Seasonal Nitrogen Fertilization Programs and Plant Growth Regulator Effects on Anthracnose Severity of Annual Bluegrass Putting Green Turf.

Anthracnose (caused by Colletotrichum cereale sensu lato Crouch, Clarke, and Hillman) is a detrimental disease of annual bluegrass (ABG; Poa annua L.) putting greens. Ethephon and trinexapac-ethyl applied together for seedhead and vegetative control can reduce anthracnose severity, although this effect has been inconsistent in previous research. Moderate nitrogen fertilization can improve ABG tolerance to anthracnose.  However, the influence of seasonal N programming on the ability of plant growth regulators to reduce anthracnose is not well understood. A two year field study was established in 2013 to evaluate potential interactions between seasonal nitrogen fertilization programs, ethephon (ET), and trinexapac-ethyl (TE) application interval on anthracnose severity of ABG putting green turf. Nitrogen treatments included spring or fall applications of 48.8 kg ha^-1, or a split application of 12.2 and 36.6 kg ha^-1 applied spring and fall respectively. Ethephon was applied at 0 or 3.81 kg a.i. ha^-1 twice in April. Trinexapac-ethyl treatment intervals consisted of none, 14 day interval, or every 200 growing degree days (GDD) base 0°C from mid-Apr through July 2013 and August 2014, applied at 0.05 kg a.i. ha^-1. Surprisingly, N had little effect on anthracnose severity during 2013. However, spring N treatments consistently reduced disease severity compared to fall only treatments from late June through early August 2014. Ethephon, initially reduced anthracnose severity, although had no effect later in the 2013 season. Ethephon treated turf consistently had reduced anthracnose severity throughout 2014.  Trinexapac-ethyl consistently reduced anthracnose severity regardless of application interval in both years. However, TE applied every 200 GDD reduced disease severity more than TE every 14-d during July and August. No consistent interactions were observed.  Results to date suggest spring rather than fall N fertilization, ET, and TE applied based on GDD model can reduce anthracnose on annual bluegrass putting green turf.