PREEMERGENCE ANNUAL BLUEGRASS CONTROL IN FAIRWAY HEIGHT ZOYSIAGRASS. P.H. Dernoeden*, University of Maryland, College Park, MD (82)
Annual bluegrass (Poa annua L.) is an intractable weed in golf course turf. Annual bluegrass becomes invasive in zoysiagrass (Zoysia japonica L.) fairways in late autumn and winter and dominates with the advent of warmer temperatures in spring. Preemergence herbicides offer a means of controlling this weed, but there has been no research conducted in Maryland to identify effective and safe materials for zoysiagrass maintained at fairway height. For decades, bensulide has been the standard annual bluegrass preemergence herbicide used on golf courses, yet there is little research to confirm its relative effectiveness, when compared to more modern preemergence herbicides. The objectives of the research were to assess various rates and timings of bensulide when targeting annual bluegrass and to compare bensulide to more modern compounds as follows: prodiamine, oxadiazon, dithiopyr , pendimethalin, cumyluron and paclobutrazol (a plant growth regulator). Bensulide 4L was evaluated in two timings as follows: single PRE application on 3 Sept. (6.25 and 12.5 lb/A) or POST 14 Oct. 2009 (12.5 lb ai/A); and sequentially at 6.25 lb ai/A (3 Sept. + 14 Oct.). Prodiamine 65DG was applied once (0.38 lb ai/A) or sequentially (0.38 + 0.38 lb ai/A) and paclobutrazol 2SC (0.25 lb ai/A) was applied sequentially (3 Sept. + 14 Oct.). Cumyluron 45% F (6 oz. product/A), dithiopyr 40WSP (0.5 lb ai/A), oxadiazon 2G (4.0 lb ai/A) and pendimethalin 3.8CS (3.0 lb ai/A) were applied once on 3 Sept.2009. Sprayable herbicides were applied in 50 GPA using a CO2 pressurized (35 psi) sprayer equipped with an 8004E flat fan nozzle. Granular formulations were applied using a shaker jar. The site received rainfall or irrigation within 24 hrs of each application. Soil was a Keyport silt loam with a pH of 5.6 and 1.4% OM. Turf was a mature stand of Zenith zoysiagrass maintained to a height of 0.5 inches, which became winter dormant in late Oct. 2010. Plots were 5 ft x 5 ft and were arranged in a randomized complete block with four replications. Percent of plot area covered with annual bluegrass and broadleaf weeds was assessed visually on a 0 to 100% scale where 0 = no weeds and 100 = entire plot area covered with weeds. Data were subjected to analysis of variance and significantly different means were separated at P ≤0.05 using Fisher’s least significant difference test. While there were few significant differences among most treatments between 23 Nov. 2009 and 11 Mar. 2010, plots treated with bensulide once or sequentially at 6.25 lb/A or at 12.5 lb/A on 14 Oct. generally had higher annual bluegrass cover, when compared to all other treatments. The period from 1 to 8 April was unusually warm and annual bluegrass plants began to tiller, expand in size and produce seedheads. Data were last collected at the time of zoysiagrass green-up on 9 Apr.2010, which was unusually early. On 9 Apr., 80% of untreated control plots had been colonized by annual bluegrass. Only bensulide applied once at 6.25 lb/A on 3 Sept. did not reduce annual bluegrass cover compared to the control. Bensulide applied at 12.5 lb/A on 14 Oct. had provided poor control (39% cover = 51% control), but had reduced annual bluegrass cover compared to the control. Bensulide applied at 12.5 lb/A on 3 Sept. (11 % cover = 86% control) and 6.25 lb/A applied sequentially on 3 Sept. + 14 Oct. (19% cover = 76% control) had provided a statistically equivalent level of control. Prodiamine applied once (0.38 lb/A) or sequentially (0.38 + 0.38 lb/A) provided 94 (4.5% cover) and 99% (0.5% cover) control, respectively. Dithiopyr provided 98% control (2% cover). Prodiamine (0.38 + 0.38 lb/A); oxadiazon, pendimethalin, and cumyluron provided exceptional control (≤ 0.5% cover = 99% control). Paclobutrazol-treated plots had 30% annual bluegrass cover (63% control) on 9 Apr, which was a level of control equivalent to bensulide applied sequentially (6.25 + 6.25 lb/A). There were low levels of the following broadleaf weed species in the study area on 9 Apr.: dandelion (Taraxacum officinale Weber ex F.H. Wigg.), mouseear chickweed (Cerastium vulgatum L.), prostrate knotweed (Polygonum arviculare L.), corn speedwell (Veronica arvensis L.) and white clover (Trifolium repens L.). Plots treated with pendimethalin and prodiamine (0.38 + 0.38 lb/A) were winter annual ( speedwell + chickwee)-free. Lowest other broadleaf weed (dandelion + clover + knotweed) populations were observed in plots treated with dithiopyr and pendimethalin.