INTRODUCTION

Quantitative resistance to diseases of potato varieties is commonly expressed as a value on a 1–9 scale in publications by testing authorities, in advisory and technical literature and in information supplied by breeders prior to official trials. It is an effective means of summarizing a wide range of characteristics, whereby a high value indicates a desirable and a low value an undesirable quality. The values indicate relative susceptibility and may be interpreted by comparing values with those of known varieties.

Valid comparisons between 1–9 values obtained by different organisations can only be made with a knowledge of:–

1. (1) the experimental procedures used,

2. (2) the method of calculating 1–9 values from the experimental data and

3. (3) the control varieties used for these calculations.

EXPERIMENTAL PROCEDURES

A summary of the procedures used at the National Institute of Agricultural Botany (NIAB) is as follows:–

Late blight (Phytophthora infestans) in foliage: Glasshouse plants are sprayed with a suspension of zoospores and incubated at 15°C and 100% rh with artificial light (Saunders 1968). The proportion of the leaf surface affected is assessed after 7 days.

Late blight in tubers: Newly harvested tubers are sprayed with a suspension of zoospores and incubated at 15°C and 100% rh in the dark (Stewart et al. 1983). The proportion of the tuber surface affected is assessed after 10 days.

Common scab (Streptomyces spp.): A soil moisture deficit to induce common scab is created by erecting polythene tunnels to cover field plots immediately prior to tuber initiation (Jellis 1975).