Should I spray hawthorns and junipers to control rust disease?

On the positive side, rust on hawthorns is rarely associated with tree death. The article you refer to was written by my colleague who is based is the U.S. The problem we face is that there are no conventional synthetic fungicides that have approval for rust control in the UK. However, you may wish to visit your local garden centre. There may be one or two garden centre products (Fungus Fighter, Systhane) that have applicability. If so, you will need to spray every 7 to 10 days for any form of control. The other key to rust control is promotion of tree vitality by mulching, irrigation, and fertilisation. Make an appointment today for a free evaluation.