Thatch Collapse: A New Disease of Golf Course Turfgrasses.

In 2010, previously unreported disease symptoms were observed on golf course putting greens located in the United Kingdom and Pennsylvania. Since this initial discovery, similar symptoms were subsequently reported in California, Michigan, South Dakota, and New Zealand on golf putting greens and fairways. Symptoms of this disease include circular patches of degraded organic matter ranging from 8 to 46 cm in diameter.  Thatch degradation results in an indentation of the turf surface and disruption in playability. Commonly found in association with the thatch degradation is a basidiomycete not previously reported within stands of any turfgrass. Fungal signs include profuse mycelium with clamp connections within the upper 2.5 cm of the soil/thatch profile and peridia similar to those produced by Sphaerobolus stellatus within the thatch and canopy of golf course putting greens. Little is known about S. stellatus within turfgrass systems or the association of this fungus to thatch collapse. Therefore, the objectives of this study were to: 1) identify the causal agent(s) of thatch collapse and document its (their) geographic distribution; 2) document the morphology of collected isolates; 3) determine the cardinal temperatures for growth of thatch collapse isolates; and 4) assess optimal temperatures for gleba production. Peridia of S. stellatus were surface disinfested and grown on antibiotic water agar prior to growth on antibiotic oatmeal agar (AOA).  After 2 to 3 weeks of growth on AOA, plates were covered with compressed white mycelium and new peridia with mature gleba were produced. Mycelial plugs were incubated in nutrient broth and the DNA of isolates (n = 11) from California, Michigan, Montana, South Dakota, and New Zealand were extracted using a DNEasy DNA extraction kit.  Amplification of the ribosomal internal transcribed spacer region (ITS) with primer set ITS4/ITS5 resulted in 695 to 724-bp sequences from all isolates with a 98% to 99% similarity to known isolates of Sphaerobolus stellatus.  To confirm the thatch degrading ability of S. stellatus, 12 samples (15 cm diameter x 5 cm deep) of mature Penn ‘A-1’ creeping bentgrass were inoculated with 0.6 g of S. stellatus-infested thatch. After 6 weeks of incubation at 12°C in continuous light, S. stellatus was successfully re-isolated from colonized thatch. Total degradation of organic matter was determined using the loss on ignition method and by direct measurements of thatch depth. When compared to the uninoculated control, plugs inoculated with S. stellatus exhibited a 21% and 25% decrease of organic matter and thatch depth, respectively. Morphological characteristics of S. stellatus isolates from turf were determined from measurement of gleba and basidiospores. The general morphology of gleba and basidiospores were within the range reported for S. iowensis and S. stellatus, respectively. The growth rate of three S. stellatus isolates in culture was compared at 6 temperatures (5, 10, 15, 21, 25, and 30°C). Differences were found among temperatures, but optimal growth rate ranged from 21 to 30°C. Gleba production was assessed for three S. stellatus isolates over 4 temperatures (10, 15, 21, and 25 °C) for 11 weeks. Gleba were produce at 10, 15, 21°C and 25°C by one isolate. Based on these results, S. stellatus is a casual agent for rapid organic matter degradation resulting in thatch collapse on golf course turfgrass species within the United States and New Zealand. Based on biological, morphological and molecular information obtained from this study, the fungus isolated from turf is similar to known mulch isolates of S. stellatus.