Turnip – White Spot Disease
Q: I have grown Purple Top turnips for many years without problems. This year though, issues have arisen. I first noticed a few small spots developing on the foliage. It has now spread throughout the entire crop. I can’t figure what ails them.
A: Looks like white spot, a common turnip disease. You’re going to have to replant if the whole crop is affected, assuming the soil is still warm enough.
My guess is that the leaves got too wet at one time or another and that let the disease spread. Next time, make a mental note to examine your turnip leaves after it rains.
Copper-containing fungicides like Kocide 101 (click for sources) are labeled for control.