INTRODUCTION

All breeding strategies, classic as well as unconventional, aim at the production of better varieties. Today this means in particular, higher levels of disease resistance. This aim is most easily achieved when both classical and unconventional techniques complement each other and increase the efficiency of the breeding processes, i.e. creation of variation, hybridization and selection. New variation may be obtained in vitro through somaclonal variation or by making use of natural meiotic segregation. For the production of clones with resistance to Phytophthora and Fusarium, spontaneous in vitro mutation can be used; this is, however, only applicable when the induction of variation is coupled with a powerful in vitro screening system. From abiotic cultures of the two fungi mentioned, exotoxins can be extracted and used for selection. This process may be also used for hybrid selection, when both fusion partners have a different resistance. In addition, for hybrid selection, hybrid vigour of the fusion products may be used. The regeneration of potatoes via another or isolated microspore culture (Wenzel et al. 1982; Uhrig 1985) provides the basis for the use of meiotic segregation in vitro. Haploids can be produced from tetraploids and from dihaploids. The use of such haploids should facilitate the incorporation of virus resistance, and of other characters, regardless of whether they are monogenically or polygenically inherited.