Hawthorn rust is caused by a nasty little fungus called Gymnosporangium globosum. It has an interesting life cycle, according to Iowa State University, that requires a juniper host and a second host, which can be apple, pear, ash or hawthorn. Spores develop on juniper hosts first and are blown on air currents to other trees. Not to be confused with other hawthorn problems such as leaf blight, hawthorn rust can be controlled with several applications of fungicide.
Observe hawthorn leaves and twigs in early summer. Look for yellowish-red spots, which will produce spores. Unfortunately, there's not much you'll be able to do about it until next spring.
Treat spots with fungicide in early spring, when the leaves and flowers begin to emerge. Fungicides containing sulphur, myclobutanil or copper soap help control hawthorn rust. Read the label carefully before use, as each type of fungicide has specific instructions for mixing and intervals for reapplication. Fungicides for hawthorn rust can be applied several times per growing season, according to Iowa State University.
Clip off any hawthorn branches that are severely infected and wilted. Dispose of them in plastic bags immediately to prevent spores from getting into the air.
Scout the junipers on your property, and, with permission, your neighbour's. Hawthorn rust begins on junipers. The spore-producing structures can be up to a few inches in diameter and have bright-orange tendrils that can be easily seen in the rainy weeks of spring. Clip these off and dispose of them in plastic bags to interrupt the life cycle of hawthorn rust.
There are rust-resistant varieties of hawthorn available at nurseries and garden centres. Investigate planting this type of hawthorn if the fungus problem is severe.
Hawthorn rust can be difficult to eradicate because spores can travel several miles on air currents.