Technical
“Golf courses in the south of the country have borne the brunt of the increase in Dollar Spot disease outbreaks…Without doubt the weather this summer has triggered the Dollar Spot problem but, if such conditions are to become increasingly common as part of our changing weather pattern, then I think greenkeepers have a worrying time ahead”
Jeff Perris, now retired agronomist speaking back in 1999
Figure 5. Dollar Spot disease cycle Disease development
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.
“….this is one of the oldest turf disease problems we know but there is still much that we need to learn about the fungi that cause it”
Dr. Kate Entwistle, The Turf Disease Centre, speaking earlier this year
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).
Conditions causing the disease to spread
“Dollar spot attacks all grasses - it is a significant disease of fine turf in the USA and they are not blessed with fescue-dominated golf courses.”
Steve Isaac, Director ‐ Sustainability at the R&A, speaking in 2007
126 I PC JUNE/JULY 2018
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.
Figure 6. The turf disease triangle
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
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156
