Response of clusters ofVitis viniferagrapes to 2,4-D and related compounds

does not appear. First page follows. Introduction The herbicidal plant regulator 2,4-dichlorophenoxyacetic acid (2,4-D) and related compounds have been demonstrated to be very toxic to the grapevine even at low concentrations (Clore and Bruns, 1953); (Weaver, Winkler, and McCune, 1958).5 It is generally recognized that clusters are extremely sensitive to the compounds, especially at young stages (Weaver and Williams, 1951). However, there have been few studies where the relative sensitivity of clusters at each of several stages of development to 2,4-D have been made. This paper presents results of experiments performed in 1958 and 1959, to elucidate and add further information on response of clusters to 2,4-D and related compounds. One objective was to develop a test method whereby one could compare toxicities of various regulators on grape clusters, or could compare varietal sensitivity of clusters to the regulators. Materials and Methods Mature vines of Thompson Seedless (syn. Sultanina), Tokay, and Ribier in an irrigated vineyard at the University of California, Davis, were used. The seedless variety, Thompson Seedless, was pruned to four canes (Winkler, 1931), while Tokay and Ribier, seeded table grapes, were cordon-trained and spur-pruned. Aqueous solutions of the plant growth regulators, obtained from commercial sources, were prepared by adding sufficient ammonia to dissolve the compounds. All concentrations are expressed in parts per million on an acid-equivalent basis. Hereafter, 2,4-dichlorophenoxyacetic acid is referred to as