Effect of Time of Spraying of Fungicide and Foliar Nutrient on Soybean Powdery Mildew

Abstract

AbstractWith the objective of determining the effects of initial infection level (% leaf area infected, % l.a.i.) at the time of fungicide and foliar nutrient application for the control of powdery mildew, a study was carried out under greenhouse conditions at Embrapa Soja, Londrina, P.R. The fungicide tebuconazole (100 g a.i./ha/application) and sulphur fungicide (2.0 kg a.i./ha/application), sprayed at 20, 30, 40 and 50% l.a.i. (growth stages V3 and V5, between R5.1 and R5.3), and three applications (at V3, V4 and R5.1, beginning at 20% l.a.i.) of tebuconazole were compared with sulphur foliar nutrient (sulphur at the dose of 125 g a.i./ha/application). The highly susceptible BR‐16 cultivar was used in a completely randomized design with 11 treatments, five replications (pots) and five plants per pot. The parameters evaluated were: % l.a.i., % yellowing leaf when the control reached growth stage R7.2 (between 50 and 75% yellow/mature leaves), % defoliation (% defol.) when the control plants reached R8.2 (pre‐harvest maturity), number of pods and seeds and seed weight (s.w.) per plant. The area under the disease progress curve (AUDPC) was determined based on disease progress. The % l.a.i. and AUDPC of the control (98% and 65.41) differed from other treatments (3.5–64.2% l.a.i. and 13.57–40.59 AUDPC). The % l.a.i. for the sulphur fungicide treatment (41.9–64.2%) and sulphur foliar nutrient (49.8%) were greater than for tebuconazole (3.5–28%). The % defol. of the control (90%) differed from tebuconazole applied at 30% initial infection (48%), with three applications (45%), and from sulphur foliar nutrient (47%). The s.w./plant of the control (2.23 g) was lower than that of tebuconazole treated at 40% IL (3.03 g) and at 20% IL (3.35 g) and for sulphur foliar nutrient (3.7 g). The initial disease severity (20–50% l.a.i.) did not affect the level of powdery mildew control and the sulphur foliar fertilizer resulted in greater number and higher s.w./plant.