Relationships between stem canker, stolon canker, black scurf (Rhizoctonia solani) and yield of potato (Solanum tuberosum) under different agronomic conditions

Relationships between diseases caused by Rhizoctonia solani on different parts of potato plants ( Solanum tuberosum ) at different stages of crop growth were investigated under selected agronomic conditions. The effects of different densities of tuber-borne inoculum, date of planting, irrigation, size of seed tubers and their interactions on the incidence of stem and stolon canker during crop growth, the incidence and severity of black scurf and the yield of progeny tubers at harvest were quantified in a multifactorial experiment. Differences in the incidence of stem canker, stolon canker and black scurf were dominated by the effect of density of inoculum on seed tubers at planting. Highly positive correlations between the disease variables indicated a close relationship between the incidence of disease at each stage of crop growth although the degree of association between variables measured at an early growth stage and those measured at progressively later stages of crop growth weakened as the time interval increased. Total yield of progeny tubers was not affected by the density of tuber-borne inoculum although there was a shift in the size distribution, with a decrease in the yield of main-sized tubers and an increase in the yield of baker- and oversized tubers at the higher density of inoculum. Of the remaining factors, the effect of season tended to be more pronounced than any of the agronomic treatments although the use of irrigation and later dates of planting did influence the incidence of infection to a limited extent.