Plant Parasitic Nematodes (PPNs)
Nematodes are animals with worldwide presence, so to find them on golf courses is completely normal. A variety of microscopic nematodes coexist in root zones where they work together, with other microorganisms, as part of the soil food web. The majority of nematodes recycle nutrients and play a beneficial role however a small number can inflict damage. While it is true that in warmer climates the damage caused by plant feeding nematodes is more severe, recent research has highlighted the need for management options in cool season regions where issues are becoming more common.
Written by: Dr. Deborah Cox
One fundamental factor contributing to increased nematode related decline in Europe is a shift to sand dominated root zones. This was originally implemented to offer a solution to common issues such as drainage, compaction, uneven water & nutrient distribution, and to ultimately improve playability. This movement inadvertently created an environment where some species of plant parasitic nematodes (PPNs) thrive because of the optimal soil pore size, oxygen availability, water availability & improved root systems upon which they feed.
In cooler regions of Europe, turf decline is usually attributed to a limited number of nematodes. Meloidogyne spp (root knot), Heterodera spp (cyst), and Subanguina spp (root gall) are three commonly encountered endoparasites on golf courses. Symptoms vary, but often small chlorotic patches can develop when populations have exceeded critical levels. Importantly, endoparasites spend almost all of their lives inside plant roots making treatment difficult and eradication almost impossible. Egg hatching (where the mobile stages of these nematodes exit the root in search of a new host) tends not to overlap meaning it is unlikely to detect them all in a single sample or treat them simultaneously. Despite the commonly held assumption that PPN activity ceases in Winter, both root knot and cyst juveniles are in fact very active during this time and they can complete more than one life cycle per year.
Helicotylenchus spp (spiral) & Tylencorynchus spp (stunt) are amongst the most commonly found PPN during Summer and they are often found together. Some species feed as ectoparasites (never entering the root), while others feed as semi-endoparasites (where the head becomes fixed in position and the rest of the body remains outside the root). These feeding habits mean that nematicide control is simpler and precision in timing treatment is less problematic. Issues can arise where their repeated & prolonged feeding close to the root tips result in shallow root systems. Symptoms often appear as turf thinning in poorly performing areas that don’t respond as expected to fertilisation, watering, or fungicides.
Because multiple PPNs are found in a typical sample, it is important to assess the burden from each type individually and the total combined pressure. This can be expressed as a nematode damage index (NDI) score which takes into account prevalence, abundance, and potential damage. While the goal is always the same (to identify and support the management of problematic nematode populations) it is not an exact science and different laboratories adopt different methodologies which can impact the assessment. Each have their own way of communicating potential damage and different thresholds exist. What tends to cause confusion is that there is no universal damage threshold that truly covers every scenario and NDI scores change over time making it a highly subjective topic.
Different turf types have different combinations of nematodes, and also different levels of tolerance. True links courses often have significant cyst nematode (Heterodera spp) pressure, and parkland courses often have significant root knot (Meloidogyne spp) issues, particularly on new constructions. Tolerance of a nematode burden is a complicated subject; while larger root systems tend to cope better (because the plant is still able to uptake the required amount of water & nutrients), more root tissue creates more potential feeding sites which in turn support PPN population growth. When high levels of nematode damage occur, it is important that the plant is encouraged to produce new roots to replace the lost tissue. Reducing stress levels in turf generally is critical to successful management of PPNs.
Reasons for developing a management strategy for nematode issues is about more than just aesthetics. Weaker, shorter, root systems are less efficient at obtaining water & nutrients which can impact the environmental strategy of a club. Reduced capability to uptake water often results in turf requiring more frequent watering. This is because roots might not reach supplies anymore, and it also introduces the possibility of fertiliser runoff because of a reduced nutrient requirement. Co-infection (with non-nematode pathogens) is common, and weakened turf presents an opportunity for fungi, such as Colletotrichum cereale, to thrive on senescent or damaged tissue. While there is a difference between ‘direct damage caused by PPNs’ and ‘PPNs weakening turf’, preventing additive pressure from opportunistic pathogens is one reason why conservative management approaches are often taken.
The biggest challenges in understanding biological patterns in turf include going back to the same spot each time, and taking the right material. Where no symptoms exist, a zig-zag pattern may be adopted taking approx. 10-12 cores (10cm deep, 2cm diameter) per 500m2. Where symptoms exist and the intention is to monitor populations, or confirm a nematode issue, approx. 8 cores may be taken covering the range of symptoms evenly in a smaller area. Composite samples are best taken in grip seal bags to prevent moisture loss. Areas of complete turf loss are less useful, however the ‘leading edge’ of a symptomatic area may be sampled using a cup-cutter. Aiming for approx. 1/3 symptoms and 2/3 healthy tissue, and wrapping a large core in foil for transport, works well. If ever in doubt, simply ask a lab for their advice.
