Is Your Couch or Zoysia Declining?

Summertime Blues: Couchgrass Decline (Part I)

While damage to couch is more prominent, especially in the USA - due to the sheer preponderance of the species on fine lawn areas - damage on zoysia, and other warm season turf species is probable, both there and elsewhere in the warmer regions of the world.

In many parts of the northern Australia and areas of South Africa couchgrass (Cynodon spp.) has suffered greatly this past summer, and symptoms may persist into autumn. Several reasons or causes have been given, but one prominent reason is – and I will use the U.S. term - bermudagrass decline. The first reported description of bermudagrass decline occurred in the southeastern United States (i.e. Florida).

Under hot humid and frequent rainfall, patches would develop on the turf and quickly coelesce forming larger non-descript areas of turf loss. The root systems turn black and die back is significant. Although, environmental factors like low light, and saturated soils no doubt contributed to the decline, the pathogen Gaeumannomyces graminis var. graminis was identified as the cause.

Originally bermudagrass decline was only associated with bermudagrass. However, patch diseases caused by the same fungus, G. graminis var graminis have now been reported on most of the warm season turfgrasses. To reflect the increasing number of turfgrasses that are infected, the name has been changed to be more encompassing by calling these root diseases of warm season turfgrasses, “Root Decline of Warm- Season Turfgrasses” (Smiley,et al. 2006).

In the United States, where kikuyugrass (Penisetum clandestinum) is limited to southern California, a patch disease similar to bermudagrass decline has been reported.

[As a side-note, and one based on my opinion with no data to back it up, I wonder if some of the problems this past summer on kikuyugrass may have been due to this pathogen or one closely associated with it? - Henk Smith, Syngenta.]

[ From Syngenta, 6 June 2007]

Watch this space..............more to come as the story unfolds further. This is of particular interest to tropical north Australia!

In the USA, this disease seems to occur most often on intensively managed 3- to 6-year-old turfs, with many of the ultra-dwarf varieties prone to infection. Excessive nitrogen fertilization, potash deficiency, sharp increases in soil pH, and thatch accumulation increase turf susceptibility to attack by the causal fungus. Low maintenance bermudagrass lawns usually have little trouble. The disease is found on all bermudagrasses but is most damaging on the newer hybrid varieties.

Exceptionally low mowing heights, poor drainage, thatch accumulation, and high soil organic matter content have been linked with the occurrence of bermudagrass decline on greens and tees.

The causal fungus of bermudagrass decline is Gaeumannomyces graminis var. graminis. Related fungi, Leptosphaeria korrae and Ophiosphaerella herpotricha, have been shown to be related to the problem in other parts of the United States.

The same organism, Gaeumannomyces, is the fungus associated with take-all patch in St. Augustinegrass.

Initial symptoms of bermudagrass decline include chlorotic patches of turf 20 - 60cm in diameter. The turf thins out and may eventually be completely killed in these patches. Chlorotic leaf blades may develop next to green shoots at the margins of the diseased area. Roots of diseased bermudagrass are brown and without feeder roots and root hairs. Signs of the fungus on the root surface appear as dark brown hypal runners. The disease occurs on weakened or damaged turf which indicates that management could be the key to control.

If disease occurs, try to identify the predisposing factors and correct them through proper management practices. On golf greens, cricket wickets or other very low cut areas, simply raising the mowing height is the most effective management practice to correct the problem. Some fungicides are effective for the control of bermuda decline caused by Gaeumannomyces graminis. Since infection is thought to occur primarily in autumn, with disease progression continuing into the winter months under cool moist conditions, autumn or end of wet season applications may be the best time for fungicides to be applied for preventative purposes. The efficacy in controlling the already established disease may be disappointing.

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