Mesotrione is the first triketone herbicide registered for use in turfgrass. Triketones prevent carotenoid biosynthesis by inhibiting the enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD). Although mesotrione controls many species of grass and broadleaf weeds, it is best know for selective control of perennial grasses like creeping bentgrass (Agrostis stolonifera L.). Field trials conducted at Virginia Tech and Blacksburg Country Club determined that several programs that integrate herbicide treatment and turf seeding effectively transitioned creeping bentgrass contaminated golf roughs back to a tall fescue [Schedonorus phoenix (Scop.) Holub] monoculture. However, mature weeds require multiple mesotrione applications for effective control. This requirement is a major limitation to mesotrione's competitiveness in turfgrass markets. Several greenhouse and laboratory studies were conducted to evaluate scenarios where mesotrione rates were titrated and applied daily to mimic ascending, descending, and intervallic time-release patterns. These patterns were applied following an initial treatment to foliage or soil to mimic a potential sprayable or granular time-release formulation. These scenarios effectively controlled five targeted weed species equivalent to the standard of two broadcast sprays, regardless of initial application placement or release pattern. However, both time-release treatments and the standard injured tall fescue based on leaf counts, plant weights, and visual phytotoxicity ratings. Additional growth chamber studies found that changes in relative humidity from 50 to 90% caused a 4- to 18-fold increase in plant phytotoxicity with a concomitant decrease in photochemical efficiency when mesotrione was applied to foliage of smooth crabgrass (Digitaria ischaemum (Schreb.) Schreb. ex Muhl.). Furthermore, white tissue was found predominately in the two youngest leaves when mesotrione was applied to soil, but in older leaves when applied only to foliage. Laboratory studies were conducted to evaluate interspecific differences in 14C mesotrione absorption and translocation between two plant species when applied to foliage or roots. Annual bluegrass (Poa annua L.) absorbed 2- to 4-fold more radioactivity than Kentucky bluegrass (Poa pratensis L.). Both species absorbed less radioactivity through roots than through foliage and root absorbed radioactivity was more often exuded into Hoagland's solution while foliar absorbed radioactivity was often found in other foliage. / Ph. D.

Advancements in cold tolerance of seeded bermudagrass and introduction of sulfonylurea herbicides have given turf managers new tools. Seedling bermudagrass response to sulfonylurea herbicides applied before or soon after seeding has not been characterized. Field observations have indicated that variability exists among sulfonylurea herbicides used for perennial ryegrass control. Objectives of the conducted research were to evaluate sulfonylurea herbicides for safety and utility while establishing seeded bermudagrass, and to elucidate variability in perennial ryegrass control with foramsulfuron. Field experiments were conducted in Blacksburg, VA to assess turfgrass and smooth crabgrass response to flazasulfuron, foramsulfuron, metsulfuron, rimsulfuron, sulfosulfuron, and trifloxysulfuron-sodium, applied 1 and 3 weeks after and before seeding. Herbicides applied 3 weeks after seeding (WAS) were generally more injurious than when applied 1 WAS. Foramsulfuron, metsulfuron, and sulfosulfuron are safe to apply 1 and 3 WAS, causing no reduction in turf cover. Herbicides applied before or after seeding injured bermudagrass in the following order from most to least injurious: flazasulfuron = trifloxysulfuron > rimsulfuron > metsulfuron = sulfosulfuron > foramsulfuron. Flazasulfuron and trifloxysulfuron-sodium are not safe to use within 3 weeks of seeding, while foramsulfuron and metsulfuron can be used anytime before or after seeding bermudagrass. Flazasulfuron, foramsulfuron, and trifloxysulfuron-sodium were evaluated for perennial ryegrass control as affected by environment. Among environmental variables collected soil temperature averaged 7 DAT correlated best with perennial ryegrass response of the three tested products. Soil temperatures below 18 C perennial ryegrass reduced control 9 WAT from 78 to 31% for foramsulfuron while flazasulfuron and trifloxysulfuron-sodium efficacy were not significantly affected. Temperature dependence on perennial ryegrass control can be ranked from most to least as follows; foramsulfuron > trifloxysulfuron-sodium > flazasulfuron. Studies were conducted to determine absorption and translocation of 14C flazasulfuron when applied to perennial ryegrass roots or foliage. Roots treated with 14C flazasulfuron absorbed 41% of recovered 14C while 25% of 14C moved from treated roots to foliage. It appears root absorption is an important component of flazasulfuron efficacy since most of the absorbed 14C remained in treated leaves and root absorbed 14C moved rapidly to foliage. / Ph. D.

Glyphosate-resistant (GR) horseweed has become a major problem in many row crop production systems in the United States. Horseweed is a winter annual weed common in no-till production systems. Fall-applied herbicides were compared with spring-applied treatments for the control of horseweed. In cotton, fall-applied trifloxysulfuron provided similar or greater control of horseweed when compared to spring-applied treatments of glyphosate + dicamba. Cotton yields with fall-applied trifloxysulfuron, clomazone, and flumioxazin were comparable to or better than spring-applied glyphosate + dicamba both years. Fall-applied cloransulam-methyl, flumetsulam, sulfentrazone, and the combination of chlorimuron-ethyl + metribuzin resulted in horseweed control and soybean yields comparable to spring-applied glyphosate + 2,4-D both years. Multiple-resistance to glyphosate and paraquat exists in a horseweed population from Mississippi. Herbicide rates of 0.066 kg ae/ha glyphosate and 0.078 kg ai/ha paraquat were required to reduce susceptible horseweed biomass 50%; whereas, rates of 0.78 kg/ha glyphosate and 0.67 kg/ha paraquat were required to reduce biomass of resistant horseweed to a similar intent. This is the first broadleaf weed species reported as exhibiting multiple-resistance to glyphosate and paraquat. The addition of metribuzin to paraquat improved control of paraquat-resistant horseweed. Paraquat at 0.84 kg/ha plus all rates of metribuzin controlled 15-cm tall horseweed at least 90% both years compared to 73% with 0.84 kg/ha paraquat alone. The addition of 1 and 2% methylated seed oil (MSO) to saflufenacil controlled horseweed 91 and 93%, respectively compared to 78% control with saflufenacil alone. The addition of saflufenacil to glyphosate improved control of GR horseweed from 50% to 100% at 21 d after treatment; control of horseweed with the combination of saflufenacil + glyphosate was additive. Saflufenacil did not affect absorption of glyphosate in glyphosate-susceptible horseweed; however, absorption increased in GR horseweed from 36 to 44% at 48 h after treatment with the addition of saflufenacil when compared to glyphosate alone treatments. Overall, the addition of saflufenacil reduced glyphosate translocation in horseweed at least 6%; however, due to the exceptional efficacy of saflufenacil on horseweed these reductions did not reduce control of GR horseweed.

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