Well-established wisteria are a dramatic addition to any landscape, evoking images of country castles and manorhouses. The gnarly vines and their plumes of cascading flowers are hardy growers that can flower throughout the spring in all regions of the UK. While fairly disease resistant, there are a few ailments that can turn leaves yellow and eventually kill even older plants.
Caused by the bacterium, Agrobacterium tumefaciena, crown gall attacks the plant's root system. By the time the malady reaches the leaves, turning them yellow and red, it may be difficult to cure. Look for swollen growths at the crown of the root. These galls vary in size and number and can have devastating effect on your wisteria. Prevent the occurrence of crown gall by planting disease-free grafts in unaffected soil. Sterilise planting tools and equipment. Treat galls with biological products such as Gallex.
Yellow spotting on your wisteria may be an indication of the mosaic virus. Transmitted to the plant by aphids, there is no effective treatment for the virus. Cut away all affected vines to prevent its spread and remove aphids with an approved herbicide or insecticidal soap.
Aphids can do more than spread virus; they can also feed on healthy wisterias and turn them sick. You can spot the tiny sucking bugs on the plant foliage but will more likely first see their handiwork in the form of yellowing and browning leaves.
Scale insects may also attack wisterias and cause their leaves to wither and die. Full infestation can defoliate plants and eventually kill them. Cut off affected areas and apply horticultural oil to kill the insects and protect remaining leaves.