I^/EED CONTROL IN PRUNES

Weed control studies were initiated in five year programs to evaluate new herbicides and to generate information on the effects of long term pre- and pcstemergence herbicide usage. Prune orchards in five counties and two U.C. experiment stations were used to esteblish the trials providing an adequate range of soil types, climatic conditions and weed species. Prune ti-ees, in the second to the f if th leaf , were retreated with ten preemergerice herbic ides (four were cons idered as standards) and seven postemergence herbicides (four evaluated as standards). The preemergence herhicides were appl ied separately and in combination for ' annual weed control, and in combination with postemergence herbicides for perennial weed control, The treatments were evaluated for herbicide efficacy and phytotoxicity to prune trees. ln addit ion, a postemergence herbicide developed for perennial weed control was appl ied preplant and on prune tree foliage to investigate phytotoxicity under these exceptional conditions. For the second year, five of the relatively new preemergence herbicides demonstrated acceptable (: 70%) weed control. The remaining preemergence herbicides were rated as unacceptable due to the presence of one or several tolerant weed species that overgrew the experimental plots. Phytotoxicity to the prune trees was not observed in any of the preemergence treatments. Four postemergence herbicides continued to provide good to excel'lent control of sensitive weed species. Weed species tolerant to these herbicides wil I necessitate an alternate weed control prograrn. The combination of a preemergence herbicide with a postemergence herbicide phytotoxic to perennials continues to provide the most effective perennial weed control . The postemergence herbicide preplant treatment exhibited no damage to prune trees. The foliar application of a postemergence herbicide on prune trees in the second leaf , while damaging al I the trees, completely destroyed ninety-five percent of those treated. Evaluations of the trials wi I I be continued for 1981.