CREEPING BENTGRASS ROOTING AS INFLUENCED BY DITHIOPYR

Dithiopyr [3,5-pyridinedicarbothioic acid, )-4(2-methylpropyl)-6-trifl uoromethyl)S,S-dimethyl ester] is a new herbicide, but its foliar safe ty and potential rooting effects need to be documented before its use on creeping bentgrass (Agrostis palustris Huds.) can be recommended. F ield studies were conducted at five separate sites in Iowa and in Mary land, where creeping bentgrass was grown on modified sandy soils and m aintained at putting green height. Three dithiopyr formulations (1.0 e mulsifiable concentrate, 0.10 granular, 0.25 granular) at rates rangin g from 0.28 to 0.84 kg ha-1 were compared with label rates or either b ensulide [0,0-bis (1-methylethyl) S-(2-[(phenylsulfonyl)aminol ethyl] phosphorodithioate] or bensulide + oxadiazon 5-(1,1-dimethy-lethyl)-1, 3,4-oxadiazol-2-(3H)-one] or DCPA [dimethyl 2,3,5,6-te-trachloro-1,4-b enzenedicarboxylate] for potential rooting and foliar discoloration ef fects. Root masses were obtained from washed soil plugs, which were co mbusted to determine root weight. In Iowa all herbicides reduced total root weight on one of four sampling dates in 2 yr. No rooting differe nces were observed among herbicide rates or formulations at soil depth s partitioned at 0 to 5, 5 to 10, 10 to 15, or 15 to 20 cm. Reduced ro ot weight was observed with all herbicides on at least one of nine sam pling dates in Maryland. Root weight reduction in Maryland studies was observed primarily within 29 d of treatment (DAT) in 1990, but not wi thin 57 DAT in 1990 or on six sampling dates in 1991. No foliar discol oration was elicited by dithiopyr applied in either April or May. Disc oloration 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 m id-June in Maryland. Overall, data indicate that dithiopyr (0.28-0.84 kg ha-1) had an acceptable safety margin for creeping bentgrass foliag e and rooting on putting greens when applied preemergence in spring.