Hello, I live in the beaches area of Toronto and I have a lovely little garden out front that has always done well. Last summer something happened with my peony ..it grew but then as it was blooming something got to is and killed it quite quickly. It just seemed to turn black from what I remember. I didn’t think too much of it at the time. These things happen every now and then. I also noticed that a few other plants (I don’t know all of the names but they were all flowering perennials in that area weren’t doing as well as they normally do. I ended up getting a bit of empty space in the front of the garden so I bought some new perennials to fill in the space (echinacea and one other I can’t remember) and none of those came back this year. In fact the echinacea also got sick and died last year. This year I notice that there is still that big empty patch and several plants around there now have leaves that are turning brown. I have attached some photos of the plants that are turning brown for you to see. I went to the local garden shop and they gave me a dormant kit and I have sprayed the garden with a combination of sulphur and horticultural oil in the hopes of killing off any fungus or pests that may be in the garden. My hostas in that area are doing fine. I put in composted matter – normally worm castings every year to add rich organic matter into the soil. I am worries that I may have an issue with the soil or perhaps a fungus or pest that is in the soil or came on the mulch I put down last year (got a Loblaws). I don’t want to buy any more new plants until I figure out what is going on in the garden. The other part of the garden seems to be doing much better – daisies and clematis are thriving. Although my Sand Cherry has a pest which (mites) which I have sprayed and the dogwood tree in the front did have one section that died but the rest seems fine.
If you can offer up any ideas or things I can do I would really appreciate it!
Since your peony was the start of things going wrong, it is possible that a fungal disease may have begun this series of problems. You may find interesting this Toronto Master Gardeners post about Botrytis, a fungal disease that afflicts peonies as well as other plants in the garden, and can remain in the soil. This piece gives good advice on sanitary cultural practices that will help to keep your garden free of fungal diseases: https://www.torontomastergardeners.ca/index.php/askagardener/my-stand-of-white-peonies-did-not-bloom-from-bud-this-year/
It is possible, since your coneflower also perished, that other fungi to which coneflowers are susceptible are to blame: Sclerotinia or Sclerotium, root or crown rots, which can originate in both the soil and the air, causing stem blotches, wilting and dieback. The fungi are especially prevalent in damp conditions, and can survive in the soil for many years.
Your photo shows what appears to be vinca with brown splotches. Vinca are also susceptible to Botrytis cinerea, also known as gray mold, and spots on leaves are also consistent with this fungal disease. (The “gray mold” of its name appears on dead plant tissues).
It is difficult to tell what, if any, of these fungal diseases you may have in your garden, but you have already been so thoughtful and vigilant in treating for fungal diseases that hopefully you will find your plants will be unaffected in the future. Good practices in terms of removal of dead plant material will also be very helpful. Once you have cleared your garden area of any dead debris, and added organic material such as compost to provide the best soil drainage possible, you could consider replanting with materials that are resistant, such as hostas and sedums.
Another note about Echinaceas, this time from Sonia Day, the garden writer who can always be counted on for an interesting take on things. Here, she talks about Echinacea and the problems some of us have in keeping this perennial alive from year to year: https://www.thestar.com/life/2015/05/08/disappointing-echinacea-and-other-perennial-problems.html
In the end, it is worth noting that you are doing everything right, and I hope your garden rebounds this year and in years to come.
PS: In terms of your Purple Sandcherry, if the pest is on the stems and looks like ‘bumps’, it may be scale instead of mites.