Introduction Symptoms Symptoms are usually first noticed in late spring and early autumm but can be observed throughout the growing season. It can be difficult to diagnose this disease by symptoms alone during the early stages. The disease begins as scattered light green patches 5 to 10 cm in diameter. These patches may then grow as large as 1 meter in diameter, and turn dull-tan to reddish-brown as . The most diagnostic of these larger patches in the lawn may exhibit a "frog-eye" pattern. In frog-eyes an apparently healthy green patch of grass is partially or completely surrounded by a ring of dead grass. Distinct streaks, crescents, and circular patterns are found in the affected lawn area.
Necrotic Ring Spot
may occur throughout the growing season, but tends to be worse
during the cooler parts of the year (April/May - Sept/Oct). If
Necrotic Ring Spot continues through the summer months, it may
cause a rot where the crown and roots become blackenend and dark
mycelium may be viewed on infected parts. Leaf lesions are often,
but not always, associated with this disease. If present, they
are variable in size, shape, and color. They usually extend across
the blade and are yellowish, dull-tan, or reddish-brown. Many
other fungal diseases can also cause leaf lesions. Microscopic
examination is often necessary to determine the cause of the problem.
Disease Cycle Correct excess soil acidity by liming annually to maintain a pH above 6.2. For most bluegrass lawns, two to five lbs of nitrogen/1000 sq.ft. is sufficient. Apply this in a fertilizer balanced by phosphorus and potassium. Do not apply even small amounts of fertilizer during the June-August stress period because this will tend to stimulate the disease. Therefore, fertilize only in autumn (September through November) and in late spring (May). Deep watering is essential for proper root growth. Water the soil under disease-prone areas to a depth of 15 to 20 cm every 7-10 days during dry periods in the summer. Soaker hoses are very useful for supplemental watering of steeper slopes where other sprinklers are inefficient. The harmful effects of excessive temperature can be reduced by a light sprinkling of the surface at mid-day. Proneness to disease in turf is increased as the cutting height is decreased. Cut lawns at 6 to 9 cm height, and do so often enough that less than 1/3 of the leaf blade is removed during each mowing. Thatch (the layer of organic matter between the mineral soil and the green grass) should be no more than 2 cm in thickness. Thatch can be removed by vertical slicing machines and/or aeration during the spring and early fall. Over a longer period thatch will be reduced by using the cultural practices discussed above. Kentucky bluegrass cultivars such as Adelphi, America, Aspen, Columbia, Eclipse, Glade, Midnight, Nassau, Parade, Ram I, Sydsport, Touchdown, Vantage, Windsor, and Victa are less susceptible to Necrotic Ring Spot than others. Blend seed of a resistant cultivar with that of one or more otherwise desirable cultivars. Blending 10-15% (by weight) of perennial ryegrass seed into bluegrass seed will prevent this disease from occurring. Ryegrass can also be seeded into existing lawns. Chemical treatment is efficient only when the previously mentioned cultural practices were first used. Furthermore, applications must be made before the crown rot develops sufficiently to cause visual symptoms of the disease. In New York State, some fungicides containing the active ingredients azoxystrobin (Heritage), thiophanate-methyl, triadimefon or myclobutanil (limited use on long Island) may be available for control of Necrotic Ring Spot in home lawns. Thoroughly water areas with a history of this disease (applying 2 to 3 cm of water) several days before applying the fungicide. Follow label directions. Updated, SLJ, 1/07 _______________________________________________________________________________ |
