Description:
Background:
Couch smut is a disease of the inflorescence (seed head) of the turf plant. The pathogen, Ustilago cynodontis (Pass.) Henn., has a worldwide distribution and is a common disease seen within a number of the green couch (Cynodon dactylon) turfgrass varieties. Couch smut has been observed in older turfgrass green couch varieties such as ‘Wintergreen’ and ‘Greenlees Park’ and even newer turf varieties such as ‘Plateau’ and ‘Grand Prix’ (observations by M. Roche). It was previously suggested that couch smut also affected hybrid green couch (Cynodon spp.) turfgrasses (Loch and Bransgrove, no date). However, in a couch smut study conducted by Geering et al. (2021), field surveys suggested that hybrid green couch cultivars (Cynodon dactylon x C. transvaalensis) are resistant to the disease. Smut (Ustilago sp.) was also identified within foliage on Qld Blue Couch (Digitaria didactyla) in Brisbane, QLD (laboratory testing by Westgate Labs, Orange NSW) following submission of a sample by Australian Sports Turf Consultants (ASTC). Couch smut was first recorded in NSW in 1907 (Carson, no date) and is prevalent in states and territories across Australia.
Further information can be seen within a Hort Innovation (Project Code: TU17002) report 'Improved capacity for integrated disease management of couch smut (Ustilago cynodontis) in turf published in 2021. Click here for access.
Symptoms:
The pathogen colonises and destroys the inflorescence arrangement generally containing 2-6 spikes, leaving only the rachis. The inflorescence spikes fail to mature or open becoming dark coloured, containing a dusty spore mass (Loch and Bransgrove, no date). The spores are mostly globose to subglobose, light yellowish brown, 5 - 8 μm in diameter with a smooth exposure (Couch, 1995). These powdery spores can cause allergies in some individuals as identified by Marchelo-d’Ragga and Misaka (2015); yet the species of green couch itself, which is used across the majority of all sports turf surfaces in Australia, is also known to be a severe allergen (Pollenlibrary.com, accessed 1 May 2017).
Damage:
The disease is a significant issue for seed production, yet a minor concern for vegetative turfgrass, especially when the disease is present in low levels.
Reputable and scientific “information on the effect of smut diseases on the growth of stolons and disease transmission in clonal grasses is very limited” (Garcia and Burdon, 1997). In glasshouse experiments conducted, Garcia and Burdon (1997) identified that couch smut reduced dry matter yield (clippings); inconsistent results were obtained in identifying if the disease effected root to shoot ratio, but they did see a reduction in stolon growth. Despite this, stolon extension in diseased individuals still appeared to be rapid enough for there to be a tendency for sections near the tip to remain healthy when seperated (Garcia and Burdon, 1997). All experiments conducted by Garcia and Burdon (1997) were in a glasshouse using green couch seed, which is knowingly less vigorous that vegetative turf varieties and it is unknown if such performance traits would be seen growing naturally in the field under managed conditions. Garcia and Burdon (1997) also reported that seed production is prevented in affected flower heads. However, the presence of the organism will not prevent healthy seed from germinating.
Shoots with infected seed heads can show a more upright habit of growth, which is particularly unsightly when left unmown. In some varieties e.g. ‘Plateau’, where couch smut is present, it may still be visible because the variety itself is low growing and the seed heads have a tendency to grow parallel to the surface.
Couch smut can become attached to machinery and people etc.as the spores are easily dislodged and disseminated. Under ideal conditions, the spores may spread to other turfgrass areas.
Control options:
Unfortunately, there is no effective chemical control for this disease at this point in time. Routine mowing of the turf surface or mowing as the flower heads appear will significantly reduce its expression.
Comments have been made about using plant growth regulators (PGR's) such as trinexapac-ethyl to suppress seedhead development. However, PGR's themselves will only reduce the vertical extension of plant material. Options for suppressing green couch seed heads in managed turfgrass systems are limited (Brosnan et. al, 2011), with the exception of routine (near daily) mowing. Experiments conducted by Brosnan et. al, (2011) using fenoxaprop, imazapic, trinexapac-ethyl and mefluidide saw a range of head suppression between <25% to 100%. However turf damage was also incurred.
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