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Biological Control

Development of a Mutant Strain of Sporothrix flocculosa with Resistance to Dodemorph-Acetate. Mohammed Benyagoub, Département de phytologie, Faculté des Sciences de l’Agriculture et de l’Alimentation, Université Laval, Qué, Canada G1K 7P4; Richard R. Bélanger, Département de phytologie, Faculté des Sciences de l’Agriculture et de l’Alimentation, Université Laval, Qué, Canada G1K 7P4. Phytopathology 85:766-770. Accepted for publication 1 April 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-766.

Several isolates of the powdery mildew biocontrol fungus Sporothrix flocculosa were screened for their resistance to the systemic fungicide dodemorph-acetate (Meltatox) by repeated exposure to increasing concentrations of the fungicide. Through this procedure, we obtained a new strain that was able to grow and form colonies on solid media amended with 300 µg of fungicide per ml, a concentration that exceeded the recommended dosage by 50% and totally inhibited the growth of the wild-type strain. This resistance trait was maintained after several subcultures on fungicide-free media. When tested for its biocontrol ability, the dodemorph-resistant strain was able to colonize conidia and mycelium of Sphaerotheca fuliginea as efficiently as the wild-type strain. In addition, the new strain was not hampered in its ability to control powdery mildew when applied in mixture with dodemorph-acetate, whereas the wild-type strain was unable to colonize the pathogen in the presence of the fungicide. This is the first report of resistance against dodemorph-acetate in a powdery mildew biocontrol agent and, to our knowledge, in a fungus. This new strain could have practical application in integrated control of rose powdery mildew.