Landscaper Pro Advice: Pests and Diseases – Focus on Earthworms and Red Thread
The impact of pests and diseases on turf are important as they will significantly reduce the aesthetic value of a lawn or amenity grassland area. In some cases the damage can be extensive leading to costly repair work. It is therefore better to be aware of the main culprits that could cause damage and how best to prevent infestations, thereby minimising the potential for turf stress.
There are a wide range of pests that can affect natural grass areas such as lawns to varying degrees, including rabbits, moles, leatherjackets (crane-fly larvae), fly larvae and chafer grubs (chafer beetle larvae). One key pest for quality lawns in cooler, wetter regions is the earthworm although the extent to which they are considered a nuisance will depend on the sward composition, cutting height and aesthetics of an area.
Are earthworms a problem?
Earthworms live within the soil environment; there are a range of species and each exhibits unique behavioural and physiological characteristics. Their distribution is influenced by climate, soil pH, soil texture, organic matter, mode of feeding, life style and their location within the soil profile.
The most important role of the earthworm is in the decomposition process within the soil. Earthworms feed on dead and decaying plant and animal material which in turn serves to control organic matter whilst also increasing nutrient availability and soil fertility. Other benefits include soil aeration leading to a modification (improvement) of soil structure, increased infiltration rates and improved rooting for plants.
A small number of earthworms produce casts made up of indigestible waste material and it is these casts that are considered undesirable by turf managers. For general amenity areas comprised of vigorous growing perennial ryegrass species mown above 25 mm such casts should not be problematic unless the area is being used as a sports pitch. Within a close mown, high quality ornamental lawn, however, these casts are considered unsightly. They will also serve to smother grass plants and encourage weeds thereby further affecting the aesthetics of an area. In such a situation the disadvantages of earthworm activity may be considered to outweigh the advantages.
Top tips for controlling earthworm activity
Good management practices can help to discourage earthworm activity.
- Organic matter is their main source of food. Controlling the level of organic material within the soil environment will reduce their food source and make an area less attractive. Therefore, box off clippings where possible, limit the use of organic fertilisers, encourage the breakdown of organic material with regular aeration and remove organic matter with a combined programme of scarification and hollow coring.
- Worms do not like acid conditions and much prefer neutral and alkaline soils. This is most useful in ornamental lawns dominated by fine bent and fescue species which also favour acid conditions. In such circumstances try to use acidic-reacting materials as opposed to alkaline ones. For example, ammonium sulphate based fertilisers and iron sulphate both provide an acidifying effect when applied and should be included within a fertiliser programme where worms are a problem.
- Take care if using lime products to raise pH as too much will increase worm activity. As well as being a soil conditioner, lime can be present in various disguises. Care should be taken when selecting sands, especially for top dressing - ensure that the material used contains no more than 0.5 % lime. Similarly, do not use limestone to backfill drain trenches. Limestone is easily broken down in acid soils significantly reducing the effectiveness of the drain but also attracting earthworms which will tend to cast into the stone backfill contaminating it with soil.
- Applications of a medium-coarse sand top dressing can also serve to reduce worm activity overtime as it tends to irritate the worms’ skin.
- Worm casts can be removed from lawn areas using a rake or brush but this should only be done in dry conditions. Trying to remove casts when they remain wet will result in smearing of the surface which may reduce surface infiltration rates and smother underlying grass.
- In some territories carbendazim-based chemical control products are available to reduce worm casting. However, they are only available to professional users with appropriate spraying qualifications.
Fungi are the most common disease-causing organisms in relation to turf but not all fungi within the soil are harmful. Many are in fact very beneficial and form important functions in relation to organic matter break down.
Those fungi responsible for disease are usually termed ‘pathogenic’. There are many different pathogenic fungi responsible for the individual diseases found on turf and they vary widely in their behavioural and physiological requirements. Most grow as thread-like structures called hyphae which can come together in large numbers to form mycelium. This mycelium may or may not be visible on the plant tissue of affected plants or within the soil.
In general, most pathogenic fungi are always present in turf but it is only when favourable environmental conditions occur that a disease will develop. Good management can often be enough to keep disease outbreaks at bay, however, the changeable and unpredictable nature of the weather often makes disease control difficult to achieve.
General amenity turf dominated by vigorous perennial ryegrass is less prone to many diseases, mainly due to the less intensive nature of the maintenance, especially at the higher height of cut. However, red thread (Laetisaria fuciformis) is probably the most prevalent disease to effect such areas as well as commonly affecting fine turf areas as well.
What are the symptoms of red thread disease?
- Grass has a general pink or red appearance turning bleached/light brown as the disease progresses.
- Fungal growth is visible either as pink, cottony flocks and/or as small red needles sticking out from diseased leaves. The needle like structures can be up to 25 mm long. They are called stromata and are the survival structures of the causal fungus.
- Infected patches of grass may be as small as 50 mm in diameter or as large as 350 mm.
- The fungus causes the grass leaves to die back from their tips but, unless grass growth is poor, damage is generally superficial.
What conditions favour the development of red thread disease?
- Red thread can occur at any time of year but is most commonly seen in late summer and autumn.
- Red thread disease can be found on any type of turf but red fescue and perennial ryegrass species are most susceptible. Different cultivars within these species will also have different levels of disease resistance.
- As for many turfgrass diseases a moist surface will aid the spread of disease and excess organic matter will serve to harbour causal fungi.
- Plants are most susceptible when grass growth is slow as a consequence of low fertility and in particular as a result of insufficient nitrogen.
Top tips for preventing and eradicating red thread disease
- Infected areas should receive an application of nitrogenous fertiliser. This should usually be enough to help the plant grow through the disease. The choice of fertiliser may depend on prevailing conditions, with a liquid being used if conditions are dry and no irrigation is available.
- Remove clippings from affected areas to minimise the spread of disease.
- Control organic matter and maintain surface drainage with regular aeration.
- Maintain a good level of fertility to aid healthy grass growth, particularly ensuring adequate levels of nitrogen are available throughout the season.
- Overseed using cultivars with a good tolerance to red thread infection.