I bought 4 small Autumn Joy plants, 2 of which I planted in the ground. The 2 in pots have a white splotchiness on the leaves. What is this and what shroud I do about it?
Without a photo, it’s difficult to make a diagnosis. Autumn Joy sedums are pretty resistant to most things. They do suffer from root rot if overwatered and get weak and floppy in insufficient sunlight. Occasionally, they are attacked by powdery mildew, which is what those spots may be. It usually begins as “chalk dust” like areas on the leaves. As the disease progresses, brown scab-like lesions appear. The leaves may also turn a purple-ish colour.
Here’s an article from the University of Arkansas, which includes a photo of advanced disease. Note that in Ontario, our rules for fungicides are different. However, the potted sedums will not survive the winter anyway, so you should destroy those plants now.