Abstract
At the end of the 1990s the apple scab-resistant variety Topaz appeared in the apple variety assortment of organic growers in Northern Europe. The variety was accepted due to its resistance to apple scab (Venturia inaequalis), its taste and appearance, but soon it became clear that it was extremely susceptible to collar rot (Phytophthora cactorum). Already after a few years high tree losses were recorded especially in orchards at replant sites. Different approaches were tested to solve this problem, and satisfactory results were achieved with both copper and Potassium-phosphite treatments. In practice, the problem was eventually solved by using adequate interstem/rootstock combinations.