PROBLEMS OF RESISTANCE IN BELGIAN STRAWBERRY CULTURE
After the first appearance of resistant strains of Botrytis cinerea Pers. to MBC-compounds in 1973, persistent resistance generalized. In 1979, after repeated applications of dicarboximides on everbearing strawberries, efficiency decreased and dicarboximide resistant Botrytis cinerea strains were isolated. Afterwards, the efficiency of dicarboximides on Botrytis decreased also in normal-bearing strawberry and in greenhouse.
In 1983, a sharp decrease in the control of Sphaerotheca macularis (Wallr. ex. Fr.) Jacz. F. spec. fragariae was observed with some triazoles and also with related fungicides such as imidazoles and pyrimidines, all Sterol Biosynthesis Inhibitors. A biological test was realized to investigate the resistance. In field trials, after repeated treatments of the dicarboximide Iprodione, a decrease of activity was observed against Alternaria alternata Nees. and strains were isolated that were no longer sensitive to 100 ppm of Iprodione. In 1986, in spite of three soil drenches, the phenylamides Metalaxyl and Oxadixyl no longer showed any activity against Phytophthora cactorum (Leb. et Cohn) Schroet. A Phytophthora cactorum strain was isolated with a level of resistance of 10,000 against some phenylamides.
Different treatment schedules were tested in the field in order to break this resistance.