Efficacy of metalaxyl for blue mould control and its persistence on tobacco

Abstract Foliar sprays of metalaxyl, benalaxyl, and cymoxanil plus mancozeb gave better control of blue mould (Peronospora tabacina) than mancozeb alone applied as a spray, or metalaxyl applied to the soil. Before flue‐curing, the mean values of metalaxyl residues in tobacco leaves were significantly higher from foliar spray treatments (5.09 μg g−1), than from soil treatments (0.93 μg g−1). Residues had decreased after flue‐curing (foliar spray treatment 2.51 μg g−1; soil treatment 0.69 μg g−1), especially on samples taken in September. Curing considerably reduced metalaxyl residues in all cases. Residues of mancozeb ranged from 59.5 to 224.2 μg g−1 before flue‐curing and from 11.0 to 22.1 μg g−1 after flue‐curing. All these residues were calculated on a dry weight basis. Imidazolidine‐2‐thione (ethylenethiourea) residues from mancozeb were always below the sensitivity limit of the method used.