Rust is the common name for an entire group of diseases, so named because their most conspicuous symptom is the breakout of visible, rust-coloured mould growth on different parts of the tree anatomy. Many rust diseases are named after the trees that they most frequently infect (cedar-apple rust, cedar-hawthorn rust, cedar-quince rust, etc.). Careful maintenance and treatment can save your trees from the harmful effects of rust diseases.
The earliest symptoms of infection from a rust disease are the titular rust-coloured spots on hawthorn leaves. As the disease progresses, tiny cuplike structures break out on the underside of leaves; these are the fruiting bodies of the fungus (Gymnosporangium globosum), called aecia. Serious infections can result in leaf wilting and premature defoliation.
Preventive and Natural Control Methods
Due to the prevalence of rust diseases, many disease-resistant cultivars of affected trees have been developed. Eliminate any apples, crabapples, cedars, junipers or cedars that have been infected with a rust disease as spores released from these trees can infect your hawthorn trees if they are planted nearby. If you do not want to remove the entire tree, at the very least clean up and discard any defoliated leaves as well as any globular, protruding orange rust galls that contain spores of the fungus.
Fungicides can also be used to treat a hawthorn infected with a rust disease. Often the most effective fungicide option is to apply a preventive or systemic fungicide early in the hawthorn's growth cycle when new buds are still pink. Fungicides containing chlorothalonil, mancozeb or sulphur are commonly marketed for control of rust diseases. Use only approved fungicides for rust disease management and never deviate from the manufacturer's recommended application instructions.
There are several different strains of rust diseases, but the one that most commonly affects hawthorn trees is called cedar-hawthorn rust because the fungus requires two host plants to complete its life cycle. In other words, Gymnosporangium globosum will not cause a rust disease unless there is a member of the rose family (like hawthorn) growing next to a juniper plant. This fact highlights the importance of carefully planning the tree species you plant in order to avoid rust diseases.