The effects of irrigation methods, calcium levels and other nutrients on soft rot (pectobacterium carotovora subspecies brasiliensis) infection of potato (solanum tuberosum l.)

Abstract

Two main experiments were carried out to study the effects of irrigation methods, calcium levels and other nutrients on soft rot infection of potato. The first experiment was an outdoor pot experiment, conducted in 2012 and 2013, during the summer season (August – December). The objective was to explore the potential for controlling tuber soft rot using calcium fertilization in combination with other nutrients: potassium, nitrogen, magnesium and phosphorus. In addition plant growth parameters were measured. The experiment was laid out as a 9 x 3 factorial design in a Randomised Complete Block Design (RCBD). Calcium was applied at 0, 8.11 and 16.22 g/ pockect calcium nitrate (15.5 % N, 19.9 % Ca) while other nutrients were magnesium sulphate (9.1 % Mg) at 1.2 and 2.7 g/ pocket, ammonium nitrate (34.5 % N) at 4.05 and 6.76 g/ pocket, single superphosphate (19.3 % P2O5) at 20.27 and 27.03 g/ pocket, potassium sulphate (50 % K2O) at 6.08 and 6.76 g/ pocket, and a nil control. The fertilizer treatment combinations did not exhibit significant difference in both the number of tubers harvested per plant and the yield in both 2012 and 2013. However, in 2012 only calcium nitrate fertilizer resulted in significant differences in the number of tubers per plant, with the application of 8.11 g/ pocket and 16.22 g/ pocket increasing the number of tubers. Harvested tubers were also analysed for tissue calcium concentration and they were not significantly affected by the fertilizer combinations. Red clay loam soils used in this experiment could have decreased the leaching of calcium nitrate to the subsurface where tubers were formed. Maceration tests were carried out on harvested tubers by inoculating with Pectobacteriumcarotovora subspecies brasiliensis. The rotting zone diameter was measured after 24 and 48 hours. However, there were no significant differences in the rotting zone diameter in all treatments. The second experiment was a field study conducted to evaluate the effects of irrigation methods and plant nutrition on the growth of potato and on the susceptibility of tubers to potato soft rot, carried out in 2012 and 2013, during the summer season (August – December) at the University of Zimbabwe, Crop Science field experiment blocks. The experiment was set up under overhead and drip irrigation using a RCBD, with seven blocks and five calcium nitrate fertilizer rates of 0, 300, 600, 900 and 1,200 kg / ha. All treatments received an equal amount of nitrogen. Tuber calcium analysis indicated that there were no significant differences in the tuber calcium content in tubers harvested from both drip and overhead irrigation in both years. Testing for the susceptibility of tubers to soft rot was carried out using maceration tests. Tubers harvested from drip irrigation in 2012 did not show any significant differences in the rotting zone diameter at both 24 and 48 hours after maceration. No rotting was observed in 2013. Tubers harvested from the overhead irrigated field did not show any signs of rotting, both in 2012 and 2013. Calcium fertilization did not influence yield or tuber size distribution, perhaps because of high inherent calcium already in the plots used and in irrigation water. This work, therefore failed to show that loss from soft rot could be reduced by tuber calcium perhaps because the calcium supplied from irrigation water was above the threshold level for such protection from infection.