CREEPING BENTGRASS ROOTING AS INFLUENCED BY DITHIOPYR

Dithiopyr [3,5-pyridinedicarbothioic acid, 2-(difluoromethyl)-4(2-methylpropyl)-6-trifluoromethyl)S,S-dimethyl ester] is a new herbicide, but its foliar safety and potential rooting effects need to be documented before its use on creeping bentgrass (Agrostis palustris Huds.) can be recommended. Field studies were conducted at five separate sites in Iowa and in Maryland, where creeping bentgrass was grown on modified sandy soils and maintained at putting green height. Three dithiopyr formulations (1.0 emulsifiable concentrate, 0.10 granular, 0.25 granular) at rates ranging from 0.28 to 0.84 kg ha-1 were compared with label rates or either bensulide [0,0-bis (1-methylethyl) S-(2-[(phenylsulfonyl)aminol ethyl] phosphorodithioate] or bensulide + oxadiazon [3-12-4-dichloro-5-(1-methylethoxy)phenyl]-5-(1,1-dimethy-lethyl)-1,3,4-oxadiazol-2-(3H)-one] or DCPA [dimethyl 2,3,5,6-te-trachloro-1,4-benzenedicarboxylate] for potential rooting and foliar discoloration effects. Root masses were obtained from washed soil plugs, which were combusted to determine root weight. In Iowa all herbicides reduced total root weight on one of four sampling dates in 2 yr. No rooting differences were observed among herbicide rates or formulations at soil depths partitioned at 0 to 5, 5 to 10, 10 to 15, or 15 to 20 cm. Reduced root weight was observed with all herbicides on at least one of nine sampling dates in Maryland. Root weight reduction in Maryland studies was observed primarily within 29 d of treatment (DAT) in 1990, but not within 57 DAT in 1990 or on six sampling dates in 1991. No foliar discoloration was elicited by dithiopyr applied in either April or May. Discoloration and cover reduction occurred where dithiopyr 1.0 EC (0.84 kg ha-1) and bensulide + oxadiazon (6.7 + 1.7 kg ha-1) were applied in mid-June in Maryland. Overall, data indicate that dithiopyr (0.28-0.84 kg ha-1) had an acceptable safety margin for creeping bentgrass foliage and rooting on putting greens when applied preemergence in spring.