Asked May 10, 2018, 6:03 PM EDT
Genesee County Michigan
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My first question is how did you determine that your peonies are suffering from Botrytis blight? Fuzzy, gray fungal spores, evident after the plants have been moistened, are a distinguishing trait of Botrytis blight. Since the problem was severe, it should have been possible to see this gray, fuzzy fungal growth on the plants after rain or overhead watering or in days with high humidity.
Generally, recommendations for control differ based on the specific disease and pathogen.
Botrytis blight can begin early in the growing season, even as new shoots are just starting to come out, so you can start looking for it now.
The best place to start with prevention of plant disease is with good cultural practices. How is the sunlight and drainage in the area where the peonies are planted? Peonies prefer full sun and good drainage. Also, good plant spacing and airflow are helpful in deterring fungal foliar disease. You did say that the plants are placed fairly close together; that could aggravate the disease issue. Watering early in the day and only at the base of the plant are also helpful in preventing foliar disease.
You are correct that fallen, diseased leaves from these plants should ideally be collected and discarded of in the trash. In fact, to help prevent disease, peonies can be cut down to the ground in the fall. All diseased plant tissue, including stems, should be bagged and disposed of in the trash, not the compost pile, where the pathogen MIGHT survive to infect other plants. Also, when plants are dry, remove dead flowers and diseased plant parts throughout the season. Remember to sanitize pruning tools with a 10% bleach solution or 70% rubbing alcohol before using the tools on healthy plants!
Botrytis blight can be caused by the fungus Botrytis cinerea, which causes disease on many plants. In this case, it could also be caused by Botrytis paeoniae, which only infects peonies.
There are fungicides that can help protect your plants from Botrytis blight. You need to look for a product that says on the label that it can be used on the specific type of plant, peony, and the specific disease, for instance, Botrytis blight. There should be products with the active ingredient “Mancozeb” or some type of copper that are labeled for use on peony against Botrytis blight. In terms of where to apply the fungicide, read and follow all label directions! I don’t think it is necessary to dig out the peonies before treatment, but follow the directions above all. Fungicides should be applied to protect against Botrytis blight early in the season.
I would suggest reading the articles below, especially the ones from the University of Minnesota and Cornell University.
Disases of peony from University of Minnesota Extension
Botrytis Blight of Peony: Botrytis paeonia from Cornell University
Peony problems from University of Illinois Extension
Please let me know if you have further questions!
Sounds good. I don't know if you know about the MSU Plant and Pest Diagnostic Services on the MSU campus in East Lansing. If the symptoms on the peonies return this year, you can send in photos and mail a plant sample to the diagnostic lab on campus, but it is $20 for an official diagnosis. Here is the website: https://pestid.msu.edu/ . You can also always use Ask an Expert again and send us photos of your personal plants. "Scouting" or looking for plant problems in your landscape throughout the season, and trying to catch things early, is a good idea!
That might not be the ideal amount of sunlight, but I guess that's what you've got for now. Conditions are rarely ideal. :-P Just think about reducing leaf wetness, as I said before. Also, I think I would continue to monitor the disease issue before moving the plants, and potentially their disease, elsewhere. Good luck. :)