Summer Fungal Disease in Grass: Leaf Spot
Fred Newcombe

In an organic program, when good cultural practices are followed, we seldom encounter summer fungal disease in grass. Although, when diseases present, leaf spot is perhaps the most common we have encountered. Leaf spot refers to a series of fungal diseases that attack many lawn grasses and are caused by many different fungi. Lesions—often brown or tan in color—form on the turf blade.  Eventually, they will spread and encircle the leaf blade   

Signs of Leaf Spots on Lawns and Athletic Fields

Grass is weak and thin with small, irregular spots/patches throughout the turf blade. Spots will vary in color and size. See image below to recognize this summer fungal disease in grass…

image of summer fungal disease in grass, leaf spot
Source: University of Minnesota Extension
Causes of Leaf Spot

Humid conditions – often accompanied with frequent irrigation usage. 

  • Improper watering – watering frequently especially late in the afternoon or evening. 
  • Poor mowing – mowing when turf is wet, mowing too frequently; mowing too short. 
  • Excessive nitrogen – while it is unlikely to introduce excessive nitrogen in an organic program, it is possible (think of the overuse of Corn Gluten Meal seeking crabgrass pre-emergent properties). 
  • Compacted soil – that does not allow water to infiltrate contributing to excess moisture around the turf plant.  
  • Acidic soils – since they tend to favor fungal dominated microbiology