INFLUENCE OF PLANTING SEED TUBERS WITH GANGRENE (PHOMA-FOVEATA) AND OF NEIGHBORING HEALTHY, DISEASED AND MISSING PLANTS ON THE YIELD AND SIZE OF POTATOES

Potato seed tubers infected or not infected with gangrene (Phoma fovea ta) were planted at Rothamsted in 1987 to measure the effect of the di sease and of neighbouring plants on yield. The experimental design was constructed so that the effect on growth of six adjacent plants (two nearest neighbours in each direction within rows and one nearest neigh bour in each direction across rows) could be estimated for each plant. Total yield, ware(> 150 g) yield and tuber number from individual pla nts were affected most by the disease but also, in decreasing importan ce, by the two plants on either side within the same row (first neighb ours), the two plants adjacent to the first neighbours (second neighbo urs) and the two adjacent plants in the rows on either side. Yield and tuber numbers increased as the different combinations of neighbouring plants contained increasing proportions of plants from diseased seed and missing plants; plants compensated for decreasing competition. Tub er size distributions showed that numbers of ware tubers decreased wit h increasing competition whereas numbers of small tubers were less aff ected. The fitted model was used to predict yields from crops planted with different proportions of diseased or missing seed tubers.