Turf Diseases: Cool Weather Diseases

Helminthosporium Leaf Spot:

This disease first appears as leaf lesions, small purplish spots which 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 frequently develops in conjunction with the leaf lesion phase of the disease which appreciably reduces both the vigor and drought tolerance of the plants.

There is a direct relationship between air temperatures and length of leaf wetness required to produce maximum infections.  When the leaf surface temperature is 70° F, the leaves must be continually wet 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.

Melting Out: 

Melting out is mainly found on bluegrasses but also can be found on ryegrasses. On the leaves, it is first seen as minute, water-soaked lesions. These soon enlarge into dark, purplish-red ovular areas 1/4 to 3/8 inch long and 1/16 to 1/8 inch wide. During severe outbreaks of the disease, bluegrass stands are commonly found with less than six leaves per square foot of turfgrass area. Melting out is usually present from April to November, but is especially prevalent in June. Moist, cloudy conditions increase the disease potential.

If melting out is present, mow at the recommended height of 2.5 to 3 inches and remove clippings if feasible. Fertilizing in the fall and late spring with a slow release nitrogen will help reduce the severity of the disease.

Snow Molds:  

Most all 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 and covered with fungus growth will appear as snow melts. Black or brown specks can be found embedded in the grass blade. Gray snow mold will appear as patches from 1 inch to 3 feet in diameter. Grass will appear 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.

Stripe Smut: 

Long yellow-green streaks develop on the leaves of the affected plants, and as the disease progresses, these streaks become gray in color.  The leaf blades later on turn light brown, wither, and die. Symptom expression is more readily apparent in late spring and early fall.  Extended periods of air temperatures in the 50-60° F range are very conducive to symptom expression.

Stripe smut diseased turf is highly vulnerable to damage from heat and drought stress, therefore, during hot dry weather it is important that particular attention be given to addressing the heightened irrigation needs of the affected area. Fertilizer should be a balanced formulation and applied at intervals that provide a uniform rate of growth throughout the season.

 

Acknowledgements:
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