INTRODUCTION

Potatoes differ from other arable crops grown in temperate countries in a number of ways which influence the conduct and logistics of breeding programmes and compound the difficulties of variety assessment. These differences can be summarized as follows.

1. The potato crop is propagated vegetatively from tubers and varieties exist as clones with the exception of crops grown from true potato seed (TPS), mainly in the tropics and subtropics. In temperate regions, however, the consequences of clonal methods include the establishment of a diversity of schemes for seed tuber multiplication, maintenance and certification of healthy stocks and of the importance given to breeding for resistance, especially to virus diseases.

2. There are more characters of economic importance in potatoes than in any other temperate arable crop. This means that the chances of selecting a variety with good performance for all the characters of importance are very small. All varieties are, therefore, compromises and perform well or less well for the range of important characters. This has contributed to the relatively slow progress made in potato breeding and partly explains why the replacement rate of old varieties by new ones is less rapid in potatoes than in other arable crops.

3. Because of the autotetraploid nature of potatoes, the F1 population raised from intercrossing two tetraploid parents is highly heterozygous. Progeny testing as a means of evaluating parental performance is laborious and has seldom been applied routinely to breeding programmes.