In the past, it seemed as if turf diseases just needed a name in order for us to be
Waitea Patch
Fact: Plant parasitic nematodes have long been known to cause significant problems on warm-season turfgrasses and they are one of the major causes of agricultural losses worldwide, estimated to be around $78 billion a year, according to the University of California. Fact: Plant parasitic nematodes can be
present in virtually all soils but it is only at high populations (or populations above their threshold values) that they are likely to cause problems in turf. Fact: The Root-knot nematode Meloidogyne is now recognised as a serious problem on both fine turf and coarse turfgrass swards. The Root-gall nematode Subanguina is a significant and widespread problem on Poa. The Lesion nematodes Pratylenchus and Pratylenchoides are increasingly being recorded as damaging both coarse and fine turfgrass swards. The Spiral nematode Helicotylenchus and the Stunt nematode Tylenchorhynchus are both regularly identified as being present in high populations in fine turf that is showing symptoms typically considered to have a fungal disease. Fact: Although we are working towards
improving our knowledge and understanding of these nematodes in amenity situations and to determine more robust threshold values in different turf situations, there is still a vast amount to learn about these organisms. One thing that we know is that the majority of plant parasitic nematodes feed on root tissues, piercing the root cells with their stylet and removing the cell content for nutrition. The length of time that the nematode spends feeding at any given site depends on the nematode species but they will all inject chemicals in to the root tissues that facilitate their feeding and cause adverse changes to the physiology of the plant. Some species of nematode keep most of their body in the rootzone whilst they are feeding, but others venture in to or remain permanently inside the root tissues as they feed. Not only do they impart chemical damage via their secretions but they also cause mechanical damage to the root structure.
86 Waitea Patch
Plants affected by the activity of nematodes, weakened by reduced water and nutrient uptake and damaged physically by the passage of the nematodes through the plant cells, would surely be more susceptible to fungal infection. It could also be argued that plants affected by fungal disease would be an easier source of nutrition for the nematode.
So, in cases where both a nematode and a fungus occur in the same area of turf, which was first to attack the plant? Although research has been done to look at this situation in agriculture, similar studies have not been done in amenity situations. However, I would argue that the situation that makes most sense is most likely. Considering that plant parasitic nematodes need a steady source of nutrition and that dying or decaying plants that have succumbed to a fungal infection are less likely than unaffected plants to provide this, the nematodes are more likely to infect newly produced and growing root tissues and initiate weakness in the plant.
Most fungi, as we saw earlier, have the ability to live as parasites or saprophytes and they have the capacity to colonise weakened plants, causing increasingly severe disease problems. I would argue that it is possible that where a rootzone supports a high population of plant parasitic nematodes, and where there is root deformity or reduced rooting depth, it is more likely that the nematodes are weakening the plant and enabling fungal disease to develop or allowing a niggling, reoccurring fungal infection that never really goes away. Plants that are dying as a result of fungal infection would, in my opinion, not be preferentially targeted as a feeding site by nematodes. Work is needed to confirm this theory in amenity situations, but there is evidence to support the general theory that nematode infected plants can show increased levels of fungal infection caused either by increased entry of the fungus through the wounds inflicted by the nematode, or entry in to a weakened plant whose physiology has been altered by the secretions pumped in to the plant
satisfied during nematode feeding.
In the past, it seemed as if turf diseases just needed a name in order for us to be satisfied and for a management programme to be implemented. Now, with our increased abilities and awareness of potential and emerging problems, we are quite rightly called upon to identify all potential contributory factors in disease development and to suggest which is the most important. But, how can we make an informed assessment of relative importance when so much is still to be learned about the organisms themselves? We need to keep a close eye on the published research that is increasingly available to us and recognise that there is still so much that we have yet to learn about something which only a couple of decades ago seemed ‘all sewn up’ and to remember to employ a modicum of common sense.
What else is waiting to be discovered as we look closer at the turf with the improved tools that are available to researchers around the world?
Dr. Kate Entwistle The Turf Disease Centre Email:
kate@theturfdiseasecentre.co.uk
Rapid Blight
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