What Causes Aster Leaf Spots?
Leaf spots on aster plants can be caused by one or more of several fungal species. These include species of the Alternaria, Ascochyta, Cercospora, and Septoria families. The fungi overwinter in plant matter on the ground and in the soil. Infection is promoted by wet conditions, especially on the leaves. Another type of fungus, Coleosporium spp., causes a similar but distinct disease on asters known as rust.
Symptoms of Leaf Spot
Asters with leaf spot will begin to develop spots mostly on leaves, although the stems and flowers of aster plants may also be impacted. You should see spots first develop on the older, lower leaves of plants. The spots progress upward to higher and younger leaves. Leaves on affected plants will also turn yellow and ultimately die. Fungi that cause rust form red or orange spores on the undersides of leaves. These look like spots and turn dark red as they develop. A severe infection will cause leaves to yellow and die back.
Managing Leaf Spot on Asters
Asters can carry the fungi that cause leaf spot in their seeds. Make sure you get certified, disease-free seeds and transplants when growing asters. Avoid overwatering plants or allowing water to collect in the soil. Also avoid watering by overhead sprinkler. Keep beds clean by picking up spent plant matter regularly and especially at the end of the season. Leaf spot on existing asters can be treated with a fungicide. You can also use a fungicide to protect healthy plants from the spread of leaf spot diseases. Plan to spray plants before it rains. Your local nursery or extension office can help you choose the right product. Note: Any recommendations pertaining to the use of chemicals are for informational purposes only. Chemical control should only be used as a last resort, as organic approaches are safer and more environmentally friendly.
