About a month ago, I noticed what seemed to be powdery mildew on the Crassula 'Campfire.' I sprayed it with the copper fungicide, which seemed to kill it. Although there is extensive scarring on the leaves, the plant is sending out new growth. There seemed to be a recurrence on some of the damaged leaves so I hit it again today, but the new leaves are unaffected so far. I received this plant in a Ramsey's order March 7.
A few days ago I noticed similar white patches on the Crassula cv. 'Buddha's Temple' so I also hit it today with the fungicide. This plant has always had a funny-looking top, and it is slumped over in its pot. In other words, it has never been a stellar performer since I got it from The Next Gardener March 17.
The conditions for powdery mildew seem rather specific, and the greenhouse only has some of them. The humidity does go up at night, but not to the 95% cited by the interweb as necessary for the mildew to take hold. And for about the past month, there has been at least one large window open all the time to provide air flow. Maybe it is something other than powdery mildew, but the fungicide seemed to greatly reduce it on the 'Campfire.'
The commonality between the plants of course is they are both Crassulas. I thought I saw something on the 'Coastal Coral' a few weeks ago but it turned out to be nothing. I've been thinking about moving that one from its plastic pot to terra cotta so it will dry out better. That wasn't a factor with the 'Campfire' since it has been in terra cotta all along. The 'Budda's Temple' is in a ceramic pot, which holds a little more moisture, but not as much as plastic.