Anthracnose

There are two types of anthracnose symptoms, a basal rot that occurs in cool, wet weather in spring and early summer, and a foliar blight that occurs under periods of heat and water stress. The foliar blight is most common in California; the basal rot anthracnose has not been reported in California.

For foliar blight symptoms, the older leaves are often attacked first, with reddish-brown to brown lesions that turn a pale tan color.

For basal rot, dark infection mats are often visible on the lower leaf sheaths and diseased crowns are often black and necrotic. Leaves are often yellow-orange.

In both cases, the fungus can produce fruiting structures (acervuli) that have fine black hair-like projections (setae) and are filled with small, crescent-shaped spores.

Anthracnose is most severe on annual bluegrass, Poa annua; it also occurs on Kentucky bluegrass, P. pratensis. Other species are only rarely affected by this disease in California.

Basal rot anthracnose is favored by cool, wet conditions (50 to 60°F) while the foliar blight is favored by higher temperatures (80 to 95°F). Both basal rot and foliar anthracnose development are favored by low soil fertility, high compaction, and high soil salinity. Extended periods of leaf wetness contribute to the development of the disease, as do practices that cause mechanical damage to the turf (top dressing and verticutting).

Anthracnose is best managed by proper cultural practices. Fungicides should be used primarily on golf course greens and other intensively managed turf.

Apply adequately balanced nutrients as necessary, concentrating on potassium and phosphorus. On golf course greens, fertilize with low rates of nitrogen (0.1–0.2 lb/1000 sq. ft.) monthly, especially during late spring and through the summer. Avoid fertilizing during periods of high temperatures (over 80°F).

Irrigate deeply and infrequently based on evapotranspiration needs of turfgrass. Allow leaves to dry between irrigations. Irrigate early in the morning rather than during late afternoon or evening. Lightly water (syringe) golf greens during the day to reduce heat and drought stress.

Reduce compaction through mechanical aerification in fall and spring. Increase mowing heights to reduce stress on affected turf. Increase air movement and reduce shading by selective pruning of trees and landscape that block air movement or light the grass. Periodically irrigate golf course greens heavily to leach salts if salinity is a problem.

Fungicides are most effective when used preventively. For areas where anthracnose is common, begin applications when the soil temperature rises above 65°F (mid to late spring) to reduce the severity of initial epidemics.

Strains of the anthracnose pathogen resistant to both QoI (Group 11) and benzimidazole (Group 1) fungicides have been documented in California. If control by fungicides from either mode-of-action group has been ineffective in the past year, switch to a fungicide with a different group number or try tank mixing with a multi-site fungicide such as chlorothalanil.

Repeated applications of fungicides with the same group number for summer patch control may contribute to the development of fungicide resistance in anthracnose because both diseases tend to occur at the same time in the season. Practice resistance management strategies for both diseases at the same time by alternating chemicals with different mode-of-action group numbers.