Turfgrass disease expert Dr Percy Wong outlines some of the new ERI diseases present in Australia, the fungal pathogens that cause them and possible strategies to manage them in various turf situations.
In the last 20 years, there has been an upsurge in the prevalence and severity of patch diseases in warm-season grasses such as couch, kikuyu and buffalo, as well as cool-season grasses such as bentgrass and wintergrass.
These patch diseases, often called ERI diseases, have been very worrying for turf managers because they have not always responded to chemical control. The most likely reason for this is that the diseases have been misdiagnosed and, therefore, the chemicals used would have been ineffective. My research has shown that the majority of these diseases are new and some of the fungal pathogens that cause them do not even have scientific names at present.
We hear a lot about ‘ERI fungi’ these days but what exactly does the term mean? ‘ERI fungi’ stands for ‘ectotrophic root-infecting fungi’ (Clarke and Gould 1993) and is a catch-all name given to fungi which cause a number of root diseases in turf.
These soil-borne fungi produce black mycelium (fungal threads) on the surface of roots and underground stems, before invading the plant tissues and causing death of roots and underground stems. They include;
All these pathogens are ERI fungi, but, as their scientific names indicate, they are very different and unrelated fungi because they come from different fungal genera (the first part of the binomial name).
As such, it would not be possible to specifically control ERI fungi as a group with certain chemicals without first identifying the exact pathogen or pathogens present, since the control measures are different for different pathogens.
For example, Bayfidan works well against take-all patch but not against spring dead patch.
Therefore, more research needs to be carried out to devise management strategies for the control of each of the ERI fungi. Chemical companies would also need to test their new chemicals against each of these pathogens in order to make useful and effective recommendations. To have a label saying a chemical is effective against ‘ERI fungi’ is meaningless because, as we have noted above, the pathogens are so varied.
Fairway patch is a serious disease that occurs on couch and kikuyu fairways of at least 20 golf clubs in Queensland, New South Wales, Victoria and Western Australia, but may have a wider distribution
What follows is a summary of some of the ERI patch diseases currently present in warm-season grasses and cool-season grasses in Australia, the fungal pathogens that cause them and notes on possible management strategies.
Fairway patch is a serious disease that occurs on couch and kikuyu fairways of at least 20 golf clubs in Queensland, New South Wales, Victoria and Western Australia, but may have a wider distribution. The disease begins as tan-coloured irregular-shaped patches (5-10cm in diameter) that progress to larger brown rings on predominantly Wintergreen couch fairways, but has been found in all the commonly grown couch cultivars.
The rings enlarge over several years to measure 50cm or more in diameter and can coalesce to form an ugly patchwork of dead and dying grass. The disease resembles take-all patch (Wong et al. 2000) or brown patch caused by Rhizoctonia solani, but all the common fungicides that have been used against these diseases have been ineffective.
The disease is caused by a ‘black sterile ERI fungus’. A ‘sterile fungus’ simply means that it has not produced spores in culture or in the field. Traditionally, we require fungal spores or other distinctive fungal structures to identify the fungus, but these days DNA techniques are increasingly used to identify fungi. By this means, the pathogen was found to be a new fungal species, which was named Phialocephala bamuru (Wong et al. 2015). The species name ‘bamuru’ is the word for ‘grass’ in the Aboriginal language of the Sydney region.
There are, at present, no satisfactory control measures for this disease. Three golf clubs in Sydney (Royal Sydney, New South Wales and Bonnie Doon) have invested in research into this disease because it had become a serious disease by around 2007/8. Chemical control was found to be ineffective. The research has led to the discovery of the cause of the disease as well as some management strategies to reduce the disfiguring impact of the disease. However, a major injection of research funds is required to employ a full-time research worker to study this and other ERI diseases for at least three years, if we are to manage these problematic diseases successfully.
For over 20 years, a patch disease called ‘summer decline’ has occurred on predominantly 328 couch greens and been plaguing golf superintendents in southern Queensland (Stirling 2001) and northern New South Wales. It now appears to be more widespread, having been found in New South Wales, Victoria and Western Australia. The disease also occurs on bowling greens.
It starts as small brown patches which enlarge and coalesce with other patches to form weakened and unsightly turf. This is most severe in the summer months when high temperatures exacerbate the symptoms, probably because the weakened root systems are unable to supply enough water to the grass and the grass dies back.
Over several years, the debilitating effect of the root disease produces a noticeable decline in turf quality, especially in summer, hence the name ‘summer decline’. Chemical control has been variable at best and it remains a difficult disease to manage.
A number of fungi were isolated consistently from the blackened diseased roots of the diseased turf (Stirling 2001), but only one of these fungi has been shown to be pathogenic to couch (Wong et al. 2012). The fungus was described as a new species, Magnaporthe griffinii, as it resembled Magnaporthe poae, the cause of ‘summer patch’ in the USA.
It is important to note that the latter pathogen has not been scientifically documented in Australia and, therefore, we do not have ‘summer patch; in this country. Further taxonomic research in the USA has shown that Magnaporthe poae was not a true Magnaporthe species and was renamed Magnaporthiopsis poae (Luo & Zhang 2013). Two years ago, taxonomists in Brisbane also determined that the summer decline pathogen was not a Magnaporthe species and created a new genus, Wongia, in my honour (Khemmuk et al. 2016). So, the pathogen is now called Wongia griffinii.
Summer decline on a couch golf green
Another disease causing couch grass to decline was first found on bowling greens in Adelaide. I isolated an ERI fungus, which caused root rotting in couch grass in glasshouse tests. As it also resembled a Magnaporthe species, I described and named it Magnaporthe garrettii (Wong et al. 2012). However, its correct name now is Wongia garrettii, as explained in the above paragraph.
