At this time of year we know only too well that managed turf is often afflicted with disease scars that spoil the appearance and playability of sports turf.
Fortunately we have an armoury of fungicides we can draw on to counter the effects of disease. However, there is a potential risk that some of these products could lose their effectiveness if the target fungi develop a resistance to them. To date there have been no confirmed cases of field resistance to turf fungicides in the UK but this has not been the case in our other agrochemical markets, where developed resistance has seen multi-million pound crop protection products toppled from the market place in the blink of an eye, as farmers and growers claimed compensation for crop losses from the manufacturers.Believe it or not, fungicides have been around for over 200 years. Early fungicides were mainly used in viticulture (grapevines) and as cereal seed treatments. Like caveman’s tools, these products were crude but effective – often quite persistent with a wide spectrum of control. Advances in chemistry have allowed more complex molecules to be synthesised as fungicides and these have tended to be more target specific, which is beneficial to the ecosystem in which they are used. Over the years a major global market has developed with products to control diseases in food crops, ornamentals, forestry, and amenity sports areas. The world market is worth around $8 billion today – so you can see there’s a lot riding on the future of these products and of course the benefits they bring to the table! Stop and think for a moment what life would be like without any fungicides to arrest the spread of disease on sports turf. It could be almost as bad as trying to live without Google!.The Fungicide Resistance Action Group-UK provides advice for combating disease resistance. They recommend the adoption of an integrated approach to controlling fungal disease that includes all aspects of cultural control, use of tolerant grasses, biological and chemical control. In this short article I will examine the important diseases of UK turf and the options available for integrated control but to begin with I shall tackle some of the terminology associated with disease resistance.TerminologyA fungicide will have a spectrum of control that includes some fungal species but not others. Those it controls are said to be ‘sensitive’ to the chemical but all other species have a ‘natural resistance’, which defines the limitations of the product. The type of resistance to chemicals that is developed by a fungus from persistent use is often referred to as ‘acquired resistance’. This is a subtle change in genetics that occurs in the cells of the fungal pathogen, protecting it from the effects of the fungicide. Studies of resistance mechanisms, the processes by which fungi adapt to chemicals, have been carried out to enable the development of new products and strategies that will help to extend the useful life of fungicides for the future.Fungicides work by interfering with natural processes at a cellular level. We refer to this as the fungicide’s ‘biochemical mode of action’. In most cases the mode of action involves blocking a biochemical pathway essential to the growth of the fungus or interfering with the natural regulation of the cell. Some products have only a ‘single site mode of action’ i.e. they disrupt the essential processes at only one point in the cells metabolism, whereas others target multiple sites and are referred to as having a ‘multi-site mode of action’.Pesticides that have similar chemical structures are often classified together, for example the ‘strobilurin’ group contains three active ingredients used in turf; azoxystrobin, pyraclostrobin and trifloxystrobin. These families of chemicals will usually have similar target sites and biochemical modes of action. Therefore, if resistance develops to one of these actives, the others in that group are likely to be tolerated as well. This phenomenon is known as ‘cross resistance’.//-ImageGallery-//
So how does the development of resistance to a chemical occur? If you were to look down a microscope at some Microdochium spores you could be forgiven for saying that they are all the same; they are not.A population of fungal spores consists of many individuals and somewhere amongst them will be one that has a genetic mutation that would allow it to survive the attack of a specific fungicide, by virtue of a minute difference in cellular chemistry. In the absence of this specific fungicide the mutation gives no benefit and so the individuality does not multiply in the population. However, when that community of fungal spores does encounter the fungicide, this one individual benefits by surviving the attack and is able to multiply and pass on this ability to all future generations, so that soon the chemical will have been rendered ineffective. This is a very simplistic illustration of one of the ways that resistance occurs but it is the same process by which all organisms evolve under natural selection.Mechanisms of fungicide resistance that have been discovered include: -
Detoxification of the chemical by producing substances that bind with it
Fungicide is altered or broken up by the fungus (detoxification)
Target site is modified to reduce the effectiveness of the fungicide
Fungus compensates for the damage to its biochemical pathway by increasing production of the target components
Low light levels from shading; e.g. from nearby hedges and trees
Susceptible grasses – Annual Meadow-grass (Poa annua)
Lack of good surface drainage caused by; excessive thatch, poor soil texture or compaction
High humidity levels – usually outside of the scope for control and often a problem on enclosed pitches (stadia, tennis courts and some bowling greens)
Excessive nitrogen levels in periods of slow growth (autumn/winter/early spring)
Sub-optimal pH of rootzone (pH well beyond the desirable range of 5.5 to 6.5)
Susceptible grasses – Annual Meadow-grass (Poa annua) especially under conditions of low fertility·
Stress from; frequent close mowing, abrasion caused by working in heavy top dressing applications, drought, and general heavy wear
Low fertility – especially low nitrogen towards the end of summer
Compaction of the rootzone
Low fertility – especially low nitrogen levels
High surface moisture
Susceptible grasses – Strong Creeping Red Fescue
Susceptible grasses – Bent grass (Agrostis spp.) are the worst affected, other grasses can become infected but some may recover
Wetness and surface alkalinity - especially following the application of lime to adjust soil pH. Prolonged or frequent use of irrigation with high pH water from the mains or a bore hole can have a similar effect to liming
The absence of antagonistic microorganisms in the soil due to the over-use of fungicides or the use of very sandy construction materials may encourage the disease to develop
Low levels of Manganese in spring
Affects almost all grasses but the most susceptible grasses are: - Red Fescue and Perennial Ryegrass
Low nutrition – especially nitrogen
Compaction in the rootzone restricting air and nutrient supply
Smooth-stalked Meadow-grass (Poa pratensis), Perennial Ryegrass (Lolium perenne) are the most affected grasses. For this reason it is not normally a problem on putting surfaces such as bowls and golf greens.
Turf mown to a height of less than 25 cm is reported to be more at risk. Unfortunately, much of the sports turf utilising these grasses is required to be less than 25cm - e.g. golf tees, cricket squares & hockey pitches.