Unfortunately, I have been unsuccessful in trying to upload photos, so I hope my description will suffice. The leaves start with small brown spots, then the leaf turns yellow then gradually rusty brown, then the whole stem dies. I have a very extensive garden but only a few plant species are affected. They are not planted close to each other and I haven't seen any insects on the plants. They include - quince, hellebores, tomatoes, and an oak-leaf mountain ash tree.