Establishing suppressive conditions against soilborne potato diseases with low rates of fish emulsion applied serially as a pre-plant soil amendment

Soil amendment with fish emulsion (FE) can suppress potato scab and verticillium wilt, but the effective broadcast rates (20 000 L ha(-1)) may not be feasible for commercial use. The aim of this 4-year study was to establish suppressive conditions against soilborne potato diseases and increase tuber yield with economically feasible rates of FE. Diluted FE (1000 and 2000 L ha(-1) or 0.05 and 0.1%) was applied to field plots or micro-plots twice a year before planting and after harvesting potatoes starting in autumn of 2007 and ending in spring of 2010. In the micro-plots, FE (0.1%) added to an infested potato soil (site BL) consistently reduced scab severity by 44.9% (2008), 44.8% (2009) and 30.9% (2010) compared with the control. Scab severity on tubers harvested from these plots was also low in 2011 when no further FE was applied, but the effect was not significant. In the field, FE (2000 L ha(-1)) consistently reduced scab severity by 45% (2008), 53% (2009), 44% (2010) and 38% (2011); reduced the percentage of tubers with deep-pitted scab by 48% (2008), 51% (2009), 66% (2010) and 77% (2011); and increased the percentage of marketable tubers by 37% (2008), 83% (2009), 20% (2010) and 8% (2011). These field plots also showed a low percentage of Verticillium dahliae-infected potato plants during the first 3 years of the field trial. Fish emulsion treatments increased total tuber yield by 720% in the first 3 years, but there was no effect in the fourth year when no FE was applied. Marketable tubers were consistently higher in the plots amended with FE (2000 L ha(-1)). Fish emulsion soil treatments increased soil bacteria but the numbers dropped to the control level in the fourth year when no FE application was made. There was no change in total fungal numbers. The results suggest that economically feasible rates of FE can provide disease suppression and enhance tuber yield and the effect may last a year or more after the last FE application.