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 differe nt parts of potato plants (Solanum tuberosum) at different stages of c rop growth were investigated under selected agronomic conditions. The effects of different densities of tuber-borne inoculum, date of planti ng, irrigation, size of seed tubers and their interactions on the inci dence of stem and stolen canker during crop growth, the incidence and severity of black scurf and the yield of progeny tubers at harvest wer e quantified in a multifactorial experiment. Differences in the incide nce of stem canker, stolon canker and black scurf were dominated by th e effect of density of inoculum on seed tubers at planting. Highly pos itive correlations between the disease variables indicated a close rel ationship between the incidence of disease at each stage of crop growt h 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 p rogeny tubers was not affected by the density of tuber-borne inoculum although there was a shift in the size distribution, with a decrease i n 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 remai ning factors, the effect of season tended to be more pronounced than a ny of the agronomic treatments although the use of irrigation and late r dates of planting did influence the incidence of infection to a limi ted extent.