Although it is unclear what causes these problems, there are several possibilities and problems that have been discounted. Analysis of samples taken from a small survey carried out on trees local to RHS Wisley, and samples sent to RHS Gardening Advice by RHS members allowed RHS pathologists to make the following conclusions:
- 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 been recorded, resulting in the bark becoming soft and smelly
- Prolonged spells of wet weather, leading to saturated soil conditions, could well be a contributory factor, stressing the trees and making them more susceptible to disease (Robinia tends to perform best on well-drained soils)
- Additionally we have consistently isolated the fungus Diaporthe oncostoma from lesions on branches of many trees examined
Until recently, D. oncostoma was considered to be saprophytic or weakly parasitic, favoured by adverse conditions for plant growth. RHS laboratory and field experiments have been unable to confirm the pathogenicity of the fungus. However, the fungus has now been reported as a causal agent of a canker and severe die-back disease of Robinia pseudoacacia (black locust) in Russia, Hungary, Italy, France and Greece.
It is possible that interactions between several factors could be involved in robinia decline in the UK.