Mildew, Measles, and Fungus Oh My!
Cassie Homan, Horticulture Agent
The weather
this year has resulted in many peonies catching the "measles" and/or
powdery mildew. These diseases are favored by warm, humid weather and this
spring the fungal diseases took off. If your peonies are crowded together with
poor air movement or in shade, you are likely to see some disease. We are
starting to notice disease symptoms with some very sad and sick looking
peonies.
Measles: Measles is a disease, also known as red spot, that causes
distinct, reddish-purple spots on the upper leaf surfaces. These spots often
coalesce and become large, reddish purple blotches on the upper leaf surfaces
but are a light brown color when viewed from the underside of the leaves. The spots
on stems will merge and form streaks that are reddish brown.
Powdery Mildew: Plants infected with powdery
mildew look like they have been dusted with flour and can lead to death of the
leaves. This disease is rarer in Kansas than Measles but does show up in wet,
humid summers.
Sanitation is
the best control for both these diseases. Remove all diseased tissue, including
stems, at the end of the growing season. The foliage can be removed in
mid-August with no harm to the plants as the plants will be essentially
dormant. Foliage that has already died should be removed now.
Mulch that
contains plant debris should also be discarded and then replaced with fresh
mulch. Reducing the source of the inoculum, (disease spores) will reduce the
chances of another severe outbreak next year. Another tip to reduce foliage
disease is to water with drip irrigation to keep the foliage as dry as
possible.
Fungicides
can be effective if applied before infection has occurred. Therefore, heavily
infected plants should not be treated as the treatment will be ineffective.
Fortunately, the disease should cause no lasting damage to the plant. Look for
individual spots to appear the following spring and then apply a recommended
fungicide before the disease has spread. Suggested fungicides include
myclobutanil (Eagle, Spectracide Immunox, Immunox Plus, Fertilome F-Stop Lawn
& Garden Fungicide), propiconazole (Banner MAXX, Fertilome Liquid Systemic
Fungicide) or tebuconazole (Bayer Disease Control for Roses, Flowers & Shrubs).
