A detailed look at gray leaf spot - a potentially devastating lawn disease.
Let's start with the fungus
In August 2020, Gray Leaf Spot (GLS) fungal disease was first confirmed as the cause of damage to a stadium field in the UK (Photo 1) and a notice was published in Grounds Management magazine to make caretakers aware of this potentially devastating disease. This article adds details to that advisory and provides information on the fungus causing the disease, describes developing symptoms, describes current control options, and discusses potential options for disease management.
All living organisms have the so-called Latin name; a scientific name that places the organism within a specific genus and species. By doing this, we group organisms that share similar characteristics so that we know that all organisms within a certain genus, for example, will look or act similarly. Within a genus, some individuals may show differences in form or function, and for that reason we assign a species name to show that there are slight differences. We can further separate species into subspecies, focusing more and more on the detailed differences of individual organisms. This ordered classification helps us understand new or previously undescribed organisms in the early days of their discovery. In the world of the living, mushrooms have been unique in that they have always been given two Latin names, one for each of their sexual and asexual stages. For the fungus that causes GLS disease, the genus name of the sexual stage is Magnaporthe and that of the asexual stage is Pyricularia. However, it was recently decided that the classification of fungi should align with all other organisms and there should be only one Latin name regardless of the sexual / asexual stage of the fungus and that meant deciding which of the two names was the most appropriate. . . Although research has confirmed that sexual reproduction of the fungus is possible in the laboratory, it has not been confirmed (to our knowledge) that the sexual stage of the fungus occurs in nature (Wei, 2015). Possibly, for that reason, it has been decided that the genus name of the causative fungus should be Pyricularia and the species that specifically infects recreational lawns should be P. oryzae.
Although the currently accepted name for the fungus that causes GLS in managed recreational lawns is Pyricularia oryzae (Tharreau et al, 2019), there are many published articles and scientific papers that have used the names Pyricularia grisea, Magnaporthe grisea, or Magnaporthe oryzae. It is likely that some authors continue to use these "alternative" names in articles about GLS on turf, but what is important to know is that they are all talking about the same disease problem.
What lawns can be affected by P. oryzae?
GLS is a fungal disease that can occur in warm and cool season lawns and was initially identified in 1991 after the outbreak of a severe blight disease on a perennial ryegrass (Lolium perenne) street in the U.S. Most notably, St Augustinegrass (Stenotaphrum secundatum) and Kikuyugrass (Pennisetum clandestinum) are the most affected warm season grasses and perennial ryegrass (L. perenne) and tall fescue (Festuca arundinacea) the most affected and commonly used cold season grasses. Due to its extensive use, not only on sports fields (such as natural fields and in various hybrid turf systems) but increasingly on golf courses and other recreation areas, L. perenne is possibly the most important grass affected for this disease. GLS can grow aggressively in young plants and 4-5 week old seedlings are extremely susceptible to infection.
What are the symptoms of infection?
Symptoms of the disease will vary slightly depending on the type of lawn affected and the age of the infected plants. The most serious outbreaks tend to develop on newly established grasses and are a major concern where areas are regularly replanted with L. perenne. It is important to note that the initial symptoms of GLS disease develop on grassy areas that receive the most sunlight and that shaded or partially shaded grassy areas are noticeably less affected.
Initial symptoms are small dark brown spots or lesions (1mm to 3mm in diameter) on leaf and stem tissues that may appear "water-soaked." These lesions have a purple margin and may also have a yellowish area of surrounding leaf tissue (Uddin et al 2003). Individual leaves can twist, although this symptom is not always easy to see on a closer mowed lawn (Photo 2).
When the disease is active, the number of lesions on a leaf will increase and individual lesions will increase rapidly. On warm-season lawns, infected leaves die and turn brown, giving the grass an overall dark hue, but on cool-season lawns, the leaf infection turns into patches of dead grass that expand and spread. merge rapidly (Butler & Kerns, 2019). In the early stages of the disease, the symptoms can be confused with those of heat stress or drought, and in the later stages, the disease can easily be misidentified as Pythium blight. In affected L. perenne grass, the stem base tissue often turns dark brown (Photo 3) and can lead to a costly misdiagnosis of Basal Rot Anthracnose.
