We have a white chalky substance accruing on our birch tree leaves, causing the leaves to resolve into high ridges. I had been hoping to have a conversation with one of the gardeners but the volunteer failed to show at Edward’s Gardens and nobody is answering the phone line. Our questions are pretty straight forward.
Are we likely to have the correct diagnosis? Is it likely this is Hamamelistes Spinosus?
Is there a home remedy that has a high probability of working?
Do we need an arborist?
Can you recommend a reliable arborist who is not prone to charge Rosedale rates for his services?
Your diagnosis of Hamamelistes Spinosus is probably correct as one of the indicators of an infestation of these spiny witch-hazel gall aphids, are raised ridges on the leaves, along with the presence of a white granular substance. The infestation on birches is typically greatest in the spring, after which time the aphids should move to witch hazels to complete their life cycle. The damage to the leaves is not usually severe enough to warrant treatment and birches will often drop the infested leaves and grow new ones. However, there are several ways you can try to treat the infestation should it persist. There may already be beneficial insects after the aphids; check the underside of the leaves. If not you can try and obtain some such as ladybugs to be released near the birches. Overall as I mentioned, leaf damage is a cosmetic problem and trees are not severely harmed. Aphid populations can be reduced by spraying the underside of the leaves with a hard stream of water.
Additionally, an alternative is the application of an insecticidal soap – which will require repeated applications, making sure that you thoroughly cover the underside of the leaves. Also, to try and prevent your birches becoming infested another year – try and locate and remove nearby witch hazels.