Pear rust

European pear rust is a fungal disease of pears, causing bright orange spots on the leaves. It also affects junipers, causing perennial canker-like swellings on the branches.

European pear rust

Quick facts

Common name European pear rust
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.

Symptoms

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 damaging perennial cankers (sunken lesions in the bark) on the stems
  • On junipers Perennial swellings on stems and branches, producing orange, horn-like outgrowths in spring following periods of high humidity

Control

Non-chemical control

  • 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 trees may cause more harm than good
  • Prune out any stem cankers on pears

Chemical control

No fungicides are currently being produced for use by home gardeners on trees from which the fruit will be consumed. The fungicides tebuconazole (Bayer Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Bayer Fungus Fighter Plus), and triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) 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.

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Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Links

Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: storing and disposing safely

Biology

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, horn-like 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 as well.

Survey

RHS Gardening Advice has recorded a steady increase in cases of this disease over the past ten years. Even though the fungus causes striking red spots and galls, our knowledge of the known distribution of Gymnosporangium sabinae on Pyrus within the UK is limited.

You can SUBMIT A RECORD of pear rust through our online survey to help monitor this disease.

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