University of Maryland Extension

Rust Diseases - Trees

Back to Common Problems - Apples

rust colored gall on cedar
 Cedar-apple rust gall

Common Rust Diseases

Cedar-apple rust, quince rust, and hawthorn rust are caused by fungi in the genus Gymnosporangium. These rust fungi require two types of plants, a pomaceous plant (apples, pears, serviceberry/shadbush, quince, and crabapples), and a cedar in which to complete their life cycle. Juniperus virginiana, the Eastern red cedar, is the most common rust-susceptible cedar in Maryland. Conspicuous symptoms are produced on both apples and cedars. Fungal spores produced on one type of host plant are carried by wind to infect the other.

These rusts damage many popular ornamental plants. Colorful leaf spots and twig and fruit malformations are produced on pomaceous plants. Peculiar round leaf galls, twig galls, cankers, and twig dieback are produced on cedar hosts.

cedar apple rust lesion on the back of leaf
Cedar apple rust on apple

Rust infected hawthorn twig
Hawthorn rust infected twig

orange cedar apple rust gall
Close-up of Cedar-apple rust gall on juniper

 orange gall on juniper
Cedar-apple rust gall on juniper

Highly susceptible, heavily infected trees may suffer leaf loss and be defoliated by midsummer. Leaf spots are conspicuous and detract from ornamental value. The loss of leaf area due to spotting and defoliation reduces the vigor of trees, decreases yields, and makes the plants more susceptible to winter injury and other diseases.

Most infected fruit drops in June, whereas the remainder may be misshapen, cracked, and subject to secondary fruit rots. Quince-rust galls on twigs may result in branch dieback and distorted growth. In moist spring weather, gelatinous masses of reddish-orange fungal spores are produced on infected twigs.

On cedar hosts, galled leaves and twigs may die back resulting in distorted growth. Perennial rust cankers may cause witches’ brooms and large swollen cankers.

Management strategies

  1. Separate alternate hosts. Do not plant susceptible apples near susceptible junipers: a distance of 1 to 2 miles greatly reduces infection. (This is obviously not practical in areas where Eastern Red cedar is common.)
  2. Hand pick and destroy cedar galls by April 1, before the spore-producing tendrils are formed. Cedar galls are most conspicuous and easy to see in wet weather when the orange spore tendrils are extruded. After the orange tendrils are produced, it is too late to prevent spore dispersal.
  3. Select resistant varieties of pomaceous plants.

 Additional Resource

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