Identification, distribution, and control of triasulfuron-resistant Italian ryegrass (Lolium multiflorum Lam.) in Central Texas

Abstract

Field studies were initiated in 1999 and 2000 at separate locations in McLennan County, Texas to determine the effectiveness of various herbicides for Italian ryegrass control in imidazolinone tolerant wheat. In the 1999 study, clodinafop and 59 g ai/ha triasulfuron controlled ryegrass greater than 80% throughout the growing-season. Pendimethalin applied pre-emergence (PRE) followed by tralkoxydim applied early post-emergence (EPOST) provided higher control than pendimethalin PRE in both years. Pendimethalin treatments provided higher season-long control in 2000 than in 1999, maintaining at least 80% control at the last rating in 2000. Clodinafop provided at least ryegrass 70% control in both years. Imazamox at 44 and 66 g/ha did not control ryegrass more than 60% in either year. Growth chamber studies were conducted to identify the presence of triasulfuron-resistant ryegrass populations in Texas and to identify alternative control measures for such populations. Ryegrass seed samples were collected from 48 wheat fields in 13 central and north Texas counties where declining ryegrass control was reported following a sulfonylurea herbicide application. Two-leaf ryegrass was screened with 150 g/ha triasulfuron, a rate five times the labeled herbicide use rate, and the injury response was compared to the untreated control from each accession. Resistance was determined by a sample's response to that of a known susceptible population. Varying levels of triasulfuron-resistance were found in samples from all 13 counties. Of the 48 accessions sampled, nine were susceptible, 10 were slightly resistant, 18 were moderately resistant, and 11 were highly resistant. Four resistant accessions plus the susceptible standard were selected for a subsequent control study. Diclofop reduced fresh weights at least 69% and clodinafop reduced fresh weights at least 71% across all accessions. Metribuzin reduced dry weights at least 62% across all accessions. No imazamox or triasulfuron treatment reduced fresh weights more than 60% in the resistant accessions. In these selected accessions, triasulfuron resistant ryegrass was cross-resistant to imazamox.