Effects of specific gravity and cultivar on susceptibility of potato (Solanum tuberosum) tubers to blackspot bruising and bacterial soft rot

Ten potato (Solanum tuberosum) cultivars were assessed for resistance to bacterial soft rot caused by Erwinia carotovora subsp. atroseptica, and to blackspot bruising. Specific gravities of individual potato tubers varied within each cultivar and between cultivars. Within all 10 cultivars tested, as tuber-specific gravity increased, the severity of bacterial soft rot decreased, and the severity of bruising increased. Among the seven New Zealand-bred cultivars, mean soft rot susceptibilities lay along a continuum, with cultivars having higher mean soft rot losses as mean specific gravity increased. Two of the three overseas cultivars exhibited very different responses from those bred in New Zealand. In contrast, for blackspot bruising, six of the New Zealand-bred cultivars and two overseas cultivars lay approximately on a continuum of increasing mean bruise score as mean specific gravity increased, similar to the within-cultivar relationships. The remaining New Zealand-bred cultivar and the overseas cultivar 'Fianna' were quite different, having much lower mean bruise scores than predicted by the trend line of the other cultivars. 'Fianna' in particular exhibited both low bruising and low soft rot incidence in this experiment.