Mitigation of Summer Stress of Roughstalk Bluegrass with Strobilurin Fungicides.

Rough bluegrass (RBG, Poa trivialis L.) is a problematic weed in tall fescue (Festuca arundinacea Schreb.) and Kentucky bluegrass (Poa pratensis L.) lawns and roughs, as well as creeping bentgrass (Agrostis stolonifera L.) fairways.  Rough bluegrass can be a contaminant included under the “other crop” category on seed labels, and is often unknowingly planted with desirable species as a result.  Rough bluegrass enters a stress-induced dormancy during summer months leaving behind brown patches that are easily mistaken for disease.  Recently, researchers observed the mitigation of summer decline of RBG with application of strobilurin fungicides and postulated that summer diseases may influence RBG decline.  Therefore, our objective was to determine the effects of several strobilurin products on measures of RBG health. Treatments on lawn-height ‘Laser' RBG included untreated, two formulations of azoxystrobin (Heritage and Heritage TL, HTL) at 610 g a.i. ha^‑1, and two formulations of pyraclostrobin (Insignia and Insignia SC, ISC) at 556 g a.i. ha^‑1 every two weeks from 21 May to 23 August in 2011.  Plots were sampled for pathogens twice in 2011.  Cover, quality, and gross photosynthesis (P_G) were monitored weekly.  A pathogen contributing to decline was not consistently detected in 2011.  No treatment resulted in acceptable RBG quality throughout the summer.  Compared to untreated, plots treated with HTL, Heritage, Insignia, and ISC averaged higher quality on 11, 4, 2, and 0 dates, respectively.  Plots treated with HTL, Heritage, Insignia, and ISC averaged greater cover than untreated plots on 12, 7, 2, and 1 dates in 2011.  Rough bluegrass treated with HTL or ISC averaged significantly greater P_G on 2 dates each from June through August in 2011.  While it is unclear if a pathogen is contributing to summer decline of RBG, strobilurin fungicide applications may increase summer quality, cover, and to a lesser extent, P_G.