In the August/September issue of Pitchcare, Dr Kate Entwistle discussed the growing problem of plant parasitic nematodes. Kate described some of the major plant nematodes found in cool season turfgrass and outlined our knowledge regarding the biology and management of these pests, as well as possible interactions with fungal pathogens.
In a short series of articles I hope to discuss some of the major issues relating to these increasingly important pests and provide an update on developments in their control and management.
One of the first questions I am asked when visiting a golf course or pitch is "Why have nematodes only become a problem in recent years?" The obvious answer is, of course, that nematodes are found in all soils and they have always caused damage to the plants we grow.
Most farmers will be familiar with a variety of plant parasitic nematodes (eelworm) causing damage to their potatoes, carrots or cereals. Similarly, any greenkeeper who has spent time working in the warmer US states will be able to relate horror stories of damaged greens, complaining golfers and seemingly endless applications of pesticide. The growing nematode problem in the European turfgrass industry may result from a number of factors:
- Nematode problems have been misidentified in the past - it has been argued that nematode damage has been overlooked in the past, with symptoms attributed to other factors including disease, insect pests and physical damage. This could be partly true, but experienced greenkeepers and groundsmen I have spoken with would argue that many current nematode problems are new to the industry
- Increasing use of sand-based constructions or heavy sand top dressing which favour nematode reproduction and increase plant vulnerability to damage - many of the nematodes that attack turfgrasses prefer sandy soils. In addition, the tolerance of grasses to nematode damage normally decreases as the sand content increases. This is due to the lower water holding capacity and higher rate of nutrient leaching in sandy soils
- Changes in nutritional management - some greenkeepers have raised the possibility that the move away from urea based fertilisers and towards foliar feeding may be affecting the soil environment and promoting an rise in nematode problems
- Climate change - the most significant driver behind the increasing level of damage in turfgrass may be soil temperature. Hotter summers and warmer winters have been the norm around the British Isles. Higher soil temperatures favour nematode reproduction and activity, and nematode damage is usually less common in regions that experience significant periods of frost. It may also be no coincidence that some of the most serious nematode damage seen in turf has been on pitches where there has been regular winter use of under soil heating.
This photo shows a rootknot nematode in the eye of a needle
Unfortunately, visible nematode symptoms in turf are fairly general and non-specific, which means it is usually impossible to confirm the presence of nematode damage without a laboratory analysis of the plants and rootzone. The nematodes themselves are microscopic and distinguishing the pests from the beneficial nematodes can be technically demanding and a job for the expert.
Subanguina root galls in the root system of a Poa plant
A look at the way nematodes actually affect plants can help understand the range of symptoms seen in turfgrass. Two major types of nematode parasitise turfgrass: ectoparasitic species migrate along root surfaces, feeding on the root cells.
Endoparasitic nematodes actually enter the root, where they migrate through the plant, before starting to feed. Some specialised endoparasitic species such as Meloidogyne (root-knot nematodes) establish and maintain complex feeding sites around the plant vascular tissue, but both ectoparasites and endoparasites can manipulate root morphology and physiology to support their parasitic way of life.
Consequently, nematode feeding causes different types of injury to plant roots:
• Reduction in root biomass (roots become shallow and sparse)
• Plant cell destruction
• Root lesions
• Abnormal root morphology
• Knot-like galls and root distortion
• Secondary infections by other pathogens
Equally important are the physiological effects seen in plants:
Chlorotic patches in a creeping bentgrass putting green caused by root knot nematodes
2. Water Uptake - commonly we see changes in water uptake, stomatal conductance, root conductance and wilting. The water shortage results in a range of effects inside the plant including decomposition of proteins and nucleic acids, disruption of plant enzymes and decreases in nutrient uptake, solute movement and chloroplast activity.
3. Nutrient Uptake - abnormal morphology and root destruction results in the disruption of the flow of micronutrients and macronutrients.
4. Plant growth regulators - nematodes can also affect the levels and distribution of many growth regulators in the plant. For example, cytokinins (required for cell division, translocation and chlorophyll synthesis) and gibberelins (required for cell elongation) are produced in the roots.
Root damage will disrupt their synthesis, so explaining the common symptoms of stunting, leaf senescence and an uneven distribution and decrease in nutrient concentration in shoots.
Plants will respond to any root-dysfunction with symptoms related to deficiencies of nutrient or water, and the following symptoms may indicate that you have a nematode problem in your turf:
Abnormal root tip swelling caused by the needle nematode. This can seriously affect root function.
• Yellowing and thinning of the turf
• Reduced turf vigour
• Premature wilt
• Turfgrass that is slow to recover from stress
• Turfgrass that does not respond to fertilisation
So, as a turf manager, how can you determine if you have a potential nematode problem in your greens or pitch?
Key Features of Nematode attack
• The distribution of nematodes in soil is usually uneven and often the nematode population is clustered into hot-spots
• Symptoms on the turf will be seen as non-uniform
• Affected plants respond poorly to stress and normal wear and tear
• Affected plants may be infected by weak fungal pathogens
• Plants may die when under high stress
What to look for above ground
• Chlorotic thinning turf
• No obvious signs of fungal disease
Pratylenchoides nematodes inside a creeping bentgrass plant root
• Poor response to nutrients
• Recovers slowly from drought
What to look for below ground
• Evidence of a stunted root system
• Abnormal root morphology
• Presence of thickenings in the roots
There are other useful questions to ask:
In turf containing mixed grass species, what types of grass are showing symptoms?
Are symptoms linked to conditions favouring nematodes? e.g. sandy soils or areas that have received heavy sand topdressing.
Sheath nematodes are important pests in ryegrass soccer pitches
It is also important to note that different turf types will contain different nematode problems. For example, while root knot nematodes are common in most turf types, stubby root nematodes and sheath nematodes are most often found in ryegrass soccer pitches.
Pratylenchoides is common in creeping bentgrass golf greens while root gall nematodes are restricted to parasitising Poa plants in golf greens.
Next time we will have a look at the range of management techniques which have been used to control nematode damage in turfgrass and get the views of some turf managers who have been actively dealing with the problems over the last few years.
Dr Colin C. Fleming
Applied Plant Science
Agri-Food and Biosciences Institute
Belfast BT9 5PX
Tel (44) 028 90 255276 (0ffice) 255262 (Laboratory)
Fax (44) 028 90 255007