Brown Patch

Ryegrasses and tall fescue are most susceptible, but Brown Patch also attacks Kentucky bluegrass and fine fescues.

Look for 6- to 20-inch diameter brown patches, sometimes with a with purplish-gray "smoke ring" border or a "frogeye" with green grass in the center. Leaves show tan lesions with brown edges.

Prevention includes reducing

Dollar spot

The name comes from the silver-dollar sized straw-colored spots this disease causes on putting greens. On longer grass, the shapes are more irregular.

When dew is on the grass, look for white cob-webby fungus. Blades of grass show straw-colored lesions with reddish-brown borders. Of the common lawn grass, bluegrasses and fescues are most affected.

Prevention includes:

• Proper mowing. Maintaining maximum height, if possible, and taking off no more than a third of the height when mowing).
  
  
• Maintaining adequate moisture without overwatering. Minimize leaf wetness by watering early in the day.
  
  
• Avoiding excess nitrogen. Dollar spot occurs less on nitrogen deficient turf.
  
  
• Choosing resistant varieties.

Fairy rings

Look for an arc or circle of lush green grass and/or toadstool or puffball mushrooms. The rings may be as large as 60 feet in diameter, though most are less than 15 feet. They usually occur in the same place each year, with the ring expanding outward.

The lush grass is from a release of nutrients (especially nitrogen) caused by the fungi. Some of the mushrooms associated with fairy rings are poisonous and should be raked up and removed regularly if small children play in the area.

Fairy rings are more of an aesthetic nuisance than threat to lawns. Sometimes there will also be a ring of dead grass caused by the fungus. More frequent mowing can minimize the difference in grass height. More frequent fertilization and heavier watering can even up grass growth and color, but may cause worse disease problems.

For more, see

Leaf spot

Leaf spots are most destructive during cold, wet, overcast weather in spring and fall. Look for gradual browning and thinning of grass. Small, dark-brown, purplish, or purplish-red colored spots appear on the leaves from the early spring to late fall. As these lesions increase in size their centers may fade to a straw or light-brown color. The spots are usually surrounded by narrow dark reddish-brown to purplish-black borders.

As the disease progresses in favorable weather, the spots run together and girdle the leaf blades. As the weather warms, the crowns, rhizomes and roots may rot. Plants lack vigor and wilt from moisture stress during the middle of the day.

Prevention includes:

• Proper mowing. Maintain lawn at maximum height and remove no more than a third of the plant when mowing.
  
  
• Keep thatch less than ½ inch.
  
  
• Don't overfertilize. Avoid applications before late May or early June. Avoid excess nitrogen, especially in spring.
  
  
• Avoid frequent light waterings. If you water during dry spells, apply enough to soak in 6 to 8 inches deep.
  
  
• Choose resistant varieties. Resistant varieties of Kentucky bluegrass and fescues are available.

Pythium

Pythium fungi can infest all commonly grown cool-season lawn grasses.

When Pythium attacks foliage, the disease is called cottony blight, grease spot, or Pythium blight. Outbreaks occur most often during hot, humid weather, and can spread quickly. Pythium can also cause root and crown rots in cool, warm or hot weather with high moisture.

Look for small patches up to 6 inches where the grass looks water-soaked and feels greasy or slimy. Patches often follow the direction of drainage or mowing.

To help prevent Pythium problems:

• Use low to moderate rates of balanced fertilizers. (High nitrogen favors the disease with some grasses.)
  
  
• Maintain soil pH in the neutral to slightly acid range.
  
  
• Avoid mowing when the grass is wet.
  
  
• Water early in the day so grass dries quickly.

Red thread occurs during humid periods in spring and fall when daytime temperatures are between 60 F and 75 F. Fine-leaved fescues and some ryegrasses are most susceptible, but it also affects Kentucky bluegrass and other fescues. Slow-growing, nitrogen-deficient turfs are particularly vulnerable.

Look for water-soaked patches of grass that turn to a bleached tan as grass dies. Patches may be round or irregularly shaped, from about an inch to a foot in diameter.

In humid weather, the fungus produces coral-pink to blood-red hyphae up to an inch long on the tips of grass blades. These red threads can disperse the disease to healthy turf by mowing and foot traffic.

To help prevent the disease,

Rust

The first signs of rust are yellow lesions. Later, spores cover the blades. They are usually orange but may also be yellow, red or brown. Wind and rain can carry the spores to cause new infections.

If the disease is severe, grass stands appear thin and discolored and the grass may eventually die. All lawn grasses are susceptible.

To prevent rust problems, keep leaves dry by

Snow mold

Snow molds appear during cold wet periods around the time of snow melt. Gray snow mold appears in roughly circular bleached patches up to 2 feet in diameter. Grass is often matted and surrounded by a white to gray fluffy halo of fungus. While unsightly, it rarely kills the turf. Infestations are worse after long, deep, compacted snow cover.

Pink snow molds appear similar to gray ones, but have a pinkish cast. They do not require heavy snow cover, and may kill turf.

To prevent snow mold:

• Avoid late fall fertilizer applications that might fuel succulent growth.
  
  
• Continue to mow turf in fall as it grows.
  
  
• Avoid compacting snow on top of turf during winter.
  
  
• Rake matted grass in mold-damaged areas in spring to encourage new growth.
  
  
• If molds are a regular problem, reseed resistant varieties.
  
  

For more, see Plant Disease Diagnostic Clinic factsheets: