Willow anthracnose, scab and canker

What is willow anthracnose, scab and canker?

The three fungal pathogens of willow in the UK are;

1. Drepanopeziza sphaerioides (willow anthracnose) – affecting leaves and stems
2. Venturia saliciperda (willow scab) – affecting leaves and stems
3. Glomerella miyabeana (willow black canker) – affecting stems only

Symptoms

Anthracnose:

• Blackish spots appear on the leaves, whose development may be distorted, and irregular black raised lesions occur on the young stems. Weeping willow is very susceptible to this disease 
• Leaves and stems are shed, leading to a sparse crown

Scab:

• Irregular black spots appear on new season's growth, causing black, shrivelled shoots

Black canker:

• Symptoms are as for scab, but appear later in the season and spread to older growth where sunken cankers appear on the stem

Control

Non-chemical control:

• 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.

Chemical control:

There are no chemical controls specifically available for these diseases and it would not be practicable to spray mature trees in any case.

Fungicides containing myclobutanil (Bayer Garden Systhane Fungus Fighter and other formulations), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra and Roseclear Ultra) are available for the control of rust diseases on ornamental plants and may be applied to willows for rust control. They will probably give useful control of these other diseases, although this is not claimed by the manufacturers, and could be valuable for protecting young, establishing trees.

Some formulations also contain insecticides but these are best avoided if no insect pest problem is specifically identified. For example, some formulations of myclobutanil (Westland Rose Rescue) contain cypermethrin, some formulations of tebuconazole (Bayer Garden Multirose Concentrate 2) contain deltamethrin, and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Biology

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 weeping willow (Salix × sepulcralis var. chrysostoma) 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.