Pythium diseases in UK turfgrass
Pythium diseases are a significant challenge in turf management, mostly unnoticed or misdiagnosed despite their widespread impact. These diseases, caused by fungal-like microorganisms, can strike at almost any time of year in the UK, affecting various turfgrass species.
Due to the variability in symptoms, Pythium infections are frequently mistaken for other issues such as Rhizoctonia, nematodes, blue-green algae or possibly gray leaf spot. These misdiagnoses are further complicated by Pythium’s interactions with other pathogens and its potential development of fungicide resistance. Moreover, plant-parasitic nematodes can exacerbate Pythium infections, creating a complex problem for turf managers.
Pythium seems to be a pervasive problem across the UK, affecting virtually all types of turf. Pathogenic DNA screenings in the spring of 2024 revealed that Pythium was present in just over 85% (30/35) of symptomatic turf samples, either as the sole pathogen or alongside other diseases. The high prevalence here may indicate Pythium to be one of the most underdiagnosed diseases in turf management. Further monitoring over time may elucidate this further.
It must be said, the year’s weather to date was perhaps conducive to the spread of Pythium diseases, with prolonged periods of wet and cool conditions interspersed with temperatures ranging from 20-30°C. These conditions created an ideal environment for Pythium diseases to thrive.
Pythium root dysfunction
Beyond the weather, certain turf management practices (especially during grow-in) also increase the risk of Pythium infections. Practices such as watering, low cutting heights, frequent mowing, high nitrogen inputs and high seeding rates are often necessary to establish turf quickly, but they also make turf more susceptible to Pythium. Understanding these risks is crucial for turf managers who are constantly pushing the boundaries to produce high-quality sports turf.
Completely eliminating a long-term risk of Pythium may be unlikely, particularly under the pressure to maintain top-tier sports turf. However, there are several strategies that can be employed to minimise the risk and impact of Pythium infections.
Pythium needs more understanding
Disease profiles, textbooks, educational fact sheets and seminars continue to address Pythium in the UK, although research of its impacts and integrated management are
limited at this time. The basic information may cover off Pythium as one of the most important diseases for crops and horticulture and that Pythium species are not true fungi but belong to a group of fungal-like water moulds in the Oomycetes class. Much of the existing literature on turf-related Pythium comes from the USA, and therefore it may not be fully applicable to the UK’s temperate climate or context of turfgrass management. There is a need for more UK, Ireland and Europe-specific
Pythium-turfgrass research to better understand and manage these diseases. Through my work in the UK, particularly at Leicester, I have observed several Pythium diseases under local growing conditions:
Pythium root dysfunction
Pythium root dysfunction may not be a different disease from Pythium root rot by some authors, though the affected turf species may differ. Pythium root rot tends to be the most frequently encountered of the Pythium diseases, due to the wide range of Pythium species and susceptible grass types. Pythium root dysfunction, however, is observed primarily in creeping bentgrass stands of less than several years old.
In one case, I have experienced a decline in turf quality that was observed as thinning, bare areas with yellowing and bronzing leaves on a 4 mm cut creeping bentgrass green. Swathes of the golf green perimeters and central corridor areas were deteriorating rapidly over 5-10 days, initially attributed to heat stress from persistent high temperatures (20-30°C). However, increased watering, the application of surfactants and raising the cutting height did not alleviate the issue. Further investigation revealed poor root condition consistent with Pythium root dysfunction. To rectify, the mowing height was raised, ceased cutting in some perimeter areas, reduced water-soluble nitrogen input, and over-seeded and re-turfed affected areas. Close monitoring was crucial in the subsequent years to prevent recurrence.
Pythium blight
Pythium blight, primarily caused by Pythium aphanidermatum and other species, affects all turfgrass species, including perennial ryegrass, bentgrasses and annual meadow grass. This disease is most likely to occur when both day and night temperatures exceed 19°C, and relative humidity remains high throughout the day. Early symptoms include small patches of darker, water-soaked leaves. As the disease progresses, the leaves collapse, leading to necrosis and the characteristic slimy “greasy spot” appearance. The affected leaves eventually dry out and turn brown, with fine white or grey cotton-like mycelium sometimes visible. Leaf die-back (from tip to stem) and leaf spots are often present.
Pythium blight is particularly severe in areas with high moisture retention, such as shaded, poorly drained and/or regularly irrigated areas. As a water mould, Pythium can spread quickly, especially via rollers or wheels. The grow-in period for turf, especially in warm weather, is an ideal opportunity for Pythium to strike, mainly as young grasses are vulnerable. Perennial ryegrass is at risk when temperatures exceed its optimal growth range of 18-20°C, particularly during extended periods of warmer summer weather.
Pythium root rot
Pythium root rot, caused by species such as P. aristosporum, P. aphanidermatum, and P. ultimum, is closely associated with moisture and wet conditions rather than temperature. This disease differs from Pythium blight in that it primarily affects the roots. Symptoms include a general decline in turf quality, with affected areas showing yellowing and possible thinning. Yellowing patches, rings, as well as general decline and swathes are also typical signs. Infected roots are thin, discoloured (often brownish), poorly anchored and lack healthy root hairs. Microscopic inspection often reveals Pythium’s double-walled spherical oospores. mycelium may be absent from the aerial parts of the plant.
While annual meadow grass and bentgrasses are commonly cited as susceptible, I have observed perennial ryegrass as equally susceptible, particularly during early establishment. Pythium root rot is associated with nematodes, especially root-knot nematodes (Meloidogyne spp.). If circular patches are observed and plant-parasitic nematodes are present, it is likely that multiple infections are contributing to the problem.
Strategies for managing Pythium diseases
Given the challenges in managing Pythium, prevention is critical. An integrated management approach that reduces stress on the turf is essential. Equally important is vigilant monitoring. Early detection is crucial, particularly during grow-in and the establishment of new turf. For bentgrass golf putting greens, this vigilance may need to extend for several years.
Effective prevention may include the use of disease modelling (e.g. Nutter and Shane, though I would be sceptical as even the best models are only about 60% reliable) or routine DNA screening of the turf to quantify infection levels (e.g. qPCR). Genetic selection is also key—choosing Pythium-resistant varieties, using the lowest possible seeding rates to achieve desired coverage, and raising cutting heights, especially in shaded areas or when environmental pressures dictate. During early establishment, aim to mow when the turf is dry, and reduce moisture levels to optimum levels. A constant field capacity may not be necessary once the roots are anchored. Avoid watering in the late afternoon or evening to reduce leaf wetness and improve air circulation to lower humidity. Surface and subsurface aeration can help move moisture through the soil profile, while deep, infrequent irrigation stimulates deeper root growth and prevents surface moisture retention.
Nitrogen fertiliser applications should be applied, but not at a rate that stimulates excessive or overly lush growth. For example, perennial ryegrass sand-based pitches of 60-100 kg N per ha per month during grow-in is acceptable (depending on weather conditions and mowing regimes), importantly the programme should facilitate a steady consistent growth without peaks and falls in growth. For creeping bentgrass golf putting greens on sand-based root zone material, the aim of consistent growth is also required, however with a typical annual input of 60-120 kg N per year.
Sequential phosphite applications, applied fortnightly, are widely recognised as beneficial for controlling water moulds and promoting root development, particularly in perennial ryegrass. Azoxystrobin is an acknowledged fungicide for Pythium control and should be watered in after application. A tank mix of azoxystrobin and phosphite may also provide effective results. Be aware of the potential for QoI-resistant Pythium, which should be considered when planning fungicide use.
Conclusion
Pythium is a complex, under-researched and often overlooked pathogen in UK turf management. However, it should be a primary focus of preventative planning throughout the year, as it is likely a contributing factor, if not the primary cause when turf declines or patches appear. By maintaining a vigilant approach and integrating effective preventive strategies, turf managers can significantly reduce the impact of Pythium and other secondary infections to maintain healthy, resilient turf.
Dr. Jonathan Knowles
Head of Sports Turf Academy at Leicester City Football Club