Factors affecting the temporal progress of stem canker (Rhizoctonia solani ) on potatoes (Solanum tuberosum)

Abstract

The effects of different densities of tuber‐borne inoculum, selected agronomic treatments (date of planting, irrigation and size of seed tubers) and their interactions on the temporal progress of stem canker (Rhizoctonia solani) on potato plants (Solanum tuberosum) were investigated in a multifactorial experiment. Data comprising stem number and the incidence and severity of stem canker from planting until tuber initiation in two consecutive growing seasons were analysed using linear contrasts, quadratic contrasts and the area under the disease progress or host growth curve. Differences in the incidence and severity of stem canker were dominated by the effect of different densities of tuber‐borne inoculum. The majority of disease progress curves were nonmonotonic for the incidence and severity of stem canker with a rapid rise in disease up to stem emergence and a decline thereafter. Most treatments affected the area under the curve and to a lesser extent the average rate of increase in disease. Of the agronomic treatments, later dates of planting and pre‐emergence irrigation reduced the levels of stem canker whereas size of seed tubers did not affect the progress of disease. Little additional information was revealed by scoring for the severity rather than the incidence of stem canker.