Effects of strip intercropping of potatoes with non-hosts on late blight severity and tuber yield in organic production

The effects of strip cropping of potatoes with cereals or a grass-clover mix in and perpendicular to the main wind direction on foliar late blight severity and tuber yield were studied in large-scale field experiments in Germany. Disease progress was assessed in 12-18 sections per plot and yields determined from the same sections. In 2000, plot size was 3 x 10 m and there were no disease reductions apparently due to interplot interference. In 2001 and 2002, with plot sizes of 6 x 18 and 6 x 36 m in strip-cropped potatoes, disease was significantly reduced by 9-20% and 4-12%, respectively, compared to pure stands of potato, with the greatest reductions in plots planted perpendicular to the wind and neighboured by grass-clover. The most important factors contributing to disease reduction were loss of inoculum outside of the plots and barrier effects of neighbouring non-potato hosts. Only 0-20% of the overall yield variation could be explained by the area under the disease progress curve, depending on cultivar and year. In one year disease effects on the yield of a moderately resistant cultivar were higher than on the yield of a susceptible cultivar. This was probably caused by differences in bulking behaviour. Nutrient limitation appeared to be more important than disease in reducing yields. Yields in the edge potato rows directly neighboured by cereals were significantly reduced in all 3 years, but competition by cereals did not change the disease-yield-loss relationship. Strip intercropping might be a useful component in an overall management strategy to reduce incoming late blight inoculum.