White Powder Plaguing Peonies Posing Perimeter Perennials Threat?

(Question)

I have a number of peonies, which bloom annually. This year, I’ve noticed one of the group has white powder on the leaves.
Should I distance it from the other flowers, besides applying something to it?

I know I haven’t watered it this year, since it’s been a later spring, but I’m resuming watering, and will add some mulch too.

There are white peonies, and a chrysanthemum on one side, and a red rose bush on the other side.

(Answer)

Our moist spring, combined with recent warm weather, has created the perfect environment for the fungus, Erysiphe polygoni, to produce Powdery Mildew on your Peonies. Your Roses and Chrysanthemums are also susceptible to Powdery Mildew.  Although it is a different species of fungus for each of these three plants, the conditions have been ideal for fungal spores- that are found commonly in our gardens, to proliferate and spread.

The best line of defence is to have vigorous, healthy plants. Make sure that your cultural practices are supporting your plants growth. These include: Check that your Peonies, Roses and Chrysanthemums are getting enough sun (a least 6 hours a day), have room for air circulation between each plant and are getting adequate water and nutrients. You may consider moving them in the fall if they have become overcrowded over time. Be careful not to wet leaves, if watering late in the day (although some species of Powdery Mildew  fungus germinate without water). If you decide to fertilize your plants, your Peonies and Roses prefer low nitrogen fertilizer. You mentioned mulching your peonies. Some mulch is good, but be careful not to mulch too close to the stems as this can create a humid environment to perpetuate the infection.

Powdery Mildew is unlikely to cause any permanent damage to your Peonies or Chrysanthemums, but can be more harmful in woody plants, such as Roses, where it can cause bud deformities and leaf curl. Remove affected leaves and buds and discard in a sealed plastic bag. Don’t put infected plant material into your compost bin, as spores can overwinter and reinfect your garden next year. Thoroughly clean gardening tools after working with infected plants, to reduce the chance of spread.

You may wish to apply a preventative fungicide. This may help to reduce the spread of the disease, but should be used in conjunction with the cultural practices recommended above.  The Ontario Ministry of Natural Resources recommends the following fungicides, used as directed to help control Powdery Mildew: Ferban, Mancozeb and Sulphur spray.

The good news is that Powdery Mildew doesn’t occur on the same plants every year, especially if you give the plants a healthy growing environment.

https://www.mnr.gov.on.ca/en/Business/Forests/2ColumnSubPage/STEL02_167012.html

https://canadianpeonysociety.blogspot.ca/p/pests-and-disease.html