How often has it been said that effective turfgrass disease management depends on early and accurate identification of the problem. Today, this is arguably more important than ever and I hope that the following article will explain why.
Diagnosing disease is not the same as identifying a pathogen. Disease diagnosis is a process that involves not only identifying the presence of potential pathogens but also understanding how those pathogens can damage the turf and recognising the progression of symptoms on affected plants. It is possible for any symptom of damage, for example chlorosis of the leaf, to have more than one cause and therefore, to recognise one symptom in isolation and to assume the cause of the damage based on that one symptom, could lead to serious errors in diagnosis. If the problem is misdiagnosed, any implemented disease management options could well provide no effective control and may even allow the damage to progress unchecked. At best, both time and money will have been wasted. At worst, the result might be lasting damage to the sward.
So what questions do you need to ask yourself when trying to identify disease?
1. What symptoms are present on the sward?
With regard to disease, it doesn't matter if the cause is in the root, crown or leaf tissue, you will ultimately see symptoms on the leaf. Are the symptoms generally affecting the whole sward (as with leaf spots) or are there discrete patches developing (as with take-all patch). In addition to the spatial distribution of the symptoms, what other characteristics are there? Are the affected plants yellowing or becoming reddened in colour, are all leaves equally affected or is it the older or younger leaves that are discoloured. Are there noticeable spots or lesions on the leaf or is the whole leaf blade equally affected. Is there any evidence of fungal mycelium or so-called watersoaked leaf tissue where the leaf loses structure and appears wet and slimy to the touch.
2. What grasses are affected?
In a mixed sward, knowing which grasses are affected can be a great help in identifying the disease. Although fungi are capable of causing infection on different grasses, most fungi will preferentially infect one grass type over another in a mixed sward and the grass preference will vary depending on the specific fungus. Also, the symptoms caused by one fungus may vary considerably on different grass types.
3. When did the symptoms first appear?
This is important to know because some fungal diseases can progress very quickly and so knowing when they appeared and what they looked like when they were initially noticed, can help to identify the type of fungus that may be causing the damage.
4. How have the symptoms changed since they were first noticed?
This is also very important because it will tell you how quickly the problem is developing or spreading and how the symptoms change with 'maturity' of the disease.
Certain products can stimulate the development of some diseases by 'weakening' the plant or affecting the pH of the rootzone, for example. Plant protection products can affect the growth of fungi, either by reducing their presence in the sward or in some cases, stimulating them. Knowing whether or not they have been applied is critical if an accurate analysis is required.
It is not unknown for symptoms of damage that strongly resemble turfgrass disease to be caused by environmental/rootzone conditions or occur as a result of product applications.
Localised areas of various grasses in a mixed sward can react differently to prevailing conditions and lead to symptoms that resemble a 'patch disease'. In these circumstances, inappropriate management using plant protection products will not only have a potentially adverse environmental impact in these situations but are also unlikely to manage the symptoms, allow the problem to progress unchecked, allow the possibility of repeated future problems and waste your resources of time and money.
Once the disease is accurately diagnosed, you can implement the most appropriate and effective management strategy. This could well involve an initial application of the most suitable plant protection product but more importantly, effect a change in your long-term cultural management plan to try and reduce the conditions that favour both initial fungal and disease development.
So what diseases should you be aware of?
On fine turf areas, take-all patch disease is being increasingly seen on Poa annua plants. It has been known for many years that the fungus responsible for this disease can infect and cause disease in P. annua as well as in Agrostis species but the disease has most frequently been seen in Agrostis species as these plants appear to be more susceptible to infection. The symptoms of take-all patch disease on P. annua will appear very similar to those on Agrostis swards but there are other, less common patch diseases that can also develop on P. annua and so it is always worth having the cause of the damage confirmed before any treatment in applied. There is increasing evidence to show that adequate availability of manganese can reduce the severity of take-all patch disease and may also lessen the development of the disease in new constructions, if added just prior to seeding. Manganese is used by the plants to produce compounds which are released around the roots and which serve as a defence against this particular root disease.
