Brown Spot

• Cool-season grass types such as fescues, ryegrasses and bluegrasses are highly susceptible to contracting brown spot. Gardeners should look for brown areas that are 6 to 12 inches in diameter, according to Cornell University. Grass blades will have tan centers with brown edges. Thick thatch or thatch that is over 1/2 an inch thick can promote brown spot. Applying too much nitrogen to the lawn will also weaken the grass and cause brown spot. A fungicide can be applied to treat the lawn area.
  
  

Leaf Spot

• Leaf spot is caused by fungal spores called bipolaris sorokiniana. This disease is one of the most destructive ailments for bluegrasses, according to North Dakota State University. You may see spots in the yard that are purple around the edges and tan in the center. If left untreated, the fungal disease can spread down to the root system or "melt out." Once it is in the root system, the entire grass blade and roots can die out. Treat leaf spot with a fungicide when you first see signs of the disease.
  
  

Rust

• Gardeners may see signs of rust when summers have been especially hot. Rust quickly spreads when air temperatures are between 85 to 95 degrees Fahrenheit, according to North Dakota State University. This disease causes the grass to turn yellowish-orange to red. You may be able to control rust by applying the proper amount of fertilizer and water to the lawn. Keeping the grass at its recommended height will help keep blades healthy enough to fight the disease.
  
  

Snow Mold

• Snow mold is identifiable right after the snow has melted off the lawn. There are two types of snow mold: gray snow mold, or typhula blight, and pink snow mold, or fusarium patch. Gray snow mold causes grass to turn straw-colored with gray mold growing on the surface. Pink snow mold will make the lawn look bleached out with pinkish mold growing on the top. You can prevent both types of snow mold by keeping the lawn mowed in the fall. Fungicide may be applied to lawns with a history of snow mold.