IMPLICATIONS OF ANNUAL BLUEGRASS CONTROL IN TALL FESCUE WITH BISPYRIBAC-SODIUM. M. Cutulle*, J.F. Derr, A. Nichols, D. McCall, and B. Horvath, Virginia Tech, Virginia Beach, VA (28)

Tall fescue is one of the most commonly-utilized turfgrasses for home lawns in the United States. Tall fescue’s popularity is attributed to a deep root system (drought tolerance), relatively low nitrogen requirements, and a resistance to most diseases. However, two pests that are problematic in tall fescue include the fungal pathogen Rhizoctonia solani (causes brown patch) and the cool season annual bluegrass (Poa annua). Rhizoctonia infects tall fescue stands during hot, humid conditions when tall fescue is under summer stress. The subsequent disease, brown patch, is aesthetically unpleasing and can thin the turfgrass stand, leading to the germination and encroachment of winter annual weeds such as annual bluegrass. Typically, tall fescue is overseeded in the fall, thus the application of preemergence herbicides for control of annual bluegrass is generally not an option. Currently, there are no selective postemergence herbicide options in the spring for control of annual bluegrass in tall fescue. A potential postemergence herbicide for control of annual bluegrass in tall fescue is bispyribac-sodium. However, previous studies report indicated that applications of bispyribac-sodium on colonial bentgrass (Agrostis capillaries L.) have increased its susceptibility to brown patch, thus promoting the sequential increase of summer disease and fall weed encroachment. Experiments evaluating timing and rates of bispyribac-sodium on annual bluegrass control and brown patch severity in tall fescue were performed at the Virginia Tech’s Agricultural Research and Extension Center in Virginia Beach, Virginia. Bispyribac-sodium was applied at rates of 37 and 75 g ai ha-1 either April 22 plus May 6 or May 22 plus June 5 in both 2009 and 2010. Plots treated with bispyribac-sodium on May 22 plus June 5, 2009 had greater than 60% brown patch cover in June 2009, while the control plots had only 40% brown patch. The earlier set of applications did not increase brown patch severity. Greater than 115 brown patch lesions per 100 leaves were recorded in plots receiving the May 22 plus June 5 herbicide application, which was greater than the 45 lesions recorded for the untreated plots. Plots treated with the high rate of bispyribac-sodium on April 22 plus May 6, 2009 had less than 2% annual bluegrass cover on June 22, 2009. On that date, plots receiving the high rate of bispyribac-sodium on May 22 and June 5 contained 12% annual bluegrass cover. Similar trends were seen in 2010 regarding brown patch severity and incidence. However, environmental conditions favored annual bluegrass establishment and recovery for the 2010 trial, thus the April 22 plus May 6 applications in 2010 were not as effective as in 2009. Overall, May 22 plus June 5 herbicide applications promoted disease progress in June regardless of herbicide rate and were not as effective at controlling annual bluegrass when compared to April 22 plus May 6 applications.