another first for me – clouds of orange dust flying off the established Asters as i cut them back. they’re in full sun, watered regulary with a drip soaker hose, Leaside. There was orange residue everywhere. YUCK! have you any idea what it is? and thank-you.
Thank you for contacting Toronto Master Gardeners.
From your picture it’s not possible to make a definitive diagnosis. The orange in the photograph suggests two possible culprits. The first possibility is aphids. Aphids are small, soft-bodied insects with long slender mouth parts that they use to pierce stems, leaves, and other tender plant parts and suck out fluids. Aphids look like orange-red bumps and often feed in dense groups on leaves or stems. It is easier to deal with the problem when it is caught early. At that point they can be knocked out with a blast of hose water or pruned out. If you think that this is an aphid infestation, try blasting the plants with water every few days first. If that doesn’t work, consider applying insecticidal soap or horticultural oil. Apply insecticidal soap after sunset to avoid harming bees. If you use a pesticide to control your pest problem, read the label to make sure you are choosing the right product for the right pest. Follow all label directions and warnings carefully. Always look for a Pest Control Products (PCP) number on the label so you know the product has been approved by Health Canada.
A longer term solution for getting rid of aphids and other harmful insects for that matter is to attract beneficial insects to your garden. For example planting Alyssum saxatilis (basket of gold) will attract hover flies and lady beetles which in turn eat aphids.
The second possibility is that your asters are infected with Coleosporium rust. Growing asters in the sun and using a drip hose for irrigation are excellent cultural practices. However you also mention that these are established asters. It’s possible that the asters are crowded causing reduced airflow. This can lead to damp conditions that stimulate fungal growth.
If an infection is caught early, the infected part of the plant can be removed and a fungal spray applied. You could also thin out the plant to increase airflow. In the fall remove all plant debris. Next spring a fungicide can be applied to the aster before the infection takes hold. Alternatively you may decide that the infection has spread too far to save the plant. If this is the case, remove all of the plant and dispose of it in the garbage. Since the fungus will remain in the soil, plant new asters away from the infected site.
Good luck with your asters.