EFFECT OF LEACHING OF A SOIL-APPLIED HERBICIDE, DIURON, ON ITS PHYTOTOXICITY IN GRAPE AND PEACH

The movement and effects of soil-applied herbicide, diuron (Karmex(R)) , in 'Kyoho' grape (Vitis vinifera L. x V. labrusca L.) vineyards and 'Hakuho' peach (Prunus persica Batsch. var. vulgaris Maxim.) orchards were examined. Field experiments were conducted at eight sites differi ng in soil texture. Mobility of diuron in soils was evaluated by bioas say using soil samples collected from every site. Herbicidal activity of diuron and fruit tree injury were related to its mobility in soils. Diuron lost its activity as it moved down in sandy loam soil. Consecu tive application of the herbicide at 4.71 kg a.i./ha severely injured the fruit trees although the compound sequentially applied at the reco mmended commercial rate of 1.57 kg a.i./ha, was not injurious to both grape vines and peach trees. Serious damage from the compound occurred in soils where it was distributed extensively and leached gradually. The injury was serious in sandy soils, but it was slight in soils with high organic matter. The exposure duration of roots to diuron in soil also seemed to be another factor causing injury. Serious injury appea red when the herbicide leached from the soil suraface into more than 1 7 cm over 1 month. Consequently, the phytotoxicity of soil-applied her bicides such as diuron on fruit trees seems to be primarily attributed to the mobility of herbicides in soil, and secondly to the duration o f their leachates in contact with the roots.