INTRODUCTION

Recurrent selection and mass intercrossing are used to enhance resistance in domestic potatoes to potato virus Y, potato leafroll virus, Verticillium wilt, Columbia root knot nematode, and deep-pitted scab. Lines resistant to one of these diseases or pests are intercrossed to enhance that single attribute. Resultant true potato seed (TPS) is bulked and sown in a field nursery where severe selection pressure will identify increased resistance to that disease or pest. Clones from selected TPS plants are retested in the same nursery and also screened in other nurseries and performance trials to select for other attributes. Surplus pollen from intercrossing within each parental group is combined with pollen from the other four groups and used to intercross all five. It is also used to pollinate a parental group of breeding lines with superior horticultural attributes. TPS from these between-group mass intercrossings is sown in a field exposed to many diseases, pests and stresses to identify clones with multi-resistance. A number of multi-resistant breeding lines have been developed and are maintained in a disease and pest-free state by meristem culturing.

CHOOSING PARENTS FOR CROSSING GROUPS

To determine the best parents, we have tested known resistant germplasm from throughout the USA, Canada and elsewhere. In addition, several hundred advanced and early-generation selections are being obtained annually from US and Canadian breeding programmes.