FACTORS AFFECTING THE TEMPORAL PROGRESS OF STEM CANKER (RHIZOCTONIA-SOLANI) ON POTATOES (SOLANUM-TUBEROSUM)

The effects of different densities of tuber-borne inoculum, selected a gronomic treatments (date of planting, irrigation and size of seed tub ers) and their interactions on the temporal progress of stem canker (R hizoctonia solani) on potato plants (Solanum tuberosum) were investiga ted in a multifactorial experiment. Data comprising stem number and th e incidence and severity of stem canker from planting until tuber init iation in two consecutive growing seasons were analysed using linear c ontrasts, quadratic contrasts and the area under the disease progress or host growth curve. Differences in the incidence and severity of ste m canker were dominated by the effect of different densities of tuber- borne inoculum. The majority of disease progress curves were nonmonoto nic for the incidence and severity of stem canker with a rapid rise in disease up to stem emergence and a decline thereafter. Most treatment s 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 preemergence irrigation reduced the levels of stem ca nker whereas size of seed tubers did not affect the progress of diseas e. Little additional information was revealed by scoring for the sever ity rather than the incidence of stem canker.