Regrettably, I can no longer recommend Aster (syn. Symphyotrichum) novae-angliae ‘Purple Dome’ due to its susceptibility to Alternaria leaf spot. Alternaria is a fungal leaf spot disease that can affect trees, as well as perennials. Although often not serious on trees, it can be particularly damaging on perennials because the fungus is saprophytic—meaning it perpetuates itself on dead plant tissue and in the soil.
The damage begins closest to the ground and works its way up the stem. The lowest leaves turn brown and feel crispy, while leaves just above turn yellow and spotted, as shown in the photo at left. Eventually the yellow, spotted leaves will also turn brown and die.
The fungus is spread by splashing water and is easily spread by normal rain showers. Due to the compact nature of ‘Purple Dome’s’ growth, it’s impossible to thin out the foliage to increase air flow without noticeably affecting the plant’s appearance. Additionally, the disease stunts the plant’s growth so that it never achieves its potential spread and height.
In order to control the disease, it is necessary to constantly remove and permanently dispose of dead plant tissue throughout the summer and fall. Even then, saprophytic fungal spores may still remain in the soil. Additionally, a systemic fungicide needs to be applied in the spring, as leaves first emerge, and then regularly throughout the summer. Although I don’t have a problem with using fungicides to bring a disease under control, any plant that requires constant chemical sprays throughout its growing season is not a superior selection. If you choose to grow ‘Purple Dome’, be aware of the time and cost involved to maintain the plant’s health and appearance.