Extension > Garden > Yard and Garden > Trees and Shrubs > Cedar apple rust and other Gymnosporangium rusts
Cedar apple rust and other Gymnosporangium rusts
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- Importance
- Pathogen and susceptible plants
- Identification of disease on:
Importance
Gymnosporangium rust fungi cause unique and fascinating diseases that require two different living plant hosts in order to complete their life cycle. Although the bright red and orange leaf spots and orange gelatinous galls symptomatic of these diseases are quick to draw attention, the disease rarely causes serious damage to its hosts and often does not require management in a home landscape. A few highly susceptible plants may suffer shoot death or defoliation from leaf spots.
Pathogen and susceptible plants
There are four diseases caused by different species of Gymnosporangium fungi in Minnesota. All four Gymnosporangium rust fungi require two different hosts to complete their life cycle; one plant from the Cupressaceae family and the other from the Rosaceae family. The telial stage of the disease occurs only on species of Juniperus of the Cupressaceae family, including junipers and eastern red cedar. The aecial stage of the disease occurs on a variety of different plant genera of the Rosaceae family including apples, crabapples, Cotoneaster, hawthorn, pear, serviceberry, and mountain ash. The four Gymnosporangium rust diseases common in Minnesota have very similar life cycles and biology but infect different species within Cupressaceae and Rosaceae families (Table 1).
Table 1. Gymnosporangium rusts and the plants they infect in Minnesota
| Disease | Rust Fungi | Telial Host - Cupressaceae | Aecial Host - Rosaecae |
|---|---|---|---|
| Cedar Apple Rust | Gymnosporangium juniperi-virginianae | Most commonly affected:
Occasionally:
|
Most commonly affected:
Rare:
|
| Hawthorn Rust | Gymnosporangium globosum | Most commonly affected: Occasionally: |
Most commonly affected: Occasionally: |
| Quince Rust | Gymnosporangium clavipes | Eastern red cedar (J. virginiana) Common juniper (J. communis) Creeping juniper (J. horizontalis) Rocky mountain juniper (J. scopulorum) Savin juniper (J. sabina) |
Infects over 480 species of Rosaceae family including: Serviceberry (Amelanchier) Chokeberry (Aronia) Hawthorn (Crataegus) Apple & crabapple (Malus) Cotoneaster Pear (Pyrus) Mountain ash (Sorbus) |
| Juniper Broom Rust | Gymnosporangium nidus-avis | Creeping juniper (J. horizontalis) Eastern red cedar (J. virginiana) Rocky Mountain juniper (J. scopulorum) |
Serviceberry (Amelanchier) Apple & crabapple (Malus) Hawthorn (Crataegus) Mountain ash (Sorbus) |
| Japanese Apple Rust | Gymnosporangium yamadae | Chinese juniper (J. chinensis) Flaky juniper (J. squamata) |
Apple & crabapple (Malus) |
Identification
Note:These diseases require two different plants in order to complete their lifecycle. Symptoms are very different on each type of plant.
Rosaceae plants (apple, crabapple, hawthorn, mountain ash, serviceberry, etc.)
Cedar-apple rust
- Leaf spots are first yellow then turn bright orange-red, often with a bright red border (Figure 1).
- Small raised black dots form in the center of leaf spots on the upper leaf surface when leaf spots mature.
- Very short ( < 1/10th inch) finger-like fungal tubes stick out from the lower surface of the leaf, directly below leaf spots (Figure 2).
- The fungal tubes appear fringed when they open at the tip to release yellow to orange powdery spores.
- Green to brown irregular spots with black dots rarely form on the fruit surface. Fruit spots do not extend deep into the fruit.
Hawthorn rust
- Leaf spots are yellow to orange. Raised black dots form in the center of the spots on the upper leaf surface as they mature (Figure 3).
- White, finger-like fungal tubes extend from the underside of leaf spots protruding up to 1/8th inch long.
- When infection is severe on hawthorn, leaves turn completely yellow and fall prematurely.