I have called the disease ‘Adelaide patch’ because the disease was first found in Adelaide. It has a limited occurrence in Australia, having only been documented in couch grass in Adelaide and Sydney. It is a serious disease which shows up in late spring every year and over several years causes the turf to thin badly and decline in quality. The patches have not responded to all chemical treatments that have been tried.
This is a serious disease of couch on golf courses and bowling greens. It was first observed at Deniliquin Golf Club, NSW, hence its name. It is caused by another black ERI fungus, which has been shown by DNA techniques to be a new genus. It has been named Budhanggurabania cynodonticola (Crous, Wingfield, Guarro et al. 2015). The name ‘Budhanggurabania’ describes a ‘black fungus or mushroom’ in the Wiradjuri language. The disease has also been found to cause severe disease on bowling greens in Dubbo (NSW), Townsville (Qld) and Darwin (NT). Again, no chemicals have successfully controlled the disease.
Adelaide patch on a couchgrass bowling green
Dempsey’s patch on a bentgrass nursery
This patch disease of buffalo and couch is caused by an ERI fungus called Gaeumannomyces wongoonoo (Wong 2002). The specific name of the fungus ‘wongoonoo’ is a Central Australian Aboriginal word for grass. So far it has only been documented in Perth, Brisbane and Griffith (NSW) but it is likely to be more widespread. The fungal pathogen may be indigenous to Australia as it has not been found overseas.
The pathogen is a close relative of the take-all pathogen (Gaeumannomyces graminis var. avenae), therefore it is not surprising that the disease symptoms are similar to those of take-all patch. There are no recommended control measures for this disease but it is likely that chemicals that control take-all patch might be effective against this disease.
This patch disease was first seen by former NSW Golf Club superintendent Gary Dempsey on his bentgrass nursery and greens. The patch symptoms begin as a purplish discolouration of the leaves. The affected area increases in size and can measure 30-50cm across. The patches are roughly circular. The plants in the patches tend to wilt and become yellow and unthrifty because the roots are diseased. However, the patches recover when the turf is adequately watered and fertilised. Therefore, by comparison with the other patch diseases, Dempsey’s patch is not such a serious disease.
The diseased plants showed typical root rotting symptoms caused by ERI fungi. I isolated a fungus from the diseased roots and it has turned out to be a new species of Magnaporthiopsis, which has been named Magnaporthiopsis agrostidis (Crous, Wingfield, Roux et al. 2015). Therefore, this fungus is related to the summer patch pathogen (Magnaporthiopsis poae) but was found to be different to it from DNA studies.
In the last few years, other unnamed Magnaporthiopsis species have been recovered from the diseased roots of Poa annua in wintergrass-dominant bentgrass greens. The disease occurs in summer as small tan-coloured patches (10-20cm in diameter) which can coalesce into larger patches. It was thought that the disease was ‘summer patch’ but it has not responded to all the available chemicals.
Similar fungi from various localities and hosts (Chewing’s fescue, couch, etc.) have been shown to be several new species of Magnaporthiopsis using DNA techniques (Wong et al. unpublished). What is clear, however, is that these species are not the same as the summer patch pathogen that occurs in the USA. Besides Magnaporthiopsis, there have been other dark coloured fungi isolated from diseased roots that have not been sufficiently studied to know their identity or if they are pathogens.
The diseases discussed above and their known hosts are summarised in Tables 1 and 2. It is obvious that much more research is required to fully investigate the identity of unnamed pathogens and, more importantly, the management of these new and problematic patch diseases.
The majority of ERI fungi have not produced spores and therefore cannot be easily identified taxonomically by traditional means. Most of these fungi appear as dark fungal threads without distinctive characteristics when viewed on diseased roots or underground stems under a dissecting microscope. As such, it would be virtually impossible to accurately identify the ERI pathogens without first isolating the pathogen(s) from the diseased tissues and studying them further in culture or using DNA techniques.
It becomes obvious that it will be difficult to rationally recommend fungicides for their control if the exact pathogen is not known. These diseases have been flagrantly misdiagnosed in the past by only examining disease specimens under a dissecting microscope. Without accurate diagnoses, we will continue to have difficulty managing these diseases.
In the last two decades, there has been a decline in technical expertise in turf disease diagnosis in Australia. The turf industry has to invest in better qualified personnel to provide a high quality diagnostic service. After all, applying an inappropriate fungicide to control a wrongly diagnosed disease is an expensive and futile exercise. It is also environmentally irresponsible.
There is also a need for research capability in the study of turf diseases at universities and research institutions to be scaled up. In the absence of local research, there has been too much reliance on overseas research, especially from the United States, for information on turf diseases. Unfortunately, many of our ERI pathogens do not occur overseas. For example, fairway patch has not been found in the USA and as such it is debatable whether we should use their management recommendations on patch diseases that visually resemble our ERI diseases.
Deniliquin patch on a Golf Course fairway
The successful management of these diseases may lie in an integrated approach that does not rely entirely on chemical control. There are indications that biological control agents, judicious fertiliser use and cultural practices such as de-thatching and higher mowing heights (where possible) may be important in reducing the devastating impacts of the diseases. The main management focus should be to keep turf root systems as healthy as possible by providing adequate nutrition, well aerated soil conditions and a lower pathogen load by regular de-thatching.
Where fungal biological agents are used, turf manager should be mindful of pesticide usage that is compatible with those agents; otherwise, the populations of the fungal antagonists would not be high enough to effectively suppress the pathogens.
Since the spread of these diseases appears to be mainly due to the transfer of infected propagating material, the development of a turf quality assurance accreditation scheme may reduce their spread from turf farms to disease-free areas. This is important for all the diseases discussed above. In the longer term, disease resistance may be the only effective way of combating these diseases but it also requires a major research effort.