These field trials were designed to measure the quality of creeping bentgrass on sand and soil-based greens following treatment with dithiopyr at three different rates. Two studies were conducted at Veenker Golf Course in Ames, IA on creeping bentgrass practice greens infested with Poa annua. One study was established on a sand-based and another on a soil-based green. Individual plot size was 5 x 5 ft and three replications were run. For both studies, Dimension (Dithiopyr) was applied at 0.25, 0.50 and 1.00 lb a.i./A on May 1, 2001. The material was applied using a 'shaker dispenser' to ensure uniform coverage.

The studies were monitored for turf quality and phytotoxicity from May 15 through September 5 (Tables 1 and 2). Turf quality was assessed using a 9 to 1 scale with 9 = best, 6 = lowest acceptable and 1 = worst quality. No phytotoxicity was observed. Control of Poa annua was recorded from May 15 through September 5 (Tables 3 and 4). Populations were determined by estimating the percentage of area per plot covered by Poa annua. The small population on the soil-based green died out and/or was overgrown by bentgrass beginning mid July. Some regrowth was observed on September 5.

To quantify possible treatment effects on Poa annua seedhead formation, differences in the percentage of Poa annua seed head formation per plot were noted for the sand-based green study on May 15, June 15, July 11, and July 24 (Table 5). The Poa annua on the soil-based green did not produce seed heads.

The soil-based green was core aerified on August 15 and large cores (approximately 3/4" diameter) were removed. There were obvious differences in recovery from the aerification and these were noted on August 24 and August 28 (Table 6). Recovery was assessed using a 9 to 1 scale with 9 = 100% recovery, 5 = 50% recovery, and 1 = 0% recovery.

Data were analyzed using the Statistical Analysis System (SAS Institute Inc., 1989-1996) and the Analysis of Variance (ANOVA) procedure. Treatment effects on Poa annua populations and visual quality were tested using Fisher's Least Significant Difference (LSD) means comparison test. In addition, orthogonal contrasts were conducted using the General Linear Model (GLM) procedure to provide means comparisons among individual treatments (Tables 7-10).

No phytotoxicity was observed on the treated bentgrass. Enhanced turf quality was observed in bentgrass treated with Dimension at 0.25 lb a.i./A on May 15 and May 23 as compared with the other treated and untreated bentgrass (Table 1). On June 6, bentgrass treated with either Dimension at 0.50 or at 1.00 lb a.i./A had better quality than the other treated and untreated turf. After June 6, quality was similar for all treated and untreated bentgrass.

There were numerical reductions in Poa annua cover among the treated and untreated plots, but the differences were statistically significant for only June 6 and August 15 (Table 3). On these dates, Dimension at 1.00 lb a.i./A provided the best numerical control but the level of control was not different from that caused by the other Dimension treatments.

There were significant differences in percentage Poa annua seedhead formation for June 15, July 11, and July 24 (Table 5). On June 15 and July 11, there were less seedheads in bentgrass treated with Dimension at 0.25 lb a.i./A but there were more in bentgrass treated with Dimension at 0.50 and 1.00 lb a.i./A as compared with the untreated controls. Mean data show that there was significantly more seedhead formation in bentgrass treated with Dimension at 0.50 or 1.00 lb a.i./A than in bentgrass treated at 0.25 lb a.i./A or untreated.

The orthogonal contrasts provide additional information on statistical differences in Poa annua cover by giving probabilities of > T values for pairwise comparisons among the treatments (Table 7). On June 1, June 15, August 15, and August 24 the differences between percentage cover in untreated bentgrass were statistically different than in bentgrass treated with Dimension at 1.00 lb a.i./A.

No phytotoxicity was observed on the treated bentgrass. Numerical improvements in turf quality were recorded through September 5 but these differences were statistically significant for only June 1, June 6, and June 15 (Table 2). On these dates, turf quality was best for bentgrass treated with Dimension at 1.00 lb a.i./A as compared with the other treated and untreated areas.

There were reductions in Poa annua cover in treated bentgrass as compared to untreated but the differences were not statistically significant (Table 4). The Poa annua population on this green was rather sporadic and appeared to die out by early August. In addition, the population within the experimental area and surrounding it did not form seedheads.

Core aerification performed on August 15 resulted in significant differences in recovery rates (Table 6). On August 24, bentgrass treated with either Dimension at 0.50 or 1.00 lb a.i./A had significantly less recovery from aerification By August 28, the cores were still only 75% regrown in bentgrass treated with Dimension at 1.00 lb a.i./A. The cores in all other treated and untreated areas had completely recovered.

Table 1. Turf quality¹ of creeping bentgrass on a sand-based green treated for the 2001 Dimension Safety study.