend of grass cultivars that have resistance to disease.  

  • Fertilize appropriately, but avoid over-fertilizing in the spring. Lush new growth is often very susceptible to disease.  

  • Mow grass two inches tall in spring and fall, and 2.5 to 3 inches in summer. Avoid mowing when the grass is wet, as this can spread fungal spores throughout the lawn.  

  • When watering is needed, water early in the day so the grass blades have time to dry before night. Water infrequently but deeply rather than frequently and shallowly.

  • Core aerate to help reduce the level of thatch (the layer of organic matter between the roots and grass blades). Thatch should be less than 0.5 inch thick.  

  • Fungicides are usually not warranted for these diseases, but may be used when outbreaks are severe. Fungicides are not effective without proper cultural practices.

Proper care of the lawn can help minimize disease and ensure that when you venture outside there is always a healthy, lush carpet of grass underfoot.

Christine Engelbrecht, Plant Pathology, (515) 294-0581,
Jean McGuire, Extension Communications and Marketing, (515) 294-7033,

Three high resolution photos are available for use with this week's column:
Rust91507.jpg Caption: Rust does not kill the grass but can weaken the grass plants, making them more sensitive to other stresses, such as drought stress or winter injury.  Photo by D. Settle.
PowderyMildew91506.jpg Caption: Powdery mildew causes white residue on grass blades.
LeafSpot91506.jpg Leaf spot causes brown spots on leaf blades.