Tall Fescue Growth after Paraquat Application to Elongating or Mature Leaves

Abstract

AbstractParaquat (1,1'‐dimethyl‐4,4'‐bipyridinium dichloride) is commonly used to kill endophyte‐infected stands of tall fescue (Festuca arundinacea Schreb.) prior to reseeding with endophyte‐free cultivars. Regrowth of infected plants, however, is often a problem, even when control initially appears to be adequate. Our objective was to evaluate the effect of leaf developmental stage on sensitivity to paraquat applications. Ten 0.5‐μL drops of paraquat at 0‐ to 60‐mM concentrations were applied to either fully elongated blades, or to emerged portions of elongating leaves of 3‐ and 6‐wk‐old seedlings growing in a growth chamber, and to endophyte‐infected plants in the field. Leaf elongation rate in the growth chamber decreased 4 h after paraquat application to elongating leaves of 6‐wk‐old plants. Paraquat did not affect leaf elongation in the field until considerable damage to the elongating leaf had occurred. Visible damage to the treated leaf also occurred faster in the growth chamber than in the field. In both environments, injury was greater when elongating compared with fully elongated leaves were treated. When fully elongated leaves were treated, damage was often limited to the treated leaf, while the shoot apex remained alive and capable of initiating new leaves and tillers. Treatment of elongating leaves, however, increased the probability that the entire plant would be killed. To increase effectiveness, paraquat should be applied to young, elongating leaves. However, these leaf blades are vertically oriented, slightly rolled, lower in leaf area, and narrower than fully elongated leaves, making their treatment more difficult.