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Rain Fastness and Persistence of Fungicides for Control of Alternaria Brown Spot of Citrus

Field persistence and simulated rain experiments were conducted to determine the residual activity and rain fastness of several fungicides used for the control of Alternaria brown spot of citrus. Fungicidal activity persisted well in simulated rain experiments, despite the high rainfall amounts tested (40 mm in 2004 and 90 mm in 2005). However, most of the fungicides showed a significant reduction of disease control on fruit of ‘Fortune’ mandarin in the orchard after natural rain in 2004. Fruit growth rate was low and did not appear to play an important role in decline of fungicide residues. Copper products showed longer residual activity on fruit and higher rain fastness than did mancozeb, difenoconazole, iprodione, famoxadone, and pyraclostrobin. Cuprous oxide and copper oxychloride SC provided satisfactory disease control on fruit through 28 days and withstood 71 mm of rainfall in the orchard. A 4-week schedule with copper compounds is proposed for disease control on Fortune fruit during periods of weather favorable for disease development. Respray to replace fungicide deposit seems to be necessary only after heavy or wind-driven rains. With this program, about half of the sprays applied per season for fruit protection could be saved compared with the current schedule in Spain.