- Infection of green stems occurs rarely and can result in thick, deformed growth of stems and cause shoot death when severe.
- Rarely found are orange to rust colored spore-filled blisters on the fruit surface (Figure 4).
Quince rust
- White finger-like fungal tubes develop all over fruit and stick out up to 1/8 inch. Tubes rip open at the tip to release powdery bright orange spores (Figure 5).
- Leaves are often unaffected. A few yellow spots may be found on leaves.
- Infected areas of stems bulge to become elongated and distorted.
Juniper broom rust
- Leaf spots are small yellow, slightly raised and may have a red border. Raised gummy yellow droplets form in the center of leaf spots and eventually turn into raised black dots.
- In highly susceptible hosts, leaf tissue around the leaf spot dies, which then turns brown with a red border and expands from the infection point to the leaf edges in a wedge shape (Figure 6).
- Infected leaf veins cause the leaf to curl and become distorted.
- Small yellow spots with red borders are found on green stems, leaf petioles and leaf veins.
- Finger-like tubes develop on leaf petioles, stems and occasionally develop on upper and lower sides of leaf veins.
Rosaceae plant image gallery
Select any image to open a slideshow.
Cupressaceae plants (eastern red-cedar and other junipers)
Cedar-apple rust
- In fall, round greenish-brown woody galls up to 2 inches in diameter can be found on twigs and small branches (Figure 7).
- In spring, these galls produce gelatinous, orange tentacle-like projections that are 1/2 inch to 3/4 inch long during wet weather (Figure 8).
- When dry, the orange gelatinous spore producing structures shrivel to dry reddish-brown projections on the gall. Galls can dry down and rehydrate several times in one spring.
Hawthorn rust
- Small reddish-brown woody galls that are round and irregularly shaped form along needles and stems. These galls are 1/10th inch to just over 1/2 inch, which is smaller compared to cedar apple rust galls.
- In spring, bright orange, gelatinous horns with straight, blunt fingers form that are shorter and fatter than cedar apple rust galls (Figure 9).
Quince rust
- Branches and twigs swell in size from infection. These later become elongated and bark begins to peel.
- In spring, small orange, gelatinous blobs emerge from swollen areas and cracks along infected branches (Figure 10 & Figure 11).
Juniper broom rust
- Rounded thicket of foliage can be seen any place in the tree canopy.
- These thickets are called witches' brooms, or clusters of many small branches that are formed by infection at the growing points.
- Swollen elongated areas with rough cracked bark can be seen on infected branches without witches' brooms.
- Orange jelly forms on needles and from cracks in infected bark during wet spring weather (Figure 12).
- Brooms usually die when small but a few survive and may reach 20-24 inches in diameter.
Cupressaceae plants image gallery
Select any image to open a slideshow.
Japanese Apple Rust
N. Gregory, Univ. of Delaware, Bugwood.org
Figure 13. Japanese apple rust.
Japanese apple rust is caused by Gymnosporangium yamadae, a fungus native to Asia. While this disease has been found in the Eastern US, it has never been identified in Minnesota.
N. Gregory, Univ. of Delaware, Bugwood.org
Figure 14. Japanese apple rust on leaves
Suspected cases of Japanese apple rust should be reported to the Minnesota Department of Agriculture's "Arrest the Pest" at 1-888-545-6684 or arrest.the.pest@state.mn.us
Biology
Gymnosporangium fungi over-winter in infected branches and galls on the Juniperus hosts. In spring, during wet weather, the galls produce orange gelatinous blobs or horns that release spores. Spores are carried by wind to susceptible Rosaceae plants. During dry weather, the orange gelatinous structures shrivel, dry and turn reddish-brown in color. Galls can rehydrate and dehydrate several times in one season in response to weather conditions. After one season of spore release however, galls of cedar apple rust and hawthorn rust die and fall off the tree. Cankers of quince rust and witches' brooms of juniper broom rust may survive for multiple years, releasing new spores each spring.
