Willow anthracnose, scab and canker
Three important diseases of willows in the UK are anthracnose, scab and black canker. They harm the trees and can make them unsightly.
Scientific name Drepanopeziza sphaerioides, Venturia saliciperda and Colletotrichum salicis
Plants affected Salix
Main causes Fungi
Timing Spring to Summer
What are willow anthracnose, scab and canker?
Three fungal pathogens cause leaf spots or blotches and shoot dieback of willow in the UK:
- Drepanopeziza sphaerioides (willow anthracnose) – affecting leaves and stems
- Venturia saliciperda (willow scab) – affecting leaves and stems
- Colletotrichum salicis (willow black canker) – affecting stems only
- Brown or black spots appear on the leaves, whose development may be distorted, and irregular raised lesions occur on the young stems. Weeping willow is very susceptible to this disease
- Affected leaves and shoots are often shed, leading to a sparse crown
- Irregular black spots appear on new season's growth, causing black, shrivelled shoots
- Symptoms appear later in the season and the shoot dieback is similar to that of scab, but it can spread to older growth where sunken cankers appear on the stem
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
- Raking up and destroying fallen leaves from anthracnose-infected trees will only be of limited value, because it is lesions remaining on the tree which are the most important source of infection
- On young trees prune out infected shoots promptly. Damage done by bark infection is proportionately more severe when the stems are small
Resistance: Salix × sepulcralis var. sepulcralis and S. 'Erythroflexuosa' are resistant to anthracnose, and other willow species are hardly affected. Salix alba var. caerulea, S. pentandra and S. purpurea are resistant to one or both of scab and black canker.
There are no chemical controls specifically available for these diseases and it would not be practicable to spray mature trees in any case.
However, the fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are labelled for the control of other diseases on ornamental plants, and may give some control of anthracnose, black canker and scab on young, establishing trees.
The following products contain a combination of both insecticide and fungicide, enabling the control of both damaging invertebrates and disease: triticonazole containing acetamiprid (Roseclear Ultra, Roseclear Ultra Gun). When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if invertebrate damage is not a problem on the plants treated.
Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
The fungi which cause these diseases produce spores on the affected tissues. They are spread in water and the diseases are therefore worst in wet weather. Severe attacks of anthracnose on the very susceptible and commonly planted golden weeping willow (Salix × sepulcralis var. chrysocoma) can leave the tree almost leafless, but the disease is of little consequence on other willows.
Anthracnose differs from scab and black canker by the fact that anthracnose-infected tissues are shed, whereas the shoots killed by the last two diseases remain on the tree. However, not all anthracnose-infected shoots are shed, and those that remain on the tree form the most important source of infection in following years, since the spores they produce are very readily dispersed around the tree in rainwater.
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