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We have a 10 year old Robinia Frisia which came out with brown spotted leaves (normal size) this year. The lower part of the crown is worse, and has hardly grown this year, the upper part has grown branches and is also affected, but not as badly. We suspect this is a fungal infection "Phloeospora robiniae". A good description is given on The Telegraph newspaper website on 04/09/2008, and of course on the RHS website. For a very good picture, type the disease name into Google and scan down to see "images for...".
What to do?
Would a pesticide treatment help? If so, which product(s) and when?
Other than that all we could find is suggestion to prune the crown to shape, and thin it to allow drying airflow, and to clear up debris.
As far as I know there is no treatment available and the tree will eventually die so the question is, do you take it out now or after watching it struggle for a few more years till it's dead?
I have one too that seems to have caught th ebug and am debating removing it or growing a rambling rose up it. Decisions decisions.
We've had the same problem with our robinia. It's only a few years old, but the last couple of summers it has looked awful. Not sure whether to prune it or cut it down altogether. Such a shame as it looked lovely when it was flourishing. Would you recommend we get rid of it?? It's sited next to our compost bin, so thought that might have something to do with it .....
Like I said, there's no known treatment and no certain cause either. Various factors could be at play as stated here - http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=401 and research is ongoing.
If yours is badly affected, get rid. Mine has had its first bad year so I will give it one more year and then probably get rid if it continues as it's certainly not attractive this year. I might replace it wit a gleditsia which has golden foliage without the problems or a gingko. The rambling rose I was pondering can go up a purpose built support with a clematis.
Could you put the new tree (eg gleditisa) in the same place as the Robinia? Or does this risk the new tree?
Would you remove the entire tree including stump out of the ground, or saw it level to the ground?
Friends of mine have already replaced two sick robinias with gleditsias. They removed the entire tree and as much of the roots as they could and then prepared the planting hole as you would for any new tree - dig it deeper and wider than the new root ball and add plenty of good garden compost and some root aid such as bonemeal and/or microrhizal fungi to the soil usued to back fill. Plant it at the same level it was in the pot, stake if large enough to need it and water well and keep it watered till established.
1. Would you treat the new hole with any kind of pesticide, to protect the new tree?
2. Are gleditsias large trees? Do different specimens come as different sizes? Could it be easily pruned to conatin it?
We would like to have a small to medium tree, so long as it can be contained to 10 metres height maximum. What would be the options ?
I don't think healthy plants need extra help aprt from maybe some Rootgrow at planting time. This is a newish product which supposedly helps roots form sybiotic relationships with bacteria in the soil and this leads to improved root function and thus a healthier plant.
Look here for all you need to know about Gleditsia. Sunburst seems to be the one for your requirements - http://apps.rhs.org.uk/plantselector/plant?plantid=884
The last two weeks here have been quite dry and my robinia is looking much better with fuller leaves so I will give it one more year and see how it goes but it's for the chop if we get another bad winter and another bad summer and it carries on looking tatty and sick.
Thanks for your suggestions, I'll look into Gleditsia.
I did email the RHS to see if they wanted any samples and they kindly replied to say that they have enough for analysis. I guess this means their research has pretty much come to a halt without anything more conclusive than just letting whatever fungus it might be take its toll.
No, I think they have enough samples already sent in and from their own plants to continue research. Other than that we have to hope for a drier summer next year. Two cool, wet summers in a row are clearly playing havoc with many plants, not just robinia.