Q What causes sudden death or dieback in wisterias?
A Wisterias do have an unfortunate habit of suddenly dying off – either a substantial branch or even the whole plant. It is not always possible to detect the cause, but likely culprits are:
Wisteria is notably susceptible to this disease, which generally proves fatal once established.
Phytophthora root rot
This fungal disease is most prevalent in wet conditions, so avoid it by choosing a well-drained spot for your plant.
Failure of the graft union
Even on mature plants, the connection between the rootstock and the grafted variety can sometimes break down. In this situation everything above the union dies. If any green shoots remain, they have probably grown up directly from the rootstock and are unlikely to flower for many years.
Drought or waterlogging
Either of these conditions can kill or seriously damage wisterias. Plants growing on walls are particularly susceptible to dry conditions as the wall not only keeps off a lot of the rain, but tends to absorb some of the available moisture itself. Always keep the soil moist, and mulch well in autumn to help retain water. Short-term waterlogging should not cause too much damage, though you may see some dieback, but wisteria will not tolerate sodden soil for long.
Severe infestations could cause dieback and might prove fatal, especially to plants that are already stressed, for example by dry weather.