I have a nice row of peonies next to my house that have done well for about...
I have a nice row of peonies next to my house that have done well for about 11 years. I've noticed that for that last 2 years that flowering has decreased by half and that the leaves turn black before fall comes. I have just placed mulch there a couple of times, no other changes. Are they related issues or ?
Even though you noticed the problems two summers ago, the problem may have started even earlier. Two years ago was a drought year, and stressed the affected peonies. These drought stressors are still being felt and will continue for quite a while. It's likely that the mulch did not contribute to the development of what is likely a fungal infection, unless the mulch was in the form of wood chips. They tend to support fungal growth.
Depending on the identification of the fungus at issue, treatment may be possible. The alternative is to remove and discard all plant material and surrounding soil, and establish a new location for new stock. Fungi are always present in soil, it's when moisture and heat conditions are optimal that they proliferate and problems occur. It may also be that making a new location is the best alternative. In order to make a decision I recommend having soil and plant tissue samples sent in for analysis, as soon as samples can be collected. Successful treatment of the problem depends on accurate diagnosis.
For more information see the following:
Peony, see Leaves with spots, then likely Phytophthora Leaf Blight:
UMN Soil Testing Lab:
UMN PLant Disease Clinic: