With a bit of time to kill over some annual leave I thought I’d carry out a review of dollar spot, focusing on the UK.
A small patch disease which can spread to form larger irregular areas. It is common on warm-season turfgrasses, especially Bermuda Grasses, and also occurs on cool-season turfgrasses.
This used to be a fairly uncommon to rare disease in the UK, however, with the intensification of turf maintenance practices, improving standards, changing working practices (for example, reducing use of well-rotted compost within topdressings) and high sand content rootzones there would, arguably, appear to be an increase in incidence of this disease and diseases in general. However, some of this could be due to misdiagnosis and confusion with red thread disease, especially during its early stages (Dawson; Escritt) or bleached out fusarium patch disease. One survey in 2001 indicated that just 3% of UK golf courses had experienced dollar spot (Mann, p21).
In the UK the disease is predominantly focused on Slender Creeping Red Fescue; although other species can be susceptible to attack. It is uncommon on Strong creeping red fescue, and rarely detrimental to Chewings fescue. Poa annua and bentgrasses may also be attacked in the UK, possibly due to climate heating, or where temperatures are consistently on the high side. However, when an attack is noticed the most likely probability is that it will be slender creeping red fescue which is infected, but other species should not be discounted.
Source: Adapted from Glenobear [Public domain], https://commons.wikimedia.org/wiki/File:Dollarspot.jpg
One well-known consultant agronomist, Jim Arthur, wrote in the 1990s about dollar spot in golf course situations, “This [disease] is in effect confined to slender creeping red fescue and is so rare that it can be ignored as being of academic interest only” (Arthur): This infers the misdiagnosis probability mentioned above. Environmental and working practice changes have occurred since then, as noted above, but there is still an argument that this disease is incorrectly identified and is actually a different disease in many situations – especially on bowling greens, which are mostly maintained by volunteer greenkeepers who may not have the training and education gained by that of qualified greenkeepers.
This disease can be identified in accordance with the following: Circular spots, initially 10-25mm in diameter, enlarging to 50mm diameter. These spots can join together to form larger infected areas, typically up to 150mm; however, the area generally maintains a spotted appearance. The infected grass is usually a light straw colour. Closer inspection of a leaf can reveal either:
- an upper bleached part with a lower green part, or,
- a central green part bordered with a lower and higher bleached part, or,
- a central bleached part bordered by green leaf either side. Where the leaf is partly bleached, a black line may separate it from the green part of the leaf.
A surface white mycelium can also often be seen in the morning on an infected turf when dew is present.
Temperatures from 15.5˚C can see the main start of this disease attack turf, although temperatures between 21˚C and 27˚C are optimum (along with high humidity), with it being active to 32˚C, for an attack from this disease. Minimum and maximum temperatures for growth have been given as from 10˚C to 35˚C, however, the Smith-Kerns Dollar Spot Prediction Model has found that “[t}emperatures consistently in excess of 30 ̊C will limit dollar spot development” Smith et al, 2018, p11. A limitation of growth of the fungus can also occur when relative humidity is below 70% (Smith et al, 2018, p9).
May to September could be considered a key time for a potential severe attack of Dollar Spot, with it easing in severity a month either side of this main period. However, much will depend upon prevailing conditions and geographic location, as well as a range of influencing factors given below. Just because temperature and humidity range are optimal it doesn’t mean than dollar spot will strike; there are many factors which significantly reduce the potential for an attack, and also extent and severity of attack. Dollar spot can also typically attack at a similar time to that of Red Thread disease, hence the reason why misdiagnosis could occur.
Earlier (in the 1980s) predictive models (R. Hall and G. Mills & J.D. Rothwell) were used to help inform fungicide application programs, however, accuracy was variable and whilst insightful, innovative and having potential for further development, one study at the time could not recommend the “forecasting systems as effective disease management tools for turfgrass managers in southern Ontario” (Burpee & Goulty).
Other favourable conditions for an attack of this disease include:
- Low nutrient levels, especially nitrogen and potassium.
- Dry soil conditions causing moisture stress.
- Excessive thatch.
- Low soil organic – micro-organism – activity.
- High relative humidity: c. >85% RH.
- Poor air circulation, particularly where shade is cast.
- Dew presence: Guttation water “supplies a nutrient-rich medium in which the fungus can grow in as it spreads from leaf to leaf.” (Vargas)
- Surface moisture, especially where evening irrigation has occurred.
- Waterlogged conditions.
- Damaged grass tissue, especially from incorrectly set mowers, scarification or verticutting.
- High frequency of mowing.
- The presence of sea-washed turf, which was historically used on some bowling greens.
A range of preventative and control methods might be considered, including:
- Maintain adequate nitrogen and potassium inputs as part of an effective fertiliser programme.
- Increase nitrogen and/or potassium levels with a fertiliser application, but not excessively and be careful of timing.
- Apply an organic fertiliser and / or organic supplement to the sward to increase diversity of soil ecology and potentially suppress the activity of the dollar spot pathogen.
- Apply top-dressing which includes composted farm yard manure, seaweed, or similar.
- Apply biostimulants as part of a preventative maintenance programme to help improve soil biodiversity.
- Remove dew early in the morning.
- Maintain soil water content to 75% or greater of field capacity.
- Keep the soil well aerated.
- Reduce thatch layers: both depth and density of them.
- Box off and remove clippings.
- Improve air circulation around the area: Consider thinning hedges and trees, removing overhanging branches.
- Use more resistant turfgrass cultivars where available.
- Improve soil ecology – micro-organisms and macro-organisms.
- Ensure mowers have sharp cutting blades and are correctly set; raise height of cut to ease stress on sward.
- Reduce mowing frequency.
- Ease up on the use of scarification and especially verticutting during dry conditions,
- Improve the drainage system;
- Use of the Smith-Kerns Dollar Spot Prediction Model – the probability of dollar spot occurrence is based on a 5-day moving average of daily relative humidity and average air temperature.
- Use of antagonistic bacteria or fungi, e.g. Bacillus licheniformis (EcoGuard) & Trichoderma harzianum (Bio-Trek), to help suppress dollar spot activity, although these do not appear to be currently available for use in the UK.
- Approved chemical control.
The disease had previously been identified as possibly being caused by different fungal pathogens including fungi in the genera Moellerodiscus and Lanzia, but alsoRutstroemia floccosum, although the established and generally accepted causal pathogen was Sclerotinia homoeocarpa.
2018 saw the publication of research titled ‘Clarireedia: A new fungal genus comprising four pathogenic species responsible for dollar spot disease of turfgrass’ (Fungal Biology Volume 122, Issue 8, August 2018, Pages 761-773) which identified the fungal genus Clarireedia as being responsible for this disease. Four species: C. homoeocarpaand C. bennettiimainly affect Festuca rubra in the UK, whilst C. jacksoniiand C. monteithianaaffect warm season and some other cool season grasses outside of the UK.
- Dawson, R.B. (1959), ‘Practical Lawn Craft and Management of Sports Turf’, pp.208-209
- Beard, J.B. (1973), ‘Turfgrass: Science and Culture’, pp. 69, 417, 581
- Escritt, J.R. (1978), ‘ABC of Turf Culture’, Kaye and Ward, pp47-49
- Burpee, L. and Goulty, L. G. (1986), ‘Evaluation of two dollarspot forecasting systems for creeping bentgrass. Can. J. Plant Sci.66: 315-351.
- Smith, J.D., Jackson, N., & Woolhouse, A.R. (1989), ‘Anthracnose and colletotrichum basal rot’, 219-226, in ‘Fungal Diseases of Amenity Turfgrasses’, E. & F.N. Spon.
- Emmons, R. (1995), ‘Turfgrass Science and Management’, 2ndEdn, pp320-321.
- Tani, T, & Beard, J.B. (1997), ‘Color Atlas of Turfgrass Diseases’, Ann Arbor Press, pp. 141-146
- Brenda Walsh, Stephanie S. Ikeda, and Greg J. Boland, (1999), ‘Biology and Management of Dollar Spot (Sclerotinia homoeocarpa); an Important Disease of Turfgrass’, HORTSCIENCE, VOL. 34(1), FEBRUARY 1999
- Arthur, J. (2003), ‘Practical Greenkeeping’, 2ndEdn, p175
- Mann, R.L. (2004) ‘A review of the main turfgrass diseases in Europe and their best management practices at present’, Journal of Turfgrass and Sports Surface Science, Vol. 80, pp19-31.
- Skorulski, J. (2014), ‘Getting the Upper Hand on Dollar-Spot Disease’, USGA Green Section Record Vol.52(18) September 5, 2014
- Allen, T.W., A. Martinez-Espinoza, and L.L. Burpee, (2005), ‘Dollar spot of turfgrass’.The Plant Health Instructor. DOI:10.1094/PHI-I-2005-0217-02. Updated 2016. American Phytopathological Society (APS)
- Vargas Jr, J.M. (2005), ‘Management of Turfgrass Diseases’, 3rdEdn, pp19-22
- Christians, N.E., Patton, A.J. & Law, Q.D. (2017), ‘Fundamentals of Turfgrass Management’, 5thEdn, pp. 356-357
- Watschke, T.L., Dernoeden, P.H., & Shetlar, D.J. (2017), ’Managing Turfgrass Pests’, 2ndEdn, pp. 108-113
- Smith DL, Kerns JP, Walker NR, Payne AF, Horvath B, Inguagiato JC, et al. (2018) Development and validation of a weather-based warning system to advise fungicide applications to control dollar spot on turfgrass. PLoS ONE 13(3): e0194216.
Chris Gray, 10th August 2019