Introduction

Susceptibility to powdery scab (Spongospora subterranea) is assessed by planting seed tubers in artificially infested soil and by controlling soil moisture during tuber initiation.

Inoculation

Inoculum for the pot test is prepared by peeling scab lesions and then drying and macerating the peelings. Seed tubers are planted in pots, c. 20-26cm in diameter, with c.0.3g of macerated scab lesions mixed into the top layer of the compost

Plot Size and Trial Design

Each plot consists of 1 pot laid out in a fully randomised block design of 7 replicates.

Cultivation

Compost is kept moist until all plants have emerged, after which it is maintained near field capacity for 4 days followed by no applications of water for 3 days. This cycle is repeated throughout the period of tuber initiation, normally until 7 weeks after emergence; thereafter, the compost is kept moist to ensure good plant growth.

Disease Assessment

The proportion of surface area affected is assessed in 7 categories shown below, using a visual key (Manual of plant growth stages and disease assessment keys, MAFF, ADAS, 1976) (Appendix 3). The diagrams are used as boundaries for the individual classes of disease severity.

Class	I	II	III	IV	V	VI
%Surface Area	0	0-5	5-10	10-25	25-50	50-100
Mid-point Value	0	2.5	7.5	17.5	37.5	75

The mean % surface area affected is calculated by dividing the sum of the number of tubers in each class multiplied by the class mid-point value by the total number of tubers assessed. In addition, the incidence of cankers is assessed according to NIAB Key 63 (Appendix 4) and an index calculated by multiplying the number of tubers in categories 1, 2 and 3 respectively. The sum is then divided by the total number of tubers assessed.