On Rosaceae plants, leaf spots and fruit infections are unique to each disease. These infections produce finger-like spore producing structures in late summer and early fall. Powdery yellow, orange or reddish-brown spores are released from these spore producing structures and infect young needles and shoots of susceptible Juniperus species. New galls may take up to two years to develop on the Juniperus host. Transfer of spores between the two plant types is what completes the complicated life cycle of these rusts.
Management of rust on Rosaceae plants
- For control of cedar apple rust on apple and edible crabapple cultivars, see the IPM guide for home apple growers at http://www.extension.umn.edu/distribution/horticulture/M1235-6.html
- Plant disease resistant cultivars of crabapple and apple when possible (see Table 2 and 3). Rust resistant cultivars of hawthorn are difficult to find. Often a cultivar will have resistance to one of the Gymnosporangium rusts (i.e cedar apple rust) and be highly susceptible to a different Gymnosporangium rust (i.e. hawthorn rust).
- Do not plant eastern red cedar and juniper plants within a few hundred yards of susceptible Rosaceae plants. This will reduce but will not completely eliminate disease problems.
- Prune and remove brown woody galls found on the eastern red cedar and juniper plants before orange gelatinous spore producing structures form in spring to reduce the level of infection on nearby Rosaceae plants.
- Prune and remove infected twigs or branches on Rosaceae plants when they occur.
- If applied before infection occurs, protective fungicides can be used to control the disease on Rosaceae plants. Typically, this occurs when flower buds first emerge and continues until spring weather becomes consistently warm and dry. Nearby infected junipers can be monitored and fungicides applied when gelatinous orange spore producing structures appear on galls and branches.
- Chemical treatments with listed active ingredients include*:
- Myclobutanil- most effective!
- Copper
- Sulfur
*Always completely read and follow all instructions on the fungicide label.
Table 2. List of cedar apple rust resistant Rosaceae cultivars that are hardy to Minnesota
| Common name | Cedar apple rust resistant cultivars |
|---|---|
| Apple (Malus spp.) | Fireside Nova Novamac |
| Crabapple (Malus spp.) | Adams
Indian Summer
Red Jewel |
Management of rust on Juniperus spp.
- Galls and witches' brooms on eastern red cedar or junipers do little harm to the tree or shrub and do not need to be managed.
- Galls and witches' brooms can be pruned off to improve the ornamental value of the tree or shrub.
- Quince rust cankers should be pruned out of the tree or shrub. The pruning cut should be made several inches below visible signs of the canker to insure complete removal of the fungus from the tree or shrub.
- Some resistant varieties of Juniperus are available and should be used in areas where Gymnosporangium rusts are a known problem (Table 3).
- Fungicides are not recommended to protect eastern red cedar or junipers from infection.
Table 3. List of disease-resistant Cupressaceae plants that are hardy to Minnesota
| Common name | Cedar apple rust resistant species, varieties and cultivars |
|---|---|
| Chinese juniper (Juniperus chinensis) |
Ames
Japonica* Plumosa* |
| Common juniper (J. communis) | Aurea*
Depressa* Saxatilist* |
| Creeping juniper (J. horizontalis) | Admirabilis*
Eximius* Livida* |
| Eastern red cedar (J. virginiana) | Aurea*
Grey Owl Pyramidalis* |
| Flaky or Himalayan juniper (J. squamata) | Alob-variegata* Farges' weeping juniper* (J. squamata var. fargesii) |
| Pfitzer juniper 'Sea green' (J. xpfitzerian) | Pfitzer juniper 'Sea green'* (J. xpfitzerian) Glauca* |
| Rocky Mountain juniper (J. scopulorum) | Medora Moonglow |
| Sargent juniper (J. chinensis var. sargentii) | Variegata* Wintergreen |
| Savin juniper (J. sabina) | Savin juniper* (J. sabina) Knap Hill* |
| * Plants also resistant to hawthorn rust | |
Reference: Tables adapted from Diseases of Tree and Shrubs, Sinclair and Lyon, 2005.
