Robinia pseudoacacia 'Frisia' is a popular yellow-leaved tree, widely grown in many gardens and public spaces. However, in recent years, this particular cultivar, 'Frisia', has suffered from a range of problems, causing the ultimate loss of the tree.
What are the problems?
In the last several years, there have been many reports to the RHS Advisory Service of 'Frisia' failing to come fully into leaf in spring. Some reports suggest that the leaves tend to lose their leaflets but the leaf rib remains on the tree. The tree commonly fails to recover and ultimately dies.
The green-leaved species has not been reported as affected.
The tree fails to come fully into leaf in spring and drops foliage prematurely. Brown spots may sometimes be present on the leaves. Shoots often arise from the trunk and boughs but, in the end, the tree does not recover.
Although it is unclear what causes these problems, there are several possibilities and problems that have been discounted. Samples sent to the Advisory Service by RHS members show a number of problems:
- There has been an increase in a leaf-spot fungus (Phloeospora robiniae), which is more prevalent in wet weather conditions, causing defoliation and weakening the trees
- There have also been cases of two root diseases, honey fungus (Armillaria) and Phytophthora root rot, both of which can cause tree death
- Bacterial rots have also been recorded, resulting in the bark becoming soft and smelly
- The unusually wet weather of the last two years could well be a contributing factor, stressing the trees and making them more susceptible to disease (Robinia tend to perform best on well-drained soils)
However, leaf-spot fungus (Phloeospora robiniae) and root rot diseases are unable to fully account for all the reports of dead robinias. It is possible that interactions between several factors could be involved in robinia decline. Investigations are still ongoing into this problem to identify the cause and major contributing factors.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.
There is no control as yet for this problem.