INCREASED ORGANIC-MATTER IN THE GROWING MEDIUM DECREASES BENLATE DF PHYTOTOXICITY

Some benzimidazole fungicides are phytotoxic to bedding plants. Organi c pesticides are bound to the organic matter fraction in the root zone and their availability to plants depends on the composition of the gr owing medium. Thus, pesticide phytotoxicity may be affected by the fra ction of organic matter in the growing medium. We conducted two studie s to examine the relationship between benzimidazole phytotoxicity and organic matter content of the growing medium. In the first study, plan ts were grown in diatomaceous earth, containing no organic matter, and drenched with different fungicides. Benlate DF reduced carbon accumul ation (growth) of the plants by 32 and 73% at the 0.5x and Ix label ra te, respectively. Carbon gain of plants drenched with either Derosal o r 3336 WP was similar to that of the control plants. Both Benlate DF a nd 3336 WP significantly decreased the number of flowers on the plants . The second study quantified the phytotoxicity of Benlate DF in media containing different amounts of organic matter. The growth of Benlate DF-treated plants was strongly affected by the amount of pear. Net ph otosynthesis decreased and the severity of visual symptoms (chlorosis) of Benlate DF phytotoxicity increased in media containing less pear. Benlate DF phytotoxicity strongly depends on the amount of organic mat ter in the growing medium, probably due to sorption of the active ingr edient of Benlate DF and/or its breakdown products to the organic matt er.