Timing of fungicide sprays for the control of late season diseases of soybean <em>Glycine max</em> (L.) Merrill

Abstract

More than 40 diseases have been identified in soybeans in Brazil and annual yield losses are estimated at 15-20% amounting to US$1.3 billion. Late season diseases (LSD) alone, caused by brown spot and Cercospora leaf blight may be responsible for over 20% losses on individual farms. Genetic resistance is the most economical and easiest way to control many diseases. In the case of LSD no reliable source of resistance is available and chemical control is the only cure. Little is known about the best time of fungicide sprays for LSD control. Research determined the best time for fungicide application based on pod-filling stages and for the control of diseases and its impact on yield. Field experiments were carried out under three tillage practices: non-tillage, minimum tillage (subsoiler) and conventional tillage. Experimental design was randomized complete block, with ten treatments (nine times of spraying and check) and five replicates each. Fungicide used was difenoconazole at 75 g active ingredient per hectare. Fungicide was applied at growth stages R5.1, R5.2, R5.3, R5.4, R5.5, R6, R5.1 + R5.4, R5.2 + R5.5 and R5.3 + R6. Parameters were: a. severity of LSD, b. percent defoliation compared to check; c. yield (kg/ha), and d. 1.000 seed weight (tsw). Due to prevailing water deficit and high temperature during the reproductive stage of soybeans, disease severity was low and no yield differences were detected among the treatments. For other parameters, significant differences were observed when difenoconazole was applied at growth stages R5.1 + R5.4, in all three soil management. Conventional tillage had greater values in all parameters when compared to no-tillage