The good news is that Mahonia shrubs are generaly pest free so aphids are not a problem if you do find any greenfly or other ashids or bugs trying to make your mahonia its home then they can usualy be treated with a pesticides there are available from most garden centres and nurseries. Pesticides usualy need to be sprayed onto the effected areas and then again two weeks later but allways read the lable for specific instructions when applying pesticides.
Unfortunatly Mahonias can suffer from Rust and Powdery Mildew, Mahonia’s effected by rust will have black spots on there leaves with yellow around the edges, if you have seen Rust on Roses then you will
now what to look for. The good news is this can also be reasly treated which a garden fungicide such as copper funicide or rose clear. Mahonias also suffer from Powdery Mildew which is also easy to spot as the leaves start to get covered with a grey/white powdery substance, this is also treatable by using garden fungicides.