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Gymnosporangium Rusts

Ravageurs et maladies
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Gymnosporangium Rust.
Photo: Jardin botanique de Montréal

Onglets

Description

Summary

The fungi in the Gymnosporangium genus have a complex life cycle that requires them to infect two different hosts. They spend part of their lives on a conifer before moving to a deciduous tree in the Rosaceae family. Infected conifers develop galls and fluorescent orange, jelly-like fructifications. Infected Rosaceae become covered with yellow to orange spots and swellings on the leaves, fruit or twigs, depending on the species of fungus. G. juniperi-virginiae grows specifically on apple and crabapple trees. Two other rust species, G. globosum and G. clavipes, may contaminate various Rosaceae in a similar fashion.

Signs and symptoms

On conifers:

  • In spring in the first year of an infection, small brownish-green, smooth swellings (galls) can be seen on twigs. At this stage, they look like berries.
  • Over the summer, the galls grow to 3 to 5 cm in diameter, becoming dark brown and covered in small circular depressions.
  • The following spring, during warm, rainy spells, the galls swell and split, giving way to fluorescent orange jelly-like fructifications.
  • These strange elongated horn-like structures are 10 to 30 mm long. They release large quantities of spores, which are carried by the wind to their secondary hosts.
  • Damage varies depending on the trees' maturity and the severity of the infection. Young seedlings and cuttings die quickly once infected.
  • In mature conifers, severely infected twigs dry out and show delayed growth. The damage is not usually fatal, however. 

On deciduous trees:

  • In early summer, small yellowish spots can be seen on upper leaf surfaces, or swellings on twigs, depending on the species.
  • Later, the leaf spots grow to about 1 cm, turn orangish and are sometimes ringed with red. Small amber-coloured specks (fructifications), which later turn black, form in the centre of the spots. Infected twigs cause buds to open prematurely and leaves to emerge small and deformed.
  • In late summer, clusters of whitish “fingers” can be seen on the underside of leaves, fruit and twigs. These fructifications produce large quantities of bright orange spores, which are then carried by the wind to conifers, their alternate hosts.
  • In a heavy infestation, trees appear to be covered with an orangish powder; galls form on leaf petioles; foliage may drop prematurely.
  • Damage varies depending on the trees' maturity and the severity of the infection. Once infected, young seedlings and cuttings die quickly. Young trees show delayed growth, and some may die. After several years of infection, mature trees gradually become weak and low-yielding.

Latin name (genus)

Gymnosporangium clavipes, G. globosum, G. juniperi-virginianae

Host plants

Various juniper species and some of their cultivars: Juniperus chinensis, J. communis, J. horizontalis, J. scopulorum, J. squamata and J. virginiana.

Various species in the Rosaceae family and some of their cultivars: apple, crabapple, hawthorn, mountain ash, pear, quince and serviceberry.

Development cycle

Description and life cycle

Fungi in the Gymnosporangium genus are members of the Basidiomycetes, fungi that produce basidia, small club-like structures. They have a complex life cycle that requires them to alternately infect two different hosts: a conifer and a deciduous tree in the Rosaceae family.

They reproduce both vegetatively (thread-like mycelia) and sexually (spores). Depending on their development stage, the fructifications produce different types of spores: aeciospores, teliospores and basidiospores.

The complete life cycle of cedar-apple rust (G. juniperi-virginianae) takes two years and requires two host trees: a juniper, where the fungus spends 18 to 20 months, and an apple or crabapple tree, where it spends 4 to 6 months.

The fungus overwinters in the form of mycelia inside galls on junipers. The galls form in the spring and summer of the first year of an infection.

The following spring, when conditions are favourable, the galls swell and split, giving way to orangish jelly-like fructifications. These produce spores, first teliospores and then basidiospores, over the next few weeks. The basidiospores can be carried several kilometres by the wind before landing on their alternate hosts, apple or crabapple trees.

Over the summer, the basidiospores germinate and the mycelia invade the young leaves and growing fruit. At first, yellowish spots with small dark specks mark the upper leaf surfaces. Long, pale fructifications then appear on the underside of leaves and on fruit and twigs, producing large quantities of bright orange spores (aeciospores).

In late summer, during dry, windy weather, the released spores are carried by the wind to juniper needles and twigs, where they cause new infections.

Prevention and control

Favourable conditions

The disease requires two host plants to be located near each other. Warm, damp weather encourages fructifications, and windy conditions help to spread the spores. Upright junipers are more susceptible than spreading junipers.

Identification

In early spring, inspect the foliage of junipers planted near susceptible deciduous trees. In summer, inspect the foliage of susceptible trees planted near junipers.

Prevention

  • Avoid planting junipers near deciduous trees in the Rosaceae family, which serve as alternate hosts for the fungus.
  • It is best to space two susceptible species at least several hundred metres apart, unless there is a windbreak or building between them.
  • Buy healthy trees that are free of galls and fructifications.
  • Choose less-susceptible species and cultivars, including:
    • Juniper: Juniperus chinensis, J. chinensis cvs. : 'Fortunei', 'Hetzii', 'Japonica', 'Leeana', 'Mas', 'Plumosa', 'Pyramidalis', etc.
      Juniperus chinensis var. sargentii, J. chinensis var. procumbens, J. communis cvs. : 'Aurea', 'Depressa', 'Suecica', etc.
      J. conferta, J. horizontalis cvs. : 'Admirabilis', 'Adpressa', 'Argenteus', 'Douglassii', 'Filicina', 'Glomerata', 'Livida', 'Plumosa', etc.
      J. rigida, J. sabina et cvs. : 'Broadmoort', 'Fastigiata', 'Hill', etc.
      J. virginiana cvs. : 'Aurea', 'Globosa', 'Pyramidalis, 'Tripartita', 'Venusta', etc.
    • Hawthorn: Crataegus crus-galli, C. intricata, C. pruinosa.
    • Crabapple: Malus cvs : 'Dolgo', 'Lollipop', 'Red Jewel', 'Coralburst', 'Morning Princess', 'Pink Spires', 'Cardinal', 'Prairie Fire', etc.
    • Apple: Malus cvs : 'Liberty', 'Nova Easygro', 'Novamac', 'Priscilla', 'Redfree', 'William's Pride', etc.
  • Keep trees healthy by pruning them properly, fertilizing them adequately and watering them during dry spells.
  • Water in the morning instead of the evening and avoid wetting the foliage; do not handle infected plants when their foliage is wet.

Physical control

  • In winter or early spring, remove and dispose of juniper branches bearing galls or fructifications. Disinfect pruning tools with a 70% isopropyl (rubbing) alcohol solution.
  • In summer and fall, rake up and destroy infected leaves and fruit as they drop.
  • Cut down and destroy severely infected trees; never compost infected plant litter.
  • In the event of a serious, repeated infection, remove one of the two species to prevent the fungus from completing its life cycle.

Biological control

None available.

Chemical control

The Montréal Botanical Garden does not recommend the use of pesticides to control this disease.

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