Cool Weather Lawn Diseases
Cool weather lawn diseases most commonly occur during cool periods, especially in the spring.
Facts About Lawn Diseases
- Soil borne fungi account for almost all of the diseases affecting turf. These fungi are most likely to infect a stressed plant.
- Fungicides suppress the damaging fungi’s effects, but also suppress beneficial fungi.
- Acid mineral fertilizers (ammonium sulfate) favor fungi, while discouraging bacteria and other fungi fighters.
- Excessive watering and foliar feeding will weaken the turf root system, while herbicides and fungicides destroy the life of the soil. Both effects allow pathogens to get the upper hand.
- Frequent low mowing especially stresses the grass.
- Most turf diseases can be eliminated by employing proper cultural practices.
Specific Cool Weather Lawn Disease
Helminthosporium Leaf Spot
This disease first appears as leaf lesions, small purplish spots that turn brown in the center as they increase in size, finally fading to a light tan with purplish-brown borders.
A severe crown and root rot – which appreciably reduces both vigor and drought tolerance – frequently develops in conjunction with the leaf lesion phase of this lawn disease.
There is a direct relationship between air temperatures and length of leaf wetness required to produce maximum disease infections. When the leaf surface temperature is 70° F, the leaves must be wet continually for 48 hours in order for a high incidence of infection to develop. However, when leaf surface temperatures are in the 80-90° F range, the same amplitude of infection occurs within 24 hours.
Most turfgrass species are affected by gray and pink snow mold.
Pink snow mold is usually seen in early spring at the edge of the spring snow thaw. Small to large patches of gray or white matted turf covered with fungus growth will appear as snow melts. Black or brown specks can be found embedded in the grass blade.
Gray snow mold appears as 1 inch to 3 feet in diameter patches. Grass appears as light yellow, straw colored, or grayish-brown turf at snow melt. Leaves appear matted together, sometimes covered with sparse to dense white to gray mycelium.
Mow into late autumn to ensure that snow does not fall onto a tall grass canopy. Avoid applications of heavy nitrogen fertilizer in the late fall. If patches appear in spring, rake affected areas to dry turf and encourage new growth. Lightly fertilize affected areas in spring to help promote new growth. Severely affected areas may need to be aerated and over-seeded.
Diseases of Turfgrass, Houston B. Couch
Compendium of Turfgrass Diseases, Richard W. Smiley, Peter H. Dernoeden, Bruce B. Clarke
University of Purdue Extension Service: G. L. Work, Warren Schultz