Friday, October 14

Gymnosporangium sabinae, aka Pear Tree Rust.

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).
 I suppose the spores will wing their way back next year.  If I knew when they were coming maybe I could cover the trees to thwart them.

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.



19 comments:

  1. Your pictures are better than mine! I wonder how the spores know where to go? I guess their journeys are all random - dictated by the direction of the wind.

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    1. I guess it's like blight spores, Mark just a matter of chance that some fall onto a suitable host plant. The atmosphere must just be a fungus soup mix. Such a pity they tend to find our plants.

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  2. Very lucky that this disease does not kill the trees - so many fruit tree diseases, including a couple that have unfortunately found their way onto a couple of my fruits trees, are a slow call to death.

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    1. Every year there just seems to be more and more pests and diseases turning up, Margaret. Soon we will have to grow everything under cover.

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  3. Urgh, that looks horrible. I don't have any pear trees but my next door neighbour does, they say they've had a good year for fruit when they dropped a bag off for me this afternoon along with a bag of apples. Looks like we've got dessert covered for a while. I shall be looking on your website shortly for information about winter wash, I hope I haven't missed the boat as my plum tree looked awful this year with its curled leaves.

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    1. It has been a good year for apples and pears< Jo. You certainly haven't missed the boat for winter wash. You must only use it when the trees are leafless. Two sprays, the first December/January and another before there is any sign of growth - no buds etc. We back this up with lures.

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  4. In one photo it looks like extra appendages are growing and the other hairy warts!! It's pretty gruesome and such a shame you can't have any control over it.

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    1. I think that is the stage when the pimples burst and the spores are released, Tanya

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  5. Oh,Sue, it looks terrible, what a fungus! I have two junipers in my garden but I've never seen any galls on juniper twigs and branches. It's pity you can't do something against.

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    1. Do any pear trees grow in your area, Nadezda?

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  6. Goodness me, what a creepy looking fungus, I haven't seen anything like that before!xxx

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  7. They look so scary! Hope it didn't make much damage to your plant!

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    1. Fortunately the trees don't appear to suffer, Mlar

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  8. Just came back to re-read this post. I knew I'd seen the fungus somewhere, and this morning I realised it was in the garden on one of my own pear trees! (But possibly at the allotment as well). I shall fill in the details on the survey form. Interesting about the juniper involvement. No doubt there are many close enough to host the fungus over the winter.

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    1. It seems to be becoming fairly common, CJ.

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  9. We've had pear rust for the first time this year and not a single pear!! All my trees are severely affected - including 3 young trees which were freshly planted! I'm hoping that the young trees survive and we get some pears next year!!

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    1. Let's hope that the claim that it doesn't affect the tree too badly are true, Gill.

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    2. Incidentally our trees have been affected for the last three years and it doesn't appear to have affected the yield.

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