Pear rust is a disease caused by the 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.

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 pear trees may do more harm than good
• Prune out any branch cankers seen on pear trees

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

No fungicides with activity against rust are currently being produced for use by home gardeners on pear trees from which the fruit will be consumed.

Download

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

Link

Chemicals: storing and disposing safely

Gymnosporangium sabinae was once almost confined to mainland Europe and very rarely recorded in the UK, but in the last twenty years or so it has become much more common in the UK, 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 from 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).