Pear rust

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

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.

Download

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

The RHS Advisory Service 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. In collaboration with TreeWatch, the RHS is undertaking a survey to monitor pear (Pyrus) trees for the presence of this pathogen and we are inviting members of the public to help by sending their reports. Visit the TreeWatch survey.