TALL FESCUE GROWTH AFTER PARAQUAT APPLICATION TO ELONGATING OR MATURELEAVES

Paraquat (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 o f infected plants, however, is often a problem, even when control init ially appears to be adequate. Our objective was to evaluate the effect of leaf developmental stage on sensitivity to paraquat applications. Ten 0.5-mu L drops of paraquat at 0- to 60-mM concentrations were appl ied to either fully elongated blades, or to emerged portions of elonga ting 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 elonga ting leaves of 6-wk-old plants. Paraquat did not affect leaf elongatio n in the field until considerable damage to the elongating leaf had oc curred. Visible damage to the treated leaf also occurred faster in the growth chamber than in the field. In both environments, injury was gr eater when elongating compared with fully elongated leaves were treate d. When fully elongated leaves were treated, damage was often limited to the treated leaf, while the shoot apex remained alive and capable o f initiating new leaves and tillers. Treatment of elongating leaves, h owever, increased the probability that the entire plant would be kille d. To increase effectiveness, paraquat should be applied to young, elo ngating leaves. However, these leaf blades are vertically oriented, sl ightly rolled, lower in leaf area, and narrower than fully elongated l eaves, making their treatment more difficult.