Powdery Mildew on Crassula
Powdery mildew favors moderate temperatures and high humidity. Do
whatever you can to keep humidity low (less than 90%) including well timed irrigation, good plant spacing, and using fans and ventilation.
If necessary, numerous fungicides are labeled for powdery mildew management in greenhouses. Use different modes of action in rotation and make sure to read the pesticide label. Follow instructions regarding application, rotations for the prevention of resistance, plant safety, and all other precautions.
Be sure to clean greenhouses between crops, eliminating infected pet plants, carryover plants, plant debris, volunteer plants, and weeds.
Source: e-gro.org