Influence of Rootstock Type and Time of Inoculation on the Resistance of Five Apple Scion Cultivars to Collar Rot Caused by Phytophthora Cactorum

SummaryIn a field experiment with young trees Cox’s Orange Pippin was the most susceptible cultivar. Cv Laxton’s Superb showed absolute resistance. In cvs Lord Lambourne. Tydeman’s Early Worcester and Worcester Pearmain most lesions were confined by host tissue reactions. In all cultivars except Superb resistance diminished with increasing vigour of the rootstock in the sequence M.9. M.2, MM.111, MM. 104, and in all cultivars except Superb resistance was least during bud-burst and flowering, increasing sharply with the start of shoot extension growth. Where scion lesions extended to the graft union, rootstock colonization reflected the relative field resistances of the rootstock clones. The results indicated that inoculations of a susceptible cultivar budded onto rootstock trees might provide a method for determining relative rootstock resistance.