We have had a dwarf patio Pear Tree (Pyrus communis) in our garden for several years. It is not a great cropper, with only a couple of fruit each year, though it is very beautiful in spring, when it is covered with a mass of flowers (see photograph below).
This year it has provided some extra interest, with bright orange patches on the uppersides of several of the leaves, suggesting some kind of rust fungus. However, the disease wasn’t listed in my guide to plant pests and diseases (“Pests, Diseases and Disorders Of Garden Plants” by Stefan Buczacki and Keith Harris, Harper Collins 2000).
As summer continued, brown, gall-like outgrowths developed on the lower leaf surfaces below the orange patches. I was now less sure that I’d found a fungus. I took some photographs:
When I took a look on the internet and searched for “pear leaf gall” I found a piece by Harry Green on the Worcestershire Biological Records Centre website, describing a gall on Pear tree leaves (Worcestershire Record No. 21 April 2007 p. 49). The article was very helpful and included photographs of the gall on Pear, caused by the fungus Gymnosporangium sabinae.
Searching for Gymnosporangium sabinae has yielded more information on the RHS website. G. sabinae is also known as the Pear Juniper Rust because its other host is Juniper. The fungus overwinters on Juniper, where it forms a perennial infection on the stem. In spring these produce orange, horn-like outgrowths, which produce wind-blown spores. The spores then reinfect Pear trees.
The Worcestershire Record article includes a photograph of the fungus on Juniperus sabina. Wikipedia also has some good pictures on both hosts. (I have since found that my plant pests and diseases book has an entry for the related Hawthorn Rust, G. clavariiforme.)
The iSpot website also has some good photographs of the fungus. iSpot is provided by The Open University as part of the OPAL project and is an excellent way of sharing and identifying natural history observations.
Gymnosporangium sabinae is on the increase in Britain. I visited Holkham Walled Garden at the end of September and found that most of the Pear trees had the galls on their leaves. As for my Pear tree, I removed and destroyed the infected leaves to contain the infection, though I’m sure it’ll be back if there are Juniper bushes in the neighbourhood.
Update April 2017: Since we moved house, our patio Pear tree has been in a much sunnier spot and now yields around fifty fruit in most years. The rust reappears every year, but we remove infected leaves as soon as we find them and the tree remains in good health.