I’ve read your other posts (as well a lot on the internet) to try to figure this out without success. I recently purchased a gardenia plant from a local garden center. There were a few of them that didn’t look too healthy, lots of yellowing, partiallyl eaten leaves, they looked like they had a pest problem. One however looked mostly healthy, 2.5 feet tall, green, and bushy with buds and an open flower – minus one bud that clearly had some bugs, some yellowing leaves on the inside, mostly near the bottom (new growth mainly), and a couple leaves with black spots at the very top. I thought I’d try to revivie it. There was no big flush of white flies flying off of it, nor can I visibly see any bugs on either side of the leaves or stems. The stems do look like they have hair on them, as though they were shedding. These seem to be the stronger older ones. I cut the bad leaves, the bud, and sprayed it thoroughly with insecticidal soap. It’s in the shade (as far away from my other plants as I can get it).
The black/brown spots, however, continued. They’re all in fairly mature leaves all at the top of the plant. I have cut them off, but I’m wondering what they are and if I need to do something else to prevent spread. What began as two, quickly became ten in a day (I wondered if it was because I sprayed? but then there should have been more). The original two started to curl (I think you can see one). I do not think this is sooty mold. It doesn’t look or feel different, and it’s quite shiny. It’s like a part of the leaf changes colour from within. A couple more today, same pattern – leaves at the very top, starting small (I’ll see if it grows). You can see in the picture an example of the few with the spots (smaller spots are more recent), versus the rest (I had just sprayed it so the small dots are liquid I believe). The yellowing leaves towards the bottome/inside are also increasing.
Any thoughts on what these issues are and how to treat? I’m hoping I can care for it towards health, knowing it wasn’t in shape when I got it.
Thank you for contacting the Toronto Master Gardeners. You have done a lot of research and worked through many possible causes for the brown spots on your gardenia leaves. Thank you for the good picture and detailed descriptions. I did think about sooty mould based on your picture. It is unsightly but won’t ultimately harm the plant. You just need to continue to look for insects that might be producing the honey dew that the mould grows on.
Another possibility is leaf spot disease. This can be either fungal (caused by Cercospora or Phyllosticta) or bacterial (caused by Pseudomonas or Xanthomonas) in origin. Small spots can coalesce and become larger blotches. They may be surrounded by a yellow ‘halo’. Both can cause leaves to drop. Some of the leaves in your photo look a bit like that, but it doesn’t sound like leaves have dropped off. Bacterial leaf spot can be helped by avoiding overhead watering and ensuring good air circulation. Fungal leaf spot can be treated with a fungicide spray on the leaves. In either case, you would remove the diseased plant material as you have done. Fungicide is a last resort and it’s best to know what you’re dealing with for certain. I suggest you contact the garden centre where you purchased the plant. It sounds as if the other gardenias for sale were also diseased and they may be able to give you a definitive diagnosis – if not a refund!
These previous Master Gardener posts may be of interest Gardenia care and Gardenia disease These sites regarding gardenia pests and disease and leaf spot disease may also be helpful Pests and Diseases; Leaf spot.
June 9, 2021