Ascochyta leaf blight or spot of turfgrasses is caused by more than 80 different fungi which can cause damage to Kentucky bluegrass, bentgrasses, Italian and perennial ryegrasses, fescues (red, meadow, sheep, and tall), redtop, and many other forage, weed, and wild grasses. The Ascochyta fungi attack grasses during much of the growing season when humidity and atmospheric moisture are high or when irrigations and mowing are frequent. The Ascochyta fungi seldom cause extensive damage.
The grass blade is usually killed from the tip down towards the base of the plant. Infection occurs through the cut end of the leaf blade. Can often see small, black pycnidia on the dead tissue just adjacent to the living tissue (use a good hand lens). Symptoms are the same on bluegrass, ryegrass, and the fescues.
This disease is common when a cool, wet spring is followed immediately by hot summer conditions. Ascochyta leaf blight is most severe when the turf becomes stressed due to a combination of high heat and improper irrigation – insufficient amount, improper frequency, or poor coverage.
This disease usually does not usually kill the grass, but, some turf thinning may occur under severe outbreaks
Fungicide are ineffective in controlling this disease and NOT recommended
Moderating temperatures usually bring control of the disease
Lawns managed with frequent irrigation are prone to this disease when temperatures exceed 90-100F for consecutive days and no rainfall occurs
Recovery may take several weeks to a month for a severe outbreak, even with cooler temperatures, rain and sufficient irrigation
Maintaining a vigorous lawn is the best prevention against Ascochyta leaf spot. Core aerate in the spring or fall to reduce or prevent thatch build up. Keep mower blades sharp. Never remove more that 1/3 of the blade at one cutting. Mow when the grass is dry to avoid spreading spores. Use a balanced fertilization program. Fungicides are available, but won't have much affect without the cultural practices previously mentioned.