(Megan Kennelly, KSU Plant Pathology)
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:
Here are some other flower diseases that carry over on debris:
Rose black spot lesions, close-up of lesions, and microscopic view showing abundant spores:
Cercospora leaf spot on hosta:
For many of these common flower-garden leaf spots, removing diseases and dead leaves can help reduce the amount of the fungi lurking over winter to cause disease next year. If composting, don’t put the compost back in the spot where you took the leaves.