Perennial Aster Disease


I have perennial aster and I think it has a disease. It is covered with a thin layer of white powder. It has this disease last year and I see it again in some of them. I have many of them. Is this a disease? If yes, how do I cure it? Thank you so much.


Unfortunately, what you describe would appear to be powdery mildew. Asters are particularly susceptible to this common fungal disease. A wide number of garden plants develop this disease to the extent that it is possible to buy plants that are mildew resistant. The best way to control this disease is to buy resistant varieties of aster. When you next visit your local nursery check to see if you can purchase mildew resistant perennial asters and replace or supplement your old diseased ones.

It’s a tough disease to eradicate because most powdery mildew fungi thrive in hot and humid conditions as well as hot, dry weather. Fortunately powdery mildew is only a cosmetic problem.  It makes the plant look unsightly but it will not completely kill it.

Ontario gardeners must abide by the Cosmetic Pesticides Ban Act which means we cannot purchase products to eradicate diseases like powdery mildew. Here are some tips in order to minimize this problem amongst your current plants. First you need to increase the space around them to create better air flow. Dense, shady or damp areas in the garden favour the development of this disease. Place plants where they will receive early morning sun and will dry out more quickly. Powdery mildew creates spores that spread through wind, insects and water runoff, which carries the disease to other plants. Water around the base of the plant and try not to get water on the leafy parts. In bad cases the leaves may turn yellow and drop off. Pick up and destroy as soon as possible.  It’s very important to clean up any plant debris around the plant in the fall. If you leave it on the ground it will harbour the fungi over winter and that will produce more spores in the next growing season.  If you have any severely infected plants remove and destroy them. Do not put any infected plant material in a compost bin.

With a bit of luck you will be able to control the spread of powdery mildew in your garden. Good luck.