Resistance to late blight in foliage (Phytophthora infestans) of potatoes assessed as true seedlings and as adult plants in the glasshouse

Summary True seedlings from crosses designed to produce progenies having high levels of field resistance to late blight (Phytophthora infestans (Mont.) de Bary) were scored in lots of 25 in a glasshouse. The results agreed with assessments subsequently obtained from plants grown from tubers from the same progenies when screened as individual whole plants in the glasshouse. The resistance of seedlings of both resistant and susceptible progenies was assessed by inoculating them 3–8 weeks after sowing. All were highly susceptible when inoculated at 3 weeks but resistance increased from 3 to 6 weeks and differed little between 6 and 8 weeks. The seedling test appears to provide a valid screen for resistance at an early stage in a breeding programme.