Question: At the end of every summer a "white film" forms on my 'Festiva Maxima' peony and eventually encompasses the plant. The branches are cut off at the ground in early November and put in the trash. What is this stuff, is it preventable and is it OK to compost infected stems?
Answer: It could be powdery mildew, a fungal disease that develops in humid, stagnant conditions. It looks like a dusting of flour on the tops and bottoms of leaves and can appear on stems as well. It is difficult to treat, but happily it is not considered a conqueror of the peony. It can be quite fatal for other perennials, however, such as phlox.
To minimize powdery mildew, try not to wet the plant's foliage if you water it, make sure it is getting plenty of sunshine (have nearby trees or shrubs grown to cast more shade?) and increase air circulation by dividing the peony. Discard the woody part at the center of the plant and divide the outer sections into several new plants. You can move the divisions to other areas of the garden or discard them. Keep one division to plant in the original spot. Air will circulate better because the plant will be smaller.
You can remove heavily mildewed leaves before the end of the season; the mildew prevents them from photosynthesizing. Seal them in garbage bags rather than composting them. Most home compost piles do not get hot enough to kill pathogens. Similarly, at the end of the season discard the stems as you've been doing, sealed in the garbage.
There is also a disease called white mold that infects peonies. Infected plants will wilt, either in whole or in part. The stems will turn light tan, and in humid weather a fluffy white substance may appear on them. If you slice the stem open lengthwise, you'll find hard black patches inside the tan section of stem. This disease arises from the soil, where it can live for many years. Your best bet is to remove the infected plant parts and seal them in garbage bags to dispose of them. Look for plants not susceptible to white mold (Sclerotinia sclerotiorum) when replanting the area. Space plants wide apart to improve air circulation, the bane of all fungal diseases.
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