Peonies are a family of perennial flowering herbaceous and woody shrubs containing dozens of species and thousands of hybrid cultivars. Native to Europe, Asia and the North American west, peonies are a low-maintenance plant that produces distinctively lush blooms on long, strong stems. They thrive in sunny locales and are not a particularly strong attractant to pests or disease but can occasionally suffer from a few that you can easily control with moderate intervention measures.
The most common diseases that peonies succumb to are derived from fungal or bacterial infiltration. Botrytis or peony blight will turn the flower buds black and cause them to drop, unfurled and shoots to brown and droop. To control blight, cut away any diseased or damaged foliage and place in the garbage, bypassing the compost bin. Keep the area around the peony plants free from dropped, decaying foliage to prevent spread. In late fall, cut down any blight-affected plants to 1 inch above the soil and throw the clippings in the trash. Cutting the plants down will break the blight cycle.
Powder mildew can also afflict peonies and appears as a white flour-like substance on the leaves, flower buds and blooms, which can sometimes be accompanied by a torquing or distorted leaf shape. When you see powder mildew, immediately apply a fungicidal spray designed for ornamental flowering plants to halt the infection and control its spread. Fungicidals such as Funginex, Daconil 2787 or Ortho Multi-Purpose Fungicide work well. If the leaves and flowers do not recover after spraying, cut them off and discard in the trash.
Two of the few consistent pests that seek out peonies are slugs and spider mites. Traditional chemical insecticides do not work effectively for slugs, so setting slug-specific bait traps is a key to getting them under control. For a homemade remedy, pour a can of beer into two or more low-lipped saucers or pie tins and nestle or bury the container in the ground so that the lip of the container is near level with the bark or soil, giving the slugs easy access. Place the beer traps around the peony plants where the slugs seem to be congregating and feeding in the evening hours. Leave the traps in place overnight, and repeat the process every night or every other night until the slugs are under control. The slugs are attracted to the beer and will seek it out and slide in for a drink and die. You can also use pellet or liquid chemical slug bait, such as Bug-Geta Snail and Slug Killer, which produce an odor that attracts the slugs, enticing them to ingest the poison.
Spider mites can appear on peonies and will draw the plant sap out through the leaves, killing the foliage and the plant, if left unchecked. If the peony leaves are shriveling and dying inexplicably, check for spider mites by holding a piece of white paper under the peony leaf while you tap gently on the leaf. If you have spider mites, they will fall onto the paper and look like pale taupe or brown skimming specks. The moment you confirm the diagnosis, spray on an insecticide containing malathion or spray the plant foliage with a stream of water from your garden sprayer, much as you would wash aphids off. Repeat insecticide spraying or power washing once a week for a month, or longer, until the symptoms abate.
Rabbits particularly enjoy eating peony blooms and they can be kept at bay by physical fencing. You can also apply natural rabbit repellent substances on the surrounding soil and lawn, as well as on the peony plants themselves. Dried blood and blood meal work well for this, but will need to be reapplied after heavy rain as their effects are washed away.