Outbreaks of Dollar Spot have become far more common since the turn of the century. Michael Fance, Technical Support Manager at Aquatrols Europe Ltd., offers some pointers into soil health and turf management for its control in the UK.
Figure 1: Classic symptoms are small bleached, straw-coloured spots which are normally no bigger than 50mm in diameter
Sports turf managers in the United Kingdom have long been aware of the existence of Dollar Spot as a turf grass disease. Until relatively recently, the disease was only regarded as being a problem on grass swards which contained a high percentage of red fescues, with slender creeping red fescue being perceived to be the most susceptible variety. It was believed that, on such turf, leaf wetness, poor nitrogen nutrition and hot day temperatures followed by warm, humid nighttime conditions would trigger the activity of the disease. As such, it was never considered to be of major importance in the broader field of fungal turf grass diseases as occurrences in the UK were not widespread.
Since the start of the millennium, outbreaks of Dollar Spot have become far more common and the disease is now causing major problems to many of those maintaining fine turf in the UK. This is not only on swards consisting of a high proportion of slender creeping red fescue but also on other turf grass species.
The classification of the fungus that causes Dollar Spot has commonly been recognised as Sclerotinia homoeocarpa. The taxonomic classification of this fungus has been under revision and one may well also see references made to either the genus Lanzia or Moellerdiscus.
Dollar Spot attacks the leaves of grass plants and is known to be active between 10 and 35OC. Pathogenicity is most severe between 15 and 25OC and it is when these temperatures are experienced that most damage occurs. Classic symptoms are small bleached, straw-coloured spots which are normally no bigger than 50mm in diameter, the size of an old silver dollar coin (Figure 1).
Figure 2. Coalescence of Dollar Spot on a golf green
Once Dollar Spot is established extensively in an area, these individual spots can coalesce and form larger, irregular patches. Unlike other common turf grass diseases, it is unusual for the individual spots to increase in size dramatically themselves (Figure 2).
Where certain climatic conditions are experienced, dense aerial mycelium can be seen on the affected turf blades early in the morning (Figure 3). Such conditions would be hot days and warm nights, together with high humidity.
As Dollar Spot has not been commonly seen in the UK until recently and, as its general appearance can be mistaken for other diseases, it can easily be misdiagnosed, but the examination of individual grass leaves can enable a more certain diagnosis. It is upon close inspection that the most recognisable visible symptoms can be seen which set it apart from other diseases.
Initial symptoms of attack are small yellow-green chlorotic spots. These spots evolve to form lesions. An infected grass leaf can be seen to be exhibiting one or more lesions, which have a bleached white centre and reddish brown borders (Figure 4). Although they can occur randomly across the leaf, these lesions are classically hourglass in shape and girdle the complete leaf blade. Older lesions may become quite long and cause blighting of the entire leaf or cut leaf end.
Figure 3. Dense aerial mycelium on affected leaf blades / Figure 4. Examples of leaf lesions exhibiting bleached white centres with reddish brown borders
A good understanding of the biology of Dollar Spot, and therefore its life cycle, is key to controlling the disease (Figure 5). The fungus is thought to survive unfavourable conditions as either dormant mycelium in infected plants or as linear black mycelial inclusions in the thatch, these are known as stromata. When daytime temperatures reach the required level, the dormant mycelium resume growth from the infected leaves and can then come into contact with nearby healthy leaves, causing new infections.
When favourable conditions for the disease persist following initial infection, such as warm nights with heavy dews, and if control measures prove to be unsuccessful, severe leaf blade damage may occur and the characteristic "dollar spots" may appear on the turf. When considering the biological aspect of this disease, it is worth noting that leaf and crown of individual plants are affected, whereas the roots and rhizomes are not, therefore, in the majority of cases, grass plants are not completely killed by an attack and can recover if correct maintenance and renovation practices are employed.
Experience and research have proved that turf grass disease development is dependent on three factors:
1. The presence of a pathogen, normally a fungus
2. Suitable environmental conditions
3. A susceptible host
If any of these factors is absent, then disease cannot develop. Dollar Spot fungal disease is no exception to the disease triangle model (Figure 6).
Figure 5. Dollar Spot disease cycle / Figure 6. The turf disease triangle
Conditions causing the disease to spread
Leaf wetness is considered to be critical to the formation and spread of Dollar Spot. Not only does the causal fungus thrive in these conditions but also its dissemination by equipment, animals or people increases. Disease spread is greatly enhanced if an infected area of turf consistently supports heavy morning dews over a number of days.
Turf growing on dry soil is less able to take up essential nutrients and water from that soil. As such, an unhealthy sward is likely to occur within which plants become susceptible hosts that are more prone to fungal attack.
In compacted soils, the grass plant is less able to produce a deep and extensive root system, which would in turn contribute to forming a strong, resilient vegetative part of the plant. Turf growing in such compacted conditions can contain plants, which become a susceptible host to the causal fungus.
The stromata associated with Dollar Spot are believed to survive unfavourable conditions, mainly over winter, within the thatch layer. Turf growing where a significant thatch layer is present is therefore more likely to be able to support the stromata and contribute to the long-term activity of the disease.
A site that is deficient in essential nutrients, especially nitrogen, is more prone to Dollar Spot attack and is much slower to recover from scarring. Without upward vegetative growth, infected leaves cannot be mown-off and taken away from the site.
