Susceptibility of apple cultivars to collar rot (Phytophthora cactorum Leb. et Cohn. Schroet.)

The susceptibility of 16 apple cultivars to collar rot caused by the fungus <i>Phytophthora cactorum</i> was tested. Three methods of evaluation were applied: a laboratory test, a greenhouse experiment and a field experiment. Two cultivars were very sensitive: 'Discovery' and 'Hibernal'. The next susceptibility group comprised the cultivars: 'Gloster', 'Florina', 'Liberty', 'Julyred', 'Paulared' and 'Jonnee'. Moderately susceptible were: 'Šampion', 'Nova Easygro', 'Jonagold' and 'Quinte'. Least susceptible were: 'Summerred', 'Nüred', 'Jonamac' and 'Spartan'. Not one of the tested cultivars was completely resistant to infection with <i>Phytophthora cactorum</i>. A high degree of correlation was noted between the results obtained in the greenhouse, in the field and in the laboratory test on annual shoots cut off at the dormant stage of the trees. Much less reliable results were obtained in the laboratory test on apple-tree shoots cut in September in the end stage of vegetation of the trees.