European pear rust is a fungal disease of pear trees, causing bright orange spots on the leaves. It also affects junipers, causing perennial canker-like swellings on the branches.
Common name Gymnosporangium sabinae
Plants affected Pyrus spp. (pears) and Juniperus spp. (junipers)
Main symptoms Bright orange spots on pear leaves
Caused by Fungus
Timing Leaf spots on pear during summer and early autumn, cankers on juniper all year round producing orange gelatinous mass of spores in spring
What is pear rust?
Pear rust is a disease caused by the rust fungus Gymnosporangium sabinae, which causes bright orange spots on the upper surfaces of pear leaves in summer and early autumn.
This fungus attacks both pears and junipers. In fact it needs both plants in order to complete its life cycle.
You may see the following:
- On pears Bright orange spots on the upper leaf surface. As summer progresses brown, gall-like outgrowths develop on the corresponding lower leaf surface. Fruit may be affected, but this is much less common. In mainland Europe the fungus can also cause perennial cankers (lesions in the bark) on the branches, which can be damaging to young trees in particular. So far, however, these have not been common in the UK
- On junipers Perennial swellings on stems and branches, producing orange, jelly-like outgrowths in spring following periods of high humidity
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
- Careful pruning of junipers to remove rust infections from the stems, or simply removing whole plants from the vicinity of pear trees will reduce the likelihood of infection, but note that the spores are airborne over quite long distances
- Removal of leaves on heavily infected pear trees may do more harm than good
- Prune out any branch cankers seen on pear trees
No fungicides are currently being produced for use by home gardeners on trees from which the fruit will be consumed. The fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are approved for the control of rust diseases on ornamental plants, and could be used on ornamental pear species from which any fruit will not be consumed.
Inclusion of a fungicide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Gymnosporangium sabinae was once almost confined to mainland Europe and very rarely recorded in the UK, but in recent years has become much more common, for reasons that are not understood. Severe infections may be capable of causing reductions in yield.
The fungus causing pear rust is, like all rusts, a biotroph: it feeds on the living cells of the host plant over an extended period without killing it. It is not able to survive on dead plant material, so must either alternate with a different, perennial host, or produce resting spores to pass the dormant season. Pear rust alternates between pears and junipers.
On pears, the brown fungal growths produced on the underside of infected leaves release spores which cannot reinfect pear, but instead are wind-dispersed and infect several juniper species, causing perennial stem infections. In spring these produce orange, gelatinous outgrowths, which in turn produce wind-blown spores that reinfect pears.
Having a perennial host, like juniper, enables the fungus to survive those periods when the alternate host is absent (if it is an annual) or dormant and leafless (as with pears), although this particular rust can also form perennial cankers on the bark of pear (these are uncommon in the UK).
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