Effect of Presprouting on Yield of Maincrop Potatoes (Solanum tuberosum L.) in Organic Farming

From 1991 to 1993 field trials at two sites (Bruchkobel and Gladbacherhof) were carried out to study the coti-tribtition of presprouting in securing the yield of main-crop potatoes (cvs. Agria. Granola. Linda and Nicola) in organic farming. In several cases presprouting diminished significantly the number of stems and tubers per plant. While in July (sample harvest) presprouting resulted in significant higher total yields with nearly all cultivars, in September (main harvest) the presprouting effect on total yield differed from year to year and partially from site to site, Presprouting reduced the percentage of undersized tubers, especially in 1992. Significant increases of the percentage of oversized tubers caused by presprouting were found especially with cv. Agria. The presprouted potatoes showed significantly higher marketable yields in 1991 at Bruchkobel (+28%), in 1992 at Bruchkobel (+ 14%) and Gladbacherhof (+ 18%) and in 1993 at Bruchkobel (+ 3%), Considerable yield increases caused by presprouting were observed when the foliage was destroyed early in the growing season by the Colorado beetle (Bruchkobel 1991) or by late blight (both sites 1992). Presprouting proved to be an important method to ensure yields in a farming system using no chemical pesticides.