EFFECT OF NEIGHBORING PLANTS ON THE YIELD OF POTATOES FROM SEED TUBERS AFFECTED WITH GANGRENE (PHOMA-FOVEATA) OR FROM PLANTS AFFECTED WITH STEM CANKER (RHIZOCTONIA-SOLANI)

Healthy seed tubers and seed tubers with gangrene or inoculated with Rhizoctonia solani to induce stem canker were planted 38 or 76 cm apart or alternately at 38 cm spacing in 2‐row (1985) or 4‐row (1986) plots at Rothamsted. At 38 cm spacing, stem canker decreased yield by 5% and 8% respectively in 1985 and 1986 and gangrene by 5% and 14%, but usually by twice these amounts when seed was spaced at 76 cm. In all experiments, yields from plots with alternate healthy and inoculated seed (50% disease) were similar to yields from healthy seed, although within these mixed populations individual plant yields from healthy seed were larger and those from diseased seed were smaller than from plots of healthy or inoculated seed respectively. Samples in August showed a similar effect on fresh weight of foliage. Disease usually decreased the number of stemdplant but in 1985 gangrene increased stem numbers and probably accounted for small amounts of compensatory growth from neighbouring healthy plants. Copyright © 1990, Wiley Blackwell. All rights reserved