A few years ago our pear trees started to develop orange spots on the leaves. Investigation revealed that this was Gymnosporangium sabinae, - a mouthful isn’t it? It's just as well that it has an easier name - Pear tree Rust.
This is a rust fungus that needs both pear and juniper trees to complete its lifecycle. This left me puzzled as to my knowledge there are no juniper trees in the vicinity. How far are the spores able to travel to find a suitable host? I read on one website that you shouldn’t grow a pear tree less than 1km (slightly over half a mile) from a juniper and that spores could actually travel 6km (nearly 4 miles). Hardly something over which the gardener has any control and so I guess this is something we just have to live with.
Fortunately, although the fungus may affect yield it will not kill the tree. The fungus can only exist on a living host and so to murder its host would be counter-productive.
The first sign of rust on a pear tree is the appearance of orange patches on the upper-side of the leaves.
The orange patch darkens in the centre as a fruiting part starts to develop on the underside of the leaf. These growths develop into something that looks like a blister from which pimples appear. Spores are dispersed from these pimples which are then blown on the wind and journey away from the pear tree in a quest to find a juniper tree on which to spend winter.
I don’t know how it appears on the juniper but from what I read it overwinters on juniper twigs and branches as galls which release spores in spring that are on the look out for suitable pear tree hosts. I wonder if they will remember the location of our plot?
The spores produced on the pear tree cannot reinfect the pear but I did try to remove as many affected leaves as I could before the pimples burst in an attempt to cut down the number of spores being produced but I guess in the grand scheme of things this was a totally fruitless exercise. (Excuse the pun).
Incidentally the RHS are carrying out a survey of the spread of this fungus and are inviting people to submit a record of any incidences they see.. The survey can be accessed here.