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Increased Organic Matter in the Growing Medium Decreases Benlate DF Phytotoxicity

Some benzimidazole fungicides are phytotoxic to bedding plants. Organic pesticides are bound to the organic matter fraction in the root zone and their availability to plants depends on the composition of the growing medium. Thus, pesticide phytotoxicity may be affected by the fraction of organic matter in the growing medium. We conducted two studies to examine the relationship between benzimidazole phytotoxicity and organic matter content of the growing medium. In the first study, plants were grown in diatomaceous earth, containing no organic matter, and drenched with different fungicides. Benlate DF reduced carbon accumulation (growth) of the plants by 32 and 73% at the 0.5× and 1× label rate, respectively. Carbon gain of plants drenched with either Derosal or 3336 WP was similar to that of the control plants. Both Benlate DF and 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 peat. Net photosynthesis decreased and the severity of visual symptoms (chlorosis) of Benlate DF phytotoxicity increased in media containing less peat. Benlate DF phytotoxicity strongly depends on the amount of organic matter in the growing medium, probably due to sorption of the active ingredient of Benlate DF and/or its breakdown products to the organic matter.