Active outbreaks of GLS can lead to partial or total destruction of a stadium field within 3 to 5 days. An unmistakable symptom of this active disease is the development of countless spores in affected plant tissues. The spores are kept away from the plant surface in specialized mycelial structures called sporophores, and each sporophor will support multiple spores. The individual spores are pear-shaped and contain three separate cells. Its characteristic shape makes the microscopic diagnosis of active disease quick and decisive. The spore mass produced by each lesion causes the development of a "velvety" gray covering on the affected tissues (Photo 4) and this symptom gives the disease its common name. Overnight incubation of infected plants in higher humidity conditions will encourage this mass of spore production and may be one way to confirm a possible disease outbreak. The symptoms of GLS can potentially be confused with those caused by Pythium fungi, but with GLS, an obvious aerial fungal mycelium will not develop in hatched plants.
How does the fungus infect the plant?
GLS is a foliar fungal disease. The spores of the causative fungus that come in contact with the leaf surface will eventually adhere to the leaf and germinate (Figure 1). The spores produce a germination tube that rapidly develops into a specialized infection structure known as appressorium (Photo 5).
As the appressorium develops, it darkens due to the accumulation of melanin that strengthens the structure and, finally, through an accumulation of pressure inside, the fungus makes its way into the plant cell to which it is attached.
Once the fungus is inside the plant cell, the mycelium of the fungus develops through the plant tissues, using the contents of the plant cell as a source of nutrition. This fungal development causes the observed "water soak" of the plant. As the infection continues, the lesions mature, darken, and expand, eventually causing the leaf to die. The fungus completes its life cycle by growing from dying plant tissues and producing spores on the plant surface. The development of spores in infected plant tissues can take between 2 and 5 days (Uddin et al. 2003) but, in general, during active disease development in the field, plant samples infected are incubated overnight in warm, humid conditions in a sealed plastic bag. will show a mass of spore development. If the samples taken for laboratory analysis are well packaged (to keep the lawn clean of any root zone contamination), confirmation of this disease can be made within minutes of the laboratory receiving the sample because the fungus will have developed during shipment. The fungal spores will move (by wind, rain, machines, humans, birds) all over the lawn and each spore has the potential to start a new cycle of infection.
What climatic conditions promote the development of diseases?
With regard to the development of diseases, it is known that the effects of temperature, the duration of leaf humidity and relative humidity are highly interdependent (Uddin et al, 2003b). As the ambient temperature increases, the duration of leaf moisture required for germination and infection of fungal spores decreases. Research has shown that at temperatures between 27 and 32° C, only 9 hours of leaf moisture are needed to facilitate infection, but at lower temperatures (20-23 ° C), more than 21 hours are required (Kerns, 2018). At temperatures below 9 ° C, fungal growth tends to cease. Actual leaf moisture allows spore germination and leaf infection, as well as rapid expansion of developing leaf lesions; however, the high relative humidity will encourage the massive production of fungal spores on the leaf surface (Uddin et al, 2003b).
Initial infections will develop in the early spring months when temperatures are relatively cold, but these initial infection levels are low and will generally go unnoticed. With each cycle of infection, the presence of fungi in the lawn will increase (through increasing amounts of spore inoculum) and eventually, during the summer months as temperatures and humidity increase, the amount of fungal inoculum it will be so overwhelming that the infection will result in symptoms of illness. Rising temperatures shortens the time it takes for the fungus to complete its life cycle and the number of infection cycles increases rapidly until late summer. At that time of year, susceptible grasses are at high risk of developing the disease and, if left untreated, the disease will completely kill the affected lawn.
The fungus is believed to survive saprophytically on decaying organic matter at the base of the turf and this could be a source of inoculum for future disease outbreaks. Annual wind-borne infections are also possible, but more research is needed to determine the relative contribution of each potential source of inoculum (Uddin et al, 2003). However, with the rapid progress being made in molecular research, it should be possible to map the population of active fungi in a specific turf area in consecutive years and compare the results to determine a more likely source of the outbreak.
How can we manage this disease?
