Biology and Management of Pythium Root Dysfunction in North Carolina.

Pythium root dysfunction (PRD) has become an important disease of creeping bentgrass putting greens in the Southeastern U.S., yet very little is known about the etiology, epidemiology, and management of this disease. Seventy-five Pythium isolates were obtained from creeping bentgrass putting greens in NC, SC, GA, and VA. Using morphological and molecular identification techniques, 59 isolates were identified as Pythium volutum and 16 were identified as P. torulosum. A subsample of P. volutum and P. torulosum isolates were tested for pathogenicity in growth chamber experiments. All isolates of P. volutum examined were highly virulent toward creeping bentgrass roots, whereas isolates of P. torulosum were non-pathogenic. Isolates of P. volutum induced drastic reductions in creeping bentgrass root depth and root mass when infected plants were exposed to a four week high temperature regime. Growth chamber experiments were conducted to determine the impact of temperature on infection of creeping bentgrass roots by P. volutum. This was conducted by varying the temperature during a four week infection period, after which the plants were exposed to a four week heat treatment. Symptoms characteristic of PRD developed in the 12C, 16C, 20C, and 24C infection temperature treatments, but not in the 28C and 32C treatments. Root depth and root mass was reduced prior to heat exposure in only the 12C, 16C, and 20C treatments. After a four week exposure to 32C/26C (day/night), considerable reductions in root depth and root mass were observed in all infection temperature treatments except for the 28C and 32C treatments. Field experiments were conducted to evaluate fungicides for preventative control of PRD. Applications of pyraclostrobin provided the best and longest lasting preventative suppression of PRD symptoms. Azoxystrobin and cyazofamid provided moderate levels of preventative suppression and the standard Pythium fungicides were not effective against PRD. In vitro assays were conducted to determine the sensitivity of P. volutum's to fungicides. Pythium volutum isolates were highly sensitive to pyraclostrobin and cyazofamid, moderately sensitive to azoxystrobin, and the least sensitive to mefenoxam. Growth chamber experiments were performed to evaluate the effects of creeping bentgrass cultivar, organic matter content, and irrigation frequency on development of PRD. 'Crenshaw', 'Syn-96', and 'G-6' were the least susceptible cultivars when compared to 'Penncross'. The popular cultivars 'A-1' and 'A-4' were moderately susceptible and 'LS-44', 'G-2' and 'Penncross' were the most susceptible cultivars. Organic matter added at the time of establishment did not have an effect on PRD development. Symptoms of PRD were most severe when creeping bentgrass was irrigated 6 times a week when environmental conditions were conducive for infection by P. volutum. When creeping bentgrass was irrigated 3 or 4 times a week, PRD symptoms were less severe and turf quality did not decline. Another series of growth chamber experiments were established to determine the effects of P. volutum infections on creeping bentgrass nitrate uptake, evapotranspiration, and photosynthesis. Nitrate uptake was elevated in creeping bentgrass plants that were infected with P. volutum when compared to the non-inoculated controls. Evapotranspiration was similar among inoculated and non-inoculated plants.