Wisterias (Wisteria spp.) are large vines from the pea family that resemble trees and shrubs in form. The deciduous foliage is divided into numerous leaflets and the spring-blooming flower clusters come in shades of pink, blue or white. Wisterias adapt well to a variety of well-drained soil types and thrive with moderate irrigation in areas of full sun. A number of factors could contribute to yellowing foliage on wisteria.
Botryosphaeria canker is among the diseases of wisteria that cause browning and yellowing foliage. The disease is caused by the botryosphaeria fungi. Inadequate watering also results in discoloured foliage. Wisterias are prone to infestation from aphids, sap-sucking pests that cause foliage discolouration.
Early symptoms of the disease include the wilting and yellowing of leaves and dieback of branches on seemingly healthy plants. An inspection of the wood below the bark reveals reddish-brown discolouration. Cankers often appear on the larger stems. Symptoms of inadequate watering include wilting and yellowing foliage that eventually starts to drop. As plants lose strength, they become more susceptible to secondary problems such as pathogenic infections and pest infestation. Aphids are soft-bodied pests that infest foliage undersides and other tender plant areas and suck plant sap. They cause distorted, curled and yellow leaves.
The fungi causing botryosphaeria are referred to as opportunistic pathogens that commonly infect plants that are already weak or are suffering from some form of cultural stress such as drought. Inspect the soil around younger plants daily in order to assess its water needs. Aphids are common in plants that are growing in poorly ventilated and overly humid sites.
Once wisteria is infected with botryosphaeria fungi, there is no fungicide that can control it. Preventive measures include avoiding injury and eliminating debris around plants. Water plants enough to keep the soil moist but not wet. This is especially important around the root zone. Mulching helps to conserve moisture in the root zone. To control aphids, introduce natural predators like lacewings, syrphid flies or lady beetles. Chemical control options include the use of pesticides containing permethrin, malathion or acephate.