Fungicide Application Timing and Row Spacing Effect on Soybean Canopy Penetration and Grain Yield

Foliar diseases, such as soybean rust (Phakopsora pachyrhizi) can cause severe defoliation and yield loss in soybean [Glycine max (L.) Merr.] and can be managed with an effective fungicide applied during the reproductive growth stages. The impact of soybean row spacing and fungicide application timing on canopy penetration and grain yield are unknown. The objectives of this experiment were (i) to quantify the impact of crop row spacing and spray application timing on penetration and coverage with a fungicide and (ii) to quantify the impact of sprayer wheel traffic during fungicide application on crop yield. The experiment was a split plot design with three replications conducted at the Northeast, Southeast, and Davis–Purdue Agricultural Centers (NEPAC, SEPAC, DPAC) in 2005 and 2006. The whole‐plot treatments were crop row spacing (19, 38, and 76 cm) and fungicide (azoxystrobin + propiconazole) [(methyl (E)‐2‐[2‐[6‐(2‐cyanophenoxy)pyrimidin‐4‐yl]oxyphenyl]‐3‐methoxy‐prop‐2‐enoate) + (1‐[[2‐(2,4‐dichlorophenyl)‐4‐propyl‐1,3‐dioxolan‐2‐yl]methyl]‐1,2,4‐triazole)] application timing (unsprayed or applications at growth stages R1, R3, R5, R3 + R5, or R1 + R3 + R5). Spray canopy coverage was quantified at 30‐cm increments within the soybean canopy. Crop row spacing affected spray canopy coverage at the R3 growth stage at the NEPAC location only. Application timing did not affect canopy coverage or spray penetration. Coverage decreased with depth in the canopy. Grain yield averaged 9% greater in the 19‐ and 38‐cm row spacing than in the 76‐cm row spacing. Fungicide application timing did not influence yield. Our research shows that fungicide application timing should not influence row spacing decisions. Growers should consider the effects of wheel‐track damage when planning spray applications during the reproductive growth stages of soybean.