The fungus, Gaeumannomyces graminis var. avenae, can effectively lock-up the manganese and make it unavailable to the plants, reducing the level of the plants defence against infection. A better result from manganese applications will generally be achieved with root uptake of the element because manganese is immobilised in the phloem (the route of primary movement of materials from the leaf to the root).
Fusarium patch remains arguably our most common and most damaging turfgrass disease and over recent years, it has occurred with increasing frequency and severity on affected swards. The fungus that causes fusarium patch, Microdochium nivale, is able to live saprophytically on dead and decaying organic material and therefore, is able to respond rapidly to ideal environmental conditions that allow it to cause disease.
The fungus will grow most actively under high pH conditions but any small increase in pH, even under generally more acidic conditions, will allow the fungus to grow more quickly than other organisms that normally ensure a healthy balance in the rootzone. In addition, this fungus is capable of growing at temperatures just above freezing and is not actually killed by frost.
The ideal conditions for rapid growth of this fungus and for fusarium patch development and repeated cycles of frost and thaw or constant cool, damp weather when turf growth is slow and the sward remains wet. Over the past few years, the winters have allowed many cases of 'aggressive' fusarium patch disease where treatments aimed at managing the disease have apparently failed.
Due to the constant presence of this fungus and its ability to dominate the turf under cool, wet conditions, it is imperative that swards that are prone to fusarium patch disease are managed throughout the year to try and minimise favourable conditions from developing in the winter. Fusarium patch disease can and does develop all year round on certain turf areas, and where this is the case, it would be well worth while looking at nutrient input, the rootzone and surface drainage as possible reasons for the constant problem.
Red thread disease appears to be increasing in occurrence on both fine and coarse turfgrass sward but in most cases, the damage is relatively superficial and restricted to the leaf. Only in extreme cases does the disease get in to the crown and cause lasting damage to the sward. In most cases it is possible to reduce the severity of the symptoms by a light fertiliser application and removal of the clippings, but it is not uncommon for red thread to develop on swards maintained under adequate nutrition. In these cases, it may be necessary to apply a plant protection product but again, remove the clippings to reduce the likely reoccurrence of the disease.
Fairy rings, although not strictly disease problems, are problems caused by fungi and therefore often referred to as diseases. These can occur on any turf area but the severity of the symptoms will depend upon the rootzone composition, the sward and the fungus present. In many cases, the associated water repellence within the rootzone can cause as much if not more of a problem than the localised disfigurement of the sward. Over recent years, I have seen many close-mown fine turf areas with superficial fairy ring disease which also have significant problems with anthracnose and / or fusarium patch disease and these diseases can cause more immediate damage to the sward than the initial problem of water-repellence.
Over the past four years, I have received an increasing number of turf samples through my laboratory that show damage to the sward resulting from nematode infections of the roots. These problems are found in both fine turf (predominantly in Poa annua swards but also creeping bentgrasses) and coarse turfgrass swards with nematode damage in stadia pitches certainly not uncommon.
New diseases are also being found. Not only do I include nematode diseases that used to be dismissed as problems of cool-season turfgrasses, but new fungal diseases are also being identified as well. Only last August, I identified the disease Rapid Blight on a golf course in the UK and I am presently working on trying to formally identify a patch disease that is affecting fescue/bentgrass greens on a golf course in Ireland that would appear to be caused by a Pythium species of fungus. We are now better able to identify the organisms that cause disease and with the increasing number of grass types available for use in amenity turf, the potential host range has also increased and new problems have arisen.
It is fair to say that the vast majority of disease problems that you are likely to encounter will be one or more of the so-called common turf diseases, fusarium patch, red thread, take-all patch, leaf spots and anthracnose but as diseases appear with unusual symptoms and new diseases are emerging as a result of maintenance practice, new grass availability and our increased ability to identify them, can you be certain enough of the cause of the symptoms to employ the most effective management? If you have any doubt at all, just ask.