We planted some peony plants earlier this year and the blooms were really great and the plants looked very healthy. However over the past week or so, all four plants are starting to have the leaves turn brown at the tips of the leaves and then ultimately completely brown. Is this a disease that I need to treat??
It's not reliable to diagnose a problem just from an image, but I'll attach a factsheet on Peony blights which might help you narrow it down. (http://www.missouribotanicalgarden.org/Portals/0/Gardening/Gardening%20Help/Visual%20Guides/Peony%20...) It does look like a possible fungal infection in which case it is not possible to treat it. Fungal attacks can be mitigated before they develop but once one sees symptoms it's too late late to treat. Keep the plants sufficiently watered, be diligent in removing all fallen leaves and clean up the area carefully in the fall. As a newly planted peony it has been very stressed, especially since it was allowed to bloom its first year. This will have weakened the plants, leaving them more susceptible to insect or disease. By encouraging healthy growth, limiting any blooms next year, they may recover and be very healthy plants down the road.
Appreciate your response, should I cut off the leaves that are infected or just leave as is and remove from around the plant once they fall off?
Sorry for the slow response. Either way is fine. The plants may still be getting some energy from the infected leaves, though leaving them on means potentially more fungal spores released... it's a judgement call depending on how badly infected each leaf is. Good luck!