Pythium root dysfunction, first observed in 1985, causes distinct patches of dead Agrostis stolonifera (creeping bentgrass) in golf course putting greens. Unlike typical Pythium root rot patterns, the affected roots lack root hairs instead of being necrotic, leading to the name “Pythium-induced root dysfunction.” The disease became more prevalent during the golf course construction boom in the late 1990s and early 2000s.
Causal agents
Pythium root dysfunction is caused primarily by P. volutum but may also be caused by P. aristosporum and P. arrhenomanes.
Primary Hosts
Agrostis stolonifera (creeping bentgrass)
Disease Cycle
Pythium root dysfunction symptoms typically appear within one year of planting newly constructed or renovated creeping bentgrass greens, although they can occur as early as three months. The source of Pythium spp. inoculum in these greens is unknown but can spread through water, soil media, or plant tissue. The causal agents are most active during fall to spring when creeping bentgrass roots are actively growing. Hyphal growth hampers water and nutrient absorption from the soil. Symptoms progress in distinct circular patches, suggesting root-to-root hyphal dispersal. Sporangia and zoospores are rarely observed in affected roots. Infected roots rapidly decline under heat stress. Symptoms tend to decrease over 5 to 10 years, possibly due to changes in soil microbial population or organic matter accumulation.>
Epidemiology
Pythium root dysfunction symptoms can occur in hot summer weather or warm, dry conditions in other seasons. Infected creeping bentgrass roots lose root hairs, reducing water and nutrient absorption. Rapid root decline occurs when soil temperatures exceed 29°C (84.9°F). Low fertility, drought stress, and low soil oxygen levels exacerbate the severity of symptoms. Most root infections occur in fall, winter, and spring when soil temperatures are favorable for pathogen growth. P. aristosporum, P. arrhenomanes, and P. volutum thrive at 25°C (77°F) but can infect roots as low as 13°C (55°F). Wet soil conditions from rainfall or over-irrigation facilitate infection during active pathogen growth.
Symptoms
Pythium root dysfunction in creeping bentgrass manifests with symptoms that are most prominent during the summer months, particularly when the turf is subjected to heat and drought stress. Initially, circular areas ranging from 4 to 16 cm in diameter exhibit signs of wilt, chlorosis, or drought stress. These areas progress into larger patches, reaching up to 50 cm in diameter, where the foliar decline takes on a yellow to orange coloration. If left untreated, these affected areas can expand further, leading to significant damage and even death of the turf.
The symptoms of Pythium root dysfunction tend to appear first on mounds and slopes, as these areas are more susceptible to drought stress. Putting greens with good air circulation and ample sunlight exposure are also prone to experiencing more severe symptoms. It has been observed that superintendents often report these symptoms after events or tournaments, particularly if the greens were mowed lower than usual or allowed to dry out.
When examining the roots infected by Pythium root dysfunction, several distinct signs can be identified. The infected roots exhibit bulbous tips, lacking the usual presence of root hairs, and appearing light tan in color. Additionally, sand does not adhere to these infected roots as it would to healthy ones, likely due to the absence of root hairs. Although the impact on root depth and density may not be noticeable early in the season, infected roots die rapidly upon exposure to heat, and reduced rooting is commonly observed in the mid- to late summer period.
Credit: Lane Tredway Syngenta
Signs
The signs of Pythium root dysfunction in creeping bentgrass are characterized by the presence of oospores and coenocytic hyphae within the cortical cells of the infected roots. These signs are particularly evident during periods of pathogen activity. Upon microscopic examination, bisporous oospores can frequently be observed along the vascular cylinders of the affected roots. It is important to note that the density of oospores present in the root tissue does not necessarily correlate with the severity of the disease. Even if the number of oospores is relatively low, the potential for root disease still exists, indicating that the presence of oospores alone does not provide a definitive measure of the disease’s intensity.
Credit: John Kaminski/Penn State University
Management
Due to the presence of this disease on relatively young creeping bentgrass greens, management of new greens is very important. Typical cultural practices include the application of nitrogen and raising of mowing heights. Like most Pythium diseases, water management is critical. Fungicides typically effective against Pythium spp. are not as effective against this disease and reliance on the QoI fungicides generally provides better control.
Cultural
To manage Pythium root dysfunction in creeping bentgrass greens, it is important to construct the greens with sufficient organic matter to enhance water and nutrient retention. Fertilization with nitrogen should be done annually at a rate of 4 to 6 lbs per 1,000 sq ft (195 to 292 kg/ha), while regular soil and tissue testing ensures proper nutrient balance. Maintaining mowing heights above 3 mm during summer helps alleviate stress.
Irrigation should be implemented before widespread wilting occurs, and areas that dry out quickly can be supplemented with syringing and hand-watering. Soil surfactants improve soil moisture uniformity. Regular cultivation and topdressing enhance oxygen levels in the soil.
Chemical
For sites with a history of the disease, preventive treatments should be applied every 21 to 28 days when soil temperatures at a 2-inch depth range from 54 to 75°F (12 to 24°C), which is the temperature range when P. volutum is most active. Fungicides are generally more effective when applied preventively rather than curatively.
When using sprayed fungicides, it is essential to apply them in at least 4 to 6 gallons of water per 1,000 square feet or follow up with 0.125 inches of irrigation immediately after application to wash the fungicide into the root zone. Granular products, on the other hand, should be applied to dry turf and watered in after application. However, care should be taken when using Koban, as it may cause phytotoxicity, especially when applied during hot weather or with low spray gallonage.
Tank-mixes of specific products like Signature 80WG + Banol 6SC or Signature 80WG + Subdue MAXX 2ME have been reported to provide good disease suppression. These mixes are most effective when applied to the foliage in 2 gallons of water per 1,000 square feet. One recommended spray program includes using Insignia 20WG, Segway 3.3SC, and Signature 80WG + Banol 6SC or Signature 80WG + Subdue MAXX 1ME at appropriate rates with irrigation. This program can be used preventively every 21 to 28 days during autumn and spring when soil temperatures are within the 54 to 75°F (12 to 24°C) range.
If applying fungicides curatively, raise the mowing height and provide appropriate nitrogen fertilization to encourage turf recovery.
Because certain fungicides like pyraclostrobin and cyazofamid are at risk of developing resistance, it is essential to rotate fungicides according to the recommended program to manage resistance effectively.