Cultural conditions that will help reduce disease development should focus on reducing the relative humidity around the lawn. Using cooling fans in stadiums, for example, will not only lower the air temperature, but will also help dry the surface of the leaves. Ensuring that the root zone does not remain wet around the base of the lawn will also help decrease moisture in the lawn. It is important, especially in root zones of pure sand or with high sand content, that the plant does not suffer from drought conditions. Irrigation time and application volume are critical factors during periods of potential water activity diseases, to ensure that the plant has access to enough water but that the leaf surface remains dry for as long as possible. Ideally, susceptible grass should not be watered after 6 p.m. so that the blade surface remains dry overnight. It may be necessary to use wetting agents to ensure that water is available deeper into the root zone and is maintained in high-sand profiles.
Nutrient availability is also a factor that we can manage. There are reports that the severity of the disease increases with increasing nitrogen availability and especially if the application is through a water-soluble nitrogen source (Madeiras, 2020). In general, balanced availability of nutrients at the rates necessary to maintain strong grass growth should reduce the plant's susceptibility to disease and allow the plant to grow through minor disease outbreaks. Care must be taken not to stimulate rapid and weak leaf development, as the leaves will be more susceptible to infection by the fungus. Observations have shown that GLS disease severity generally increases with rapidly increasing amounts of nitrogen (N), while controlled-release fertilizers can help reduce the risk of disease. Low doses of nitrogen applied at shorter intervals are less likely to promote disease and ideally application rates should be kept below 1.25 g N / m² / application (Braitmaier, personal communication).
The lawn should be kept at an optimal height for the type of lawn, but when symptoms of the disease begin to develop, the cutting height can be lowered slightly and clippings removed. This reduction in cutting height is the opposite of what would be recommended to minimize other leaf spot diseases (usually caused by fungi Drechslera spp. Or Bipolaris sp.) But for all leaf shoots, it is better to remove cuttings and thus remove potential inoculum. It is worth mentioning that if GLS is severely affecting the lawn, the removal of clippings is unlikely to make a significant difference in disease progression (such is the amount of fungal spore inoculum during high disease pressure ) (Bonos et al, 2006).
We know that young plants are susceptible to infection and therefore when disease has been previously confirmed at a site, prepare for possible infection 4-5 weeks after emergence. There has been much interest from turfgrass breeders around the world to produce turfgrass varieties (cultivars) that show reduced susceptibility to GLS disease and several of these varieties of L. perenne are now available. Since 2000, Lolium varieties have been developed and tested in the US for GLS tolerance at the various NTEP (National Turf Assessment Program: www.ntep.org) sites and at Rutgers University in New Brunswick (NJ) (https: // turf.rutgers.edu/research/reports/). Breeders in the US talk about so-called "GLS resistant" varieties, but these varieties are not 100% resistant to GLS, they simply have a reduced susceptibility to the disease.
When this disease has previously developed or when there is concern that it could potentially develop, the use of these GLS-tolerant varieties is strongly recommended. It should be noted that varieties showing reduced susceptibility to GLS have a darker green leaf color than many L. perennial varieties and may not blend very well into an existing lawn. That said, in Germany (since 2017) and in Austria (since 2018) many previously affected stadiums have used the dark GLS tolerant varieties after their Koro-Renovation. High seed purity is essential for these tolerant varieties because entry of pale green Poa annua or P. trivialis will be even more obvious through the lawn.
Another option on disease-prone lawns is to use grasses that are not affected by this disease. For a sports field, the use of Poa pratensis could be considered, but there are differences in germination and establishment rates between P. pratensis and L. perenne that could make it an inappropriate option in certain cases. Some gardeners have been able to limit GLS infection by initially sowing with 100% Poa pratensis (dark varieties) and a few weeks later, replanting with 100% GLS-tolerant L. perenne. Other gardeners are using a hybrid system consisting of 30% Poa pratensis and 70% a mixture of GLS tolerant L. perenne varieties for summer renovation (Braitmaier, personal experience).
Fungicide applications will be effective against the causative fungus, but as the disease becomes increasingly aggressive during the summer months, the relative efficacy of the fungicides is likely to be reduced. Early or preventive applications will be required to effectively manage summer outbreaks of the disease, but cases of resistance or reduced susceptibility to the fungicides strobilurin and DMI, respectively, have already been reported (Bonos et al, 2006). Products with a multisite mode of action are likely to offer more reliable long-term control and, when available, such actives should be included in any fungicide program.
