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
  • Promptly remove and destroy infected pear leaves as soon as they are noticed, to reduce spore production
  • Prune out any stem cankers on pears

Chemical control

The fungicide difenoconazole (Westland Plant Rescue Fungus Control concentrate) is labelled to control pear rust.

NB: This product was withdrawn in 2014: remaining stocks must be used or disposed of before the 30th June 2015.

Fungicides containing myclobutanil (Bayer Garden Systhane Fungus Fighter concentrate*) can be used on pear to control scab and may have an incidental affect.

*NB: The following products are being withdrawn:

Myclobutanil (Bayer Garden Systhane Fungus Fighter concentrate and Bayer Garden Fungus Fighter Disease Control). These products cannot be sold after the 30th November 2015 and remaining stocks must be used or disposed of before the 30th November 2016.

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

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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