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Extension > Garden > Yard and Garden > Trees and Shrubs > Cedar apple rust and other Gymnosporangium rusts

Cedar apple rust and other Gymnosporangium rusts

Rebecca Koetter and Michelle Grabowski

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:
Eastern red cedar (Juniperus virginiana)
Rocky mountain juniper (J. scopulorum)

Occasionally:
Chinese Juniper (J. chinensis)
Creeping juniper (J. horizontalis)
Low juniper (J. communis var. depressa)

Most commonly affected:
Apple & crabapple (Malus)

Rare:
Hawthorn (Crataegus)

Hawthorn Rust Gymnosporangium globosum

Most commonly affected:
Eastern red cedar (J. virginiana)
Rocky mountain juniper (J. scopulorum)

Occasionally:
Chinese Juniper (J. chinensis)
Creeping juniper (J. horizontalis)
Low juniper (J. communis var. depressa)
Savin juniper (J. sabina)

Most commonly affected:
Hawthorn (Crataegus)
Apple & crabapple (Malus)

Occasionally:
Serviceberry (Amelanchier)
Quince (Cydonia)
Pear (Pyrus)

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

Hawthorn rust

Quince rust

Juniper broom rust

Rosaceae plant image gallery

Select any image to open a slideshow.

Cupressaceae plants (eastern red-cedar and other junipers)

Cedar-apple rust

Hawthorn rust

Quince rust

Juniper broom rust

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

Like other Gymnosporangium rusts, this exotic disease spends part of its life on apples or crabapples and the other part on Juniperus spp. Japanese apple rust produces round woody galls on the branches of Juniperus spp. similar to cedar apple rust. In spring, Japanese apple rust galls produce orange gelatinous projections that stick out like rubbery shelves but do not dangle like the horns produced on galls by cedar apple rust (Figure 13). On apple trees, leaf spots are bright red with a pale cream to white center. Long finger-like fungal spore producing structures emerge from the underside of leaf spots in mid to late summer and release chestnut brown powdery spores (Figure 14). Infection on apple fruit is rare.

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

*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
Freedom
Liberty

Nova
Easygro

Novamac
Redfree

Crabapple (Malus spp.)

Adams
Adirondack
Beverly
Candied Apple
Dolgo
Donald Wyman
Eleyi
Ellwangerina
Henry Kohankie

Indian Summer
Liset
Lollipop
Mt. Arbor
Narragansett
Ormiston Roy
Persicifolia
Purple Prince
Red Baron

Red Jewel
Robinson
Robusta
Royalty
Sargent cv. Tina
Snowdrift
Special Radiant
Zumi

Management of rust on Juniperus spp.

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
Blue point*
Foemina*
Fortunei*
Hetzii*
Hetzii Columnaris
Iowa

Japonica*
Keteleer
Leeana*
Maney
Mas*
Mountbatten
Perfecta

Plumosa*
Aurea*
Pyramidalis*
Robusta Green
Spartan
J. chinensis var. Procumbens*

Common juniper (J. communis)

Aurea*
Aureospica*
Cracovia*

Depressa*
Hibernica*
Oblonga Pendula*

Saxatilist*
Suecica*
Suecica Nana*

Creeping juniper (J. horizontalis)

Admirabilis*
Adpressa*
Argenteus*
Douglasii*

Eximius*
Filicina*
Glomerata*

Livida*
Petraea*
Plumosa*

Eastern red cedar (J. virginiana)

Aurea*
Berg's Rust Resistant*
Blue Mountain
Burkii*
Globosa*

Grey Owl
Hillspire
Kosteri*
Pseudocupressus*

Pyramidalis*
Skyrocket*
Tripartita*
Venusta*

Flaky or Himalayan juniper (J. squamata)

Alob-variegata*
Meyeri*
Wilsonii*

Farges' weeping juniper* (J. squamata var. fargesii)

Pfitzer juniper 'Sea green' (J. xpfitzerian)

Pfitzer juniper 'Sea green'* (J. xpfitzerian)
Compacta*

Glauca*

Rocky Mountain juniper (J. scopulorum)

Medora

Moonglow

Sargent juniper (J. chinensis var. sargentii)

Variegata*
Wateri*

Wintergreen

Savin juniper (J. sabina)

Savin juniper* (J. sabina)
Broadmoort*
Fastigiata*

Knap Hill*
Tamarix juniper* (J. sabina var. tamariscifolia)

* Plants also resistant to hawthorn rust

Reference: Tables adapted from Diseases of Tree and Shrubs, Sinclair and Lyon, 2005.

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