Research has been carried out on the possible biological control of GLS through the use of populations of bacteria or Trichoderma sp. formulations to naturally antagonize the pathogen population (Dammie, N., 2017). It is fair to say that against such an aggressive and potentially harmful turfgrass disease, limited and inconsistent evidence of any meaningful controls would mean that, at this time, this option should not be relied upon to produce an effective management option. In the future, refinement of products or formulations could result in products with improved efficacy and scientific backing.
When environmental conditions threaten the potential for serious illness, it is wise to try to cool the turf or reduce the humidity around the turf canopy, especially in stadium situations, by using electric fans on the field. Turf cooling is important because sustained elevated temperatures above 28 ° C promote rapid disease development. Integrated evaporative cooling fans can be used to reduce the stadium air temperature to approximally 6-10 ° C below room temperature. With the flexibility of being able to turn off the cooling system, the fan will only help with overall air movement and sheet drying.
In recent years, UVC has been widely used as an option to manage the inoculum of fungal spores (mainly Microdochium nivale spores) on the surface of turf blades and could also be of potential benefit in an integrated anti-fungal program. Pyricularia spores. Although UVC is unlikely to target inoculum around the grass base or fungal mycelium in plant debris, any positive reduction in spore activity on the leaf surface will reduce infection rates and, therefore, it will limit the severity of the disease.
In stages or in turf areas of L. perenne where this disease has not yet established itself, it is worth investing in all options that aim to minimize the possible introduction of fungal spores or infected plant material that will act as the primary inoculum for the illness. . Cleaning of all equipment used at different sites / courts, either during renovations or for general lawn maintenance, must be thoroughly cleaned and disinfected with special disinfection systems before allowing passage to a new lawn area (Photo 6).
Even the smallest amount of infected plant material could trigger a disease epidemic that could result in the total loss of the lawn. Similarly, it is prudent to ensure that all footwear is cleaned before the lawn is allowed (Photo 7).
The spores of this fungus can easily move through the debris of the affected areas. Sanitizing mats can be a useful consideration for use in especially sensitive sites or where foot traffic is difficult to control. With the movement of airborne spores, these mats alone will not guarantee disease prevention, but can be used as one more piece in an integrated management program.
With recent advances in molecular biology, there is a possibility in the future for near "real-time" analysis of microbial communities. The new technology can detect extremely small amounts of DNA belonging to a particular genus or species and could be used to confirm the presence of pathogens on or within a lawn that appears strong and healthy (Bronzato et al., 2018; Villari et al. , 2017). , Kumar et al, 2021). It is also possible to analyze spores collected in sticky traps using the same technologies and assess the risk to turf from airborne sources of infection. Molecular biology has the clear advantage of species-level specificity and excellent sensitivity. Therefore, it is well suited to surveillance programs and preventive strategies. Although new approaches to pathogen surveillance and validation of new technologies will take time, an early warning system could be as simple as cleaning the lawn and evaluating the fungus DNA on the cleaned swab.
GLS is a fungal disease that poses a potentially serious threat to L. perenne managed turfgrass. If left unchecked and under ideal weather conditions for fungal growth, the disease could kill a stadium field in a couple of days. It is essential that the lawn is monitored for the first signs of infection and that possible disease outbreaks are confirmed by analysis. Due to the risk of confusion with various other diseases, p. Eg Pythium and with drought stress, a quick and accurate diagnosis is required when the first symptoms appear because every minute it has this aggressive disease. Once present in any area of the lawn, the fungus will complete successive cycles of infection, increasing the level of spore inoculum with each cycle until, eventually, the lawn simply dies. Managing turf strength (through appropriate nutrient input), turf quality (using cultivars with reduced susceptibility, monitoring cutting height, minimizing blade moisture duration and where possible, reducing temperature air / relative humidity) and where possible, by implementing a fungicide program (early / preventive application followed by a timed approach that includes different modes of fungicide action), the possibility of serious disease can be minimized. An integrated approach to disease management is recommended for all fungal disease problems, but is especially important when faced with a fungal disease that is potent. Mainly as harmful as GLS.
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The writers can be contacted via the following email addresses: