Many plant pathogens like to survive the winter in infected crop debris. One example is iris leaf spot.
Here is a zoom – the black spots are structures where the fungus produces spores:
So what can we do? Here are some tips from the Horticulture News (http://www.ksuhortnewsletter.org/):
Iris are known for a couple of common problems: a fungus disease known as iris leaf spot and an insect named iris borer. Though both cause problems in the spring, now is the time to start control measures. Both the fungus and eggs of the borer overwinter on old, dead leaves. Remove dead leaves and cut back healthy leaves by ½ this fall to reduce populations of these pests. Also remove other garden debris from the iris bed. This can significantly cut down on problems next spring. (Ward Upham)
Another disease that lurks over the winter is peony leaf blotch (also known as red spot or measles) and you can find info on that disease here on the Common Plant Problems website.
(Photos by Megan Kennelly)