The leaves of my cabbages and purple sprouting ...

Cabbage is a cool season crop, and thrives in cooler temperatures. Hot and humid conditions will cause cabbage to rot. It is actually, one of the earliest crops that can be planted in the spring, but not too early. Cabbage plants will survive a hard frost but not a freeze. Several days or nights of colder temperatures will cause cabbage plants to produce an elongated terminal stem, a condition known as bolting that eventually produces a seed head.

Since Broccoli is a member of the Cabbage family, your question may be answered with one suggestion. But first I need to know, did the plant rot or die off? If this is the case,perhaps you should check your soil. In Wisconsin, the yellows is very common there. It is caused by a soil fungus (a species of fusarium), the common form in Wisconsin. The reccomendation is to plant in another spot or grow varieties marked "yellows-resistant" from where you purchase your seeds.

I hope this is somewhat helpful to your problem.