DEVELOPMENT OF A MUTANT STRAIN OF SPOROTHRIX FLOCCULOSA WITH RESISTANCE TO DODEMORPH-ACETATE

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 mu g of fungicide per mi, 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.