Physiological and Environmental Basis of Turfgrass and Weed Response to Mesotrione Formulations

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.