Tuber size distribution in cv. Sebago and quantitative effects ofRhizoctonia solani on yield

Abstract

Plants dug at random from a commerical crop, cv. Sebago, were scored just prior to normal harvest date in two seasons for stem and stolon infection caused byR. solani. Tuber weights relative to stolon position were recorded. Yield was directly related to haulm fresh weight (F) and it was considered essential to includeF when interpreting yield reduction from stolon pruning. The probability of tuber size distribution fitted a proportional odds model; tuber sizes were not uniformly distributed down the stem profile and the top and bottom stolons had the lowest probabilities of bearing, marketable tubers (>45 g), but commonly bore them when stolons in the middle of the stem profile were pruned. Stem canker did not significantly affect haulm fresh weight or tuber yield and there was a stolon pruning threshold (18%) below which yield was not reduced. For every 20% increase in pruning above this threshold, yield was reduced by only 6%. The effects ofR. solani infection on yield were determined from a model based on an assumption of assimilate flow.