Detection of the rapid blight pathogen Labyrinthula terrestris on non-symptomatic Poa trivialis

Rapid blight is a new disease of cool season turf grasses caused by Labyrinthula terrestris. It is problematic in Arizona and ten other states in cool season turfgrasses at sites with elevated salinity of soil and/or irrigation water. L. terrestris colonizes Tifgreen bermudagrasses in the field, but causes no apparent disease. Laboratory trials have shown that as concentrations of sodium chloride in irrigation water increase, disease severity increases, and when calcium and potassium salts are used to increase salinity, disease is greatly reduced or not observed. In preliminary field assays of cool-season turfgrasses irrigated with effluent, L. terrestris was observed in laboratory cultures from non-symptomatic turfgrass. To further substantiate if L. terrestris and/or other Labyrinthula species were present in non-symptomatic turfgrass in the field and to determine if disease could be induced by increased salinity, a trial was conducted at the Karsten Turfgrass Research Facility of The University of Arizona. In August 2006, field plots were established in bermudagrass “Tifway 419” and overseeded with Poa trivialis “Laser” in October. Plots were treated with potassium chloride, potassium sulfate or sodium chloride salts to increase soil salinity. Other plots treated with fungicides that are ineffective in controlling rapid blight as well as a sulfur treatment also were included in the assays. Poa trivialis was sampled in December 2006 and April 200. In laboratory assays using a semi-selective medium, Labyrinthula was detected in all treatments. Incidence was significantly higher in the untreated control and fungicide treated plots than in the salt treated plots. Results show that increasing soil salinity did not induce disease or result in an increase in detection of Labyrinthula at this site. Results of this study on Poa trivialis and previous studies on Tifgreen bermudagrass suggest that Labyrinthula may be widespread in non-symptomatic turfgrasses.