An area of turf that lacks air movement across it is less likely to dry naturally at the canopy. Whatever causes impeded air movement or indeed the fact that the wind is still for a prolonged time; the natural drying process to the leaf is considerably compromised. Likewise, an area affected by shade, especially from the early morning sun, is less likely to dry naturally. Periods of prolonged high humidity also impact on the drying process to the leaf.
It is also true to say that any damage to grass leaf tissue can make the plant far more susceptible. Such damage is often associated with blunt or badly adjusted mower blades that cause leaf tear, thus exposing an open wound to infection.
Over the past two decades, attacks have been noted in the UK from mid June to early October. It is during this period of the year that turf managers in many sports are under ever increasing pressure to produce the best quality swards possible.
Controlling the disease
Residues left by fungicides that were used legally prior to their revocation may have also been controlling outbreaks of this disease, even if Dollar Spot wasn't on the label. Falling levels of these residues may have contributed to the increase in Dollar Spot.
Integrated Pest Management (IPM) programmes should now be the turf manager's standard approach to dealing with all turfgrass fungal diseases, and Dollar Spot is no exception. An IPM approach will consider the following methods - cultural, mechanical, biological, genetic and chemical. One, some or all of these methods may be necessary, but chemical control should always be considered as the last resort.
To culturally prevent or control Dollar Spot, sound management practices should be followed. These will promote a dense, durable, resilient and healthy sward less likely to contain plants that would be susceptible hosts to the fungus. If there is a history of Dollar Spot on a certain site, then these measures should be given more thorough consideration.
As duration of leaf wetness is such an issue when considering Dollar Spot, every effort should be made to reduce it to a minimum. Removal of early morning dews by mowing, switching or brushing should be considered. When rainfall is insufficient, irrigation should be deep and infrequent, as light sprinklings using the "little and often" approach often exacerbate the situation. Automatic irrigation should take place early in the morning rather than in the evening in order to reduce the time between water applications and physical dew removal.
Following a programmed approach using proven surfactant technology can assist with the function of dew removal as well as addressing issues associated with soil dryness. Deep, infrequent irrigation, in conjunction with the application of the correct water management technology, will maintain uniform moisture levels in the soil profile. This will enable the plant to take up water plus essential nutrients that will, in turn, increase plant health, limit plant stress and make the sward less prone to attack.
Every effort should be made to maintain an open, well-aerated and non-compacted profile. Regular aeration using different tines at varying depths will result in a healthy rootzone within which a vigorous sward can survive. All operations should take place when soil conditions are consistent with those required to ensure the desired results.
Management of any thatch layer is paramount in the prevention and the control of Dollar Spot. Although correct aeration operations will help to ensure an environment where beneficial micro-organisms can aid in the breakdown of thatch, its mechanical removal is often necessary. Regular verti-cutting can prevent its build up, whilst deeper scarification operations remove lower lying thatch layers. The control of both the depth and density of any thatch layer will not only promote a healthier sward, but will also limit the available environment needed by the stromata associated with Dollar Spot to survive unfavourable conditions for disease activity.
A balanced nutrient input should be adhered to with adequate nitrogen applications being key. A sward that is growing will not only facilitate the removal of infected blades in boxed-off grass clippings, but will also ensure quicker recovery from an outbreak.
Mowing practices are also considered to be key in the management of this disease. Firstly, it is essential that mower blades are sharp and give a clean cut to the leaf blades as these wounds soon heal over. Such wounds are far less readily infected by mycelium. Secondly, it is good practice to box-off clippings from an infected area and dispose of them accordingly. Letting infected clippings fly in this situation only serves to spread the disease further.
The use of Dollar Spot resistant cultivars should be considered when carrying out overseeding to a previously infected area, or indeed when seeding or turfing a new area on a site that has a history of outbreaks. Certain cultivars of Agrostis stolonifera (creeping bentgrass), namely L-93, A-1, Providence and Pennlinks have shown superior tolerance to Dollar Spot in the US (Hurley, 1999).
Figure 7. The affect of concentrated humus applications on Dollar Spot incidence (Virginia Tech-1997)
Ongoing trials work
Although the exact mechanism of suppression is not fully understood, there has been research in the US to suggest that certain commercially available organic fertilisers have given some degree of control of Dollar Spot (Liu et al.,1995; Nelson and Craft, 1991).
The results from the 1997 study at Virginia Tech suggest that applications of concentrated humus can reduce Dollar Spot outbreaks (Figure 7). They suggest that the concentrated humus created a more favourable environment for plant growth, that it strengthened the plant's natural defences and that it possibly promoted the growth of beneficial fungi and bacterial species in the soil that could discourage the pathogen through competition.
Aquatrols research trials will be looking to emulate this work with our recently launched range of products containing Aquavita technology with regards to the control of Dollar Spot as the disease raises its profile in the UK.
As with most aspects of turf grass management, a pro-active approach is always the best practice. In the case of Dollar Spot, both the formulation as well as the implementation of an IPM programme is crucial to the prevention and control of this disease. There are many options, be they cultural, mechanical, biological or genetic, before the need to apply a chemical control.
In conclusion, Dollar Spot appears to be on the increase and could possibly become a widespread problem for managers of fine turf across the UK. If this proves to be correct, it is certain that much more research will be carried out into the causal fungi as well as both prevention and control strategies.
Michael Fance, Technical Support Manager, Aquatrols Europe Ltd
Pitchcare are an appointed distributor of Aquatrols products visit the Pitchcare Store or call 01902 440258 for advice or quotations on Aquatrols wetting agents and nutrient products.