The effects of Gaeumannomyces graminis var. graminis, and cultural and environmental effects on the growth and development of dwarf bermudagrass

Bermudagrass decline is a devastating disease of bermudagrass on golf course greens in the southern U.S. Numerous environmental stresses predispose dwarf bermudagrasses to infection. In this study, the influence of temperature, host nutrition, and pathogen activity on developmental morphology of the host bermudagrass was evaluated. The effects of light levels and temperature on the developmental morphology of Tifdwarf bermudagrass also was evaluated. N source and rate influenced disease severity as indicated by turf quality, crop growth rate, axillary bud appearance rate, and shoot density. Increasing N from 16.2 to 48.8 kg ha-1 year-1, decreased disease severity, increased crop growth rate, shoot density, and generally increased the rate of axillary bud formation. Effects were more pronounced with the ammonium sulfate (AS) N source versus isobutylidine diurea (IBDU). The phyllochron required 10% fewer growing degree days when N was supplied as IBDU compared to AS and as N increased from 16.2 to 32.6 kg ha -1 year-1. In controlled environment studies, increasing N increased bermudagrass internode and lamina length and shoot weights. Generally, increasing N decreased the chronological appearance of successive leaves and axillary buds. The phyllochron increased with day/night temperature but internode and lamina length decreased 30% and 70%, respectively, and shoot weights decreased 3 fold with increased temperature regime. Decreasing light levels caused temperature-regulated increases in leaf and internode length. Internode length increased from 20 to 40 mm as light levels decreased from 50 to 10% of full sun at 27/19°C but increased from 10 to 22 mm long at 35/27°C. Lamina lengths responded similarly. The alteration in growth form occurred within 3--4 days of treatment initiation. Results indicated that temperature as well as light levels regulated expression of dwarfness in Tifdwarf bermudagrass. The data from these studies suggest that N source and amount affected bermudagrass developmental morphology and may have contributed to disease escape. In regions affected by long periods of overcast, rainy weather, growth habit of dwarf bermudagrasses may change dramatically in response to low light and lower temperatures, making Tifdwarf less likely to tolerate close mowing heights. Thus, mowing stress could predispose the grass to bermudagrass decline.