Stem canker ( Rhizoctonia solani ) of maincrop potatoes.

SUMMARY In two years, potato plants were sampled at 1 ‐ or 2‐weekly intervals from plots planted with seed tubers bearing sclerotia of Rhizoctonia solani (black scurf) and with seed without sclerotia either infested or not with cultures of R. solani at planting. Sprouted King Edward seed was used in 1981 and sprouted or non‐sprouted King Edward and Pentland Crown in 1982. Stem canker decreased the number of aerial stems and decreased plant height by a constant amount throughout growth. The length but not the number of lateral stems (branches) was increased. Total fresh weight of foliage was decreased although few differences were significant, but the weight of foliage on main stems was decreased and the weight of laterals increased. Total leaf area was decreased by a constant amount throughout growth and the proportion of the leaf area on lateral stems increased. Infection increased % dry matter of main stems and dry weight per unit area of leaves and decreased total dry weights of stem bases and roots. Tuber production was delayed on infected plants although more tubers were initiated later, and at harvest in 1981 infected plants had more tubers < 5 cm than healthy plants but similar numbers of larger tubers. Fresh weight of tubers increased at a similar rate on healthy and infected plants, but the infected plants yielded 50 g less than healthy throughout tuber bulking in 1981 and 20 or 80 g (King Edward, respectively sprouted and non‐sprouted seed) or 110 or 170 g (Pentland Crown) less than healthy in 1982. In both years, neither seed‐borne nor soil‐applied inoculum significantly decreased total or ware (>40 mm) tuber yields at harvest. In both years soil‐applied inoculum had greater effects on plant growth and yield than seed‐borne inoculum.