This yellow-leaved tree is widely grown in gardens and other amenity situations on different soil types and in a range of aspects. It is propagated by budding and grafting the ‘Frisia’ cultivar on to seedling rootstocks of the species, R. pseudoacacia.
Problems
In the last two years, there have been many reports of the tree failing to come fully into leaf in spring but, there are shoots arising from the trunk and boughs. The leaves tend to loose their leaflets but the leaf rib remains on the tree. The tree commonly fails to recover fully and ultimately dies.
Cause
It is unclear what causes these problems. The symptoms are consistent with a wilt fungus blocking the water-carrying vessels in the trunk. This would suggest that the pathogen can cross the graft barrier, which would be unusual. The green-leaved species has not been reported as affected.
The RHS plant pathologists have found no evidence of this potential pathogen so far, but have found many instances of fungal leaf-spot disease that can defoliate and weaken trees. This is most likely to be most prevalent in wet weather. Some instances of honey fungus and wood-rotting fungi have also been found. Neither the leaf spot or the other diseases are likely to fully account for the current damage seen in Robinia pseudoacacia ‘Frisia’.
Control
There is no control yet for this problem.
RHS Members can help RHS plant pathologists look into this matter further by sending in samples of ailing Robinia. The samples required are:
- Full details of soil, site, cultivation regime, history of the plant and the problem
- Samples of the dying branches) cut back to healthy tissue to see if a fungus is inside the connective vessels
- Sample of leaves attached to the stem showing typical symptoms
- Details about the health of the root system: Dead and live roots can be differentiated as the latter are creamy-white and juicy inside, are often bendy and will have small, hair-like roots coming off their length, while dead roots, are brown to black both inside and out, are often brittle and are missing the smaller side-roots. If possible, three or four samples of dead roots should be extracted from different points around the base of the tree. The roots should be of approximately pencil thickness, have some associated soil included (a cupful is sufficient) and be sealed in a labelled polythene bag.
- Any rotting material found beneath the bark at the base of the tree
- Send the samples, full details and your membership number to ‘Robinia Problem’, Advisory Services, RHS GardenWisley, Woking, Surrey GU23 6QB.
- Photographs of the tree and any affected areas – digital ones are ideal (maximum file size 250KB) – can be emailed with your membership number to gardeningadvice@rhs.org.uk and please put ‘Robinia Problem’ in the subject line.