Calcium soil amendment increases resistance of potato to blackleg and soft rot pathogens

Dickeya and Pectobacterium species cause blackleg / soft rot disease complex on potato in Zimbabwe. The disease is seedborne and difficult to control. This research focused on ways of increasing the inherent resistance of potato plants and tubers to blackleg and soft rot. Two field experiments were conducted at the University of Zimbabwe campus plots in 2008/9 and 2009/10 seasons in order to determine reproducibility of the results. Sprouted tubers of cv. BP1 were inoculated with a mixture of Pectobacterium atrosepticum, Pectobacterium carotovorum subsp. brasiliense and Dickeya dadantii. The inoculated tubers were planted in plots treated with different fertilizer combinations. The treatments were: 1) compound S (7N: 21P: 8K) + ammonium nitrate (34 %N); 2) compound D (7N: 14P: 7K) + calcium nitrate (19 Ca: 15.5N); 3) compound S + calcium nitrate and 4) compound D + ammonium nitrate. Blackleg disease incidence and severity were recorded fortnightly starting from 2 weeks after crop emergence (WACE) while soft rot disease was assessed at physiological maturity. Blackleg incidences and severity were significantly lower (P < 0.05) in the plots where calcium nitrate was applied for both experiments. Blackleg disease incidence was reduced by more than 20% in both experiments 1 and 2. Soft rot incidence in the progeny tubers was also significantly reduced by the calcium treatment. In addition, calcium amendment significantly reduced (P < 0.05) soft rot losses of tubers in storage. Calcium nitrate was effective in reducing blackleg and soft rot diseases in combination with either compound D or compound S. The yield was significantly higher in plots where compound S was applied as a basal fertilizer than that recorded in plots treated with compound D fertilizer. This study shows that calcium soil amendments reduce blackleg and soft rot diseases under Zimbabwe’s growing seasons in red fersiallitic soils. Compound S produces better results in potato production than compound D and farmers should be encouraged to use compound S when growing potatoes.Key words: potato, disease resistance, blackleg, soft rot, storage losses, calcium: potato, disease resistance, blackleg, soft rot, storage losses, calciu