Garden Pests and Diseases: what is wrong with my fern leaf buckthorn?, 2 by sweitzmath

sweitzmath wrote:
Hello all, I have a fernleaf buckthorn (Rhamnus frangula "fine line") that I planted in the Fall of 2010. It did beautifully last season and seemed to be doing well this season, but entire branches worth of leaves have been dying off during the last few weeks. Please take a look at the attached photos.

I can't figure out what's bothering it. My soil is naturally clay but I've heavily amended it with compost over the years; I fed the entire bed a helping of compost in April. Nothing else in the bed is showing die-off. I am battling a powdery mildew on a few other plants in the bed (using an organic biofungicide), but there is no sign of mildew on the buckthorn. It's in a part-sun to part-shade spot, which is supposed to be good for it. We did have a period of unseasonable heat, but the die-off has continued since the heat ended.

I don't know where else to go from here; everything that I find on cultivating Rhamnus frangula says that it is a pretty tough plant. I would hate to lose it because it's the perfect, just perfect, size shape and texture for the spot it's in.

All ideas welcome! Thank you in advance; I am a beginner (maybe advanced beginner now, after a few years) and need the help.

Lisa