Effect of Calcium Fertilizer and Water Status on Tolerance of Creeping Bentgrass (Agrostis stolonifera L.) to Dollar Spot (Sclerotinia homoeocarpa Bennett).

Creeping bentgrass (Agrostis stolonifera L.) is the most popular turfgrass species on golf course putting greens throughout the world due to its fine texture, recuperative ability and dense growth habit. However, many commonly used creeping bentgrass cultivars are susceptible to dollar spot caused by the fungus Sclerotinia homoeocarpa (Bennett). Even though dollar spot symptoms are confined to aerial parts of turfgrass plants, S. homoeocarpa produces a metabolite that is toxic to bentgrass roots. The toxin causes roots to thicken, cease to elongate, and become devoid of root hairs. Recent studies have identified several tetranorditerpenoid compounds that could be responsible for the root-browning and exhibit extremely phytotoxic properties. A preliminary field experiment indicated improved dollar spot tolerance when calcium fertilizer was used, especially when irrigation supplies were sub-optimal. The objective of this study was therefore to evaluate creeping bentgrass tolerance to dollar spot under influences of calcium fertilizer at various water status. A growth chamber experiment was carried out as a 2 x 2 x 2 factorial with five replications. Creeping bentgrass “L-93” core samples (7.6 cm diameter) collected from a USGA green were transplanted into cone-tainers (7.6 cm diameter and 35.6 cm depth) filled with USGA-spec sand. After three weeks’ growth in the green house, plants were transferred to a growth chamber for one week acclimation before treatment application. Treatments included with or without calcium fertilizer at a rate of 244 kg/ha (MicroPel^®), 50% or 100% water supplies based on evapotranspiration (ET) water loss every 24 hours, and with or without inoculation of S. homoeocarpa. The growth chamber was maintained at light intensity of 600 µmole/m²/s for 12 hours and temperature at 25/20 ^oC (day/night). During this experiment, plants were maintained at 2.5 cm by mowing twice every week. Data collected included weekly assessment of turf quality and color based on a 1-9 scale, clipping biomass at 1, 2 and 3 weeks after initial treatment application, and weekly assessment of disease severity throughout this experiment. This experiment is still ongoing, and results will be discussed in the presentation.