Hawthorns are attractive shrubs or small trees with dark green foliage and pretty white flowers. Some hawthorn varieties are susceptible to diseases, called rust, caused by three types of Gymnosporangium fungus. Rust often causes brown spots to appear on hawthorn leaves or attacks the fruit of the plant. Treatments are available to combat rust on hawthorns, but you need to know which fungus to fight.
Caused by the fungus Gymnosporangium globosum, this disease affects the leaves of the hawthorn plant, causing yellow spots that turn orange and develop black centres. Hawthorns are infected when fungal spores from infected cedar or juniper trees land on the hawthorn and set up housekeeping. As the disease progresses, small white tubes develop on the underside of the leaves. These tubes, called aecia, produce more spores. Cedar-hawthorn rust can make the leaves fall off, but generally does not affect the fruit, flowers or branches of the plant. Keep hawthorns as far away from cedars and junipers as possible.
The most damaging rust disease is cedar-quince rust (Gymnosporangium clavipes), which attacks the hawthorn fruit and may cause cankers on thorns and new growth twigs. This fungus also causes aecia tubes to form, but on the fruit instead of the leaves. The fruit eventually turns orange as the rust spores completely cover it. Do not plant hawthorns near cedars.
Cedar-apple rust (Gymnosporangium juniperi-virginianae) attacks hawthorns less frequently than other types. This rust attacks the leaves and may also attack the fruit and twigs. Healthy hawthorns usually withstand cedar-apple rust attacks easily. Hawthorns already suffering from stress such as trunk damage, drought or other pests may need treatment to eliminate the rust. Cedar-apple rust is generally prevalent only in the eastern United States. Keep hawthorns away from cedars and apple trees.
Always make sure the hawthorn trees you buy are rust-resistant varieties. Make sure cedars, junipers and cultivated crabapples and apple trees are at least two miles away. If these trees are already in your landscape, check the trees for galls and cankers, remove any you find and destroy them. Galls look like tumours and produce orange "fingers" that produce spores.
Use a fungicide just before the hawthorn flowers. Make sure the fungicide label states it treats the type of rust infecting your hawthorns and follow the manufacturer's instructions for applying the product. You may need to repeat the application, but do not apply more than the label indicates. Drench the foliage with the fungicide to ensure total coverage. Instead of using a fungicide, if the damage is not too widespread, you may opt to prune way as much of the diseased leaves, twigs or fruit as possible. A healthy plant should recover on its own. Prevention is the best treatment. In addition to keeping the hawthorn away from plants that spread disease, make sure the hawthorn is getting enough air circulation, sunlight, water and fertiliser to stay in peak condition. You may have to replace the infected hawthorns with disease-resistant varieties.