Control of Rose Blackspot in the Field with Benomyl1,2

Abstract

Abstract Rose plants grown in large circular tiles sealed internally and sunk in a field were sprayed with an aqueous suspension of conidia of Diplocarpon rosae Wolf, 2 weeks after initial applications of methyl l-butylcarbamoyl-2-benzimidazolecarbamate (benomyl). Weekly benomyl spray applications confined foliar infections to 35 ± 5% of the leaflets after 4 months compared with almost 100% infection in the untreated controls. Benomyl, applied once to the soil surface (either 1 or 2 lb. active ingredients/25 ft2), held infections to approximately 60%. One pound of the active ingredient per 25 ft2 of soil surface, applied 3 times at one-month intervals, held infections to approximately 35% on ‘Pink Radiance’, 40% on ‘Nocturne’, and 50% on ‘Red Radiance’. Blackspots on plants in soil treated with benomyl usually were small, linear, and interveinal compared with typical oval to oblong cross-veinal spots on sprayed or untreated plants. After wintering outside, all benomyl treated plants, but only 50% of the untreated plants were alive. The treated plants were larger and more vigorous than the untreated plants. At this time, foliar symptoms were most prevalent on plants that had been unsprayed or sprayed with benomyl and were least on those that had received 3 soil applications of benomyl during the preceding summer.