Willows love water, and so do the fungi that attack them. A wet spring can bring out leaf blights, spots and cankers that make a willow look far worse than it really is. Some of these diseases are cosmetic and need nothing more than a rake. Others, like willow blight and trunk cankers, can kill branches or even the tree. Here is how to tell them apart and what each one needs.
The short answer: most willow fungus problems are managed with sanitation rather than sprays: prune out infected twigs and branches, disinfect your pruners between cuts, rake up and dispose of fallen leaves, improve air circulation and avoid wetting the leaves. Willow scab and black canker (willow blight) are the most damaging leaf and twig diseases. Leaf spots, tar spot, rust and powdery mildew are usually minor. Cankers on the main trunk are serious and need an arborist’s assessment.
Willow scab
The University of Illinois explains that willow scab, caused by Venturia saliciperda, infects leaves as they expand in early spring. Washington State University describes brown to dark spots along the leaf midrib; young leaves look scorched, wither and drop, sometimes heavily, especially near branch tips. The fungus then grows into the twigs, forming small brown to black cankers that girdle them and cause dieback. Look for velvety, olive-brown spore masses along the veins on leaf undersides.
The disease spreads by splashing water and is favored by wet weather. Older leaves and twigs are less susceptible.
What helps
Washington State University recommends these non-chemical steps first:
- Avoid overhead watering.
- Space plantings and prune to improve air circulation.
- Rake and destroy fallen leaves before the spring infection period.
- Prune and destroy diseased twigs and branches.
- When replanting, choose less susceptible willows; WSU lists bay-leaved, osier, purple and weeping willows.
WSU also notes that fungicide sprays in spring can protect new leaves, but homeowners should not spray trees over 10 feet tall; that is a job for a licensed applicator.
Black canker
Black canker, caused by Glomerella miyabeana, often follows scab. The University of Illinois notes symptoms start in late spring to early summer as irregular brown to black leaf lesions. Leaves shrivel as the infection moves into the twigs, where small sunken cankers form near the leaf stalks. Montana State University adds that girdled twigs may droop or crook at the tips.
When a tree has both scab and black canker, the combination is called willow blight and can cause extensive leaf loss and dieback.
What helps
Montana State University recommends pruning out and disposing of infected branches, disinfecting tools with 70% alcohol or a household disinfectant between cuts, improving air circulation, supporting tree vigor with adequate water, and avoiding long periods of wet foliage. It notes fungicides are usually not recommended in urban settings. The University of Florida notes weeping willow appears to be resistant to black canker.
Cankers on branches and trunk
The University of Florida notes that many fungi cause cankers on willow. A canker is a dead area of bark and wood, often sunken, cracked or discolored. Infected branches should be pruned out. If a canker girdles the main trunk, the tree will die.
There is no spray that cures a canker. The best defense is a vigorous tree: water in drought, avoid mower and string-trimmer wounds, and prune properly to avoid leaving stubs. If you find a large canker on the trunk, have a certified arborist judge whether the tree is still safe.
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Rust
The University of Florida describes willow rust as yellow spots on the lower leaf surfaces that, if severe, cause leaf drop. It usually appears later in the season. Rake up and destroy leaves from infected trees. Rust is rarely life-threatening. See why weeping willows lose leaves for how rust fits into late-season leaf drop.
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Leaf spots and tar spot
Many fungi cause small leaf spots on willow. The University of Florida says they are usually not serious enough to warrant preventive sprays; rake up fallen leaves in fall. Tar spot causes raised black spots on the leaves. UF calls them harmless; again, raking and disposing of fallen leaves is the fix.
Powdery mildew
Powdery mildew shows as a white coating on leaves. UF notes the disease is usually not serious, so it rarely needs treatment. Good air flow through the canopy helps.
Root rot and wood decay
UC IPM lists Phytophthora root and crown rot and wood decay fungi among willow problems, and UF notes root rot can occasionally cause decline. Mushrooms or shelf-like conks at the base of a willow usually mean internal decay. That is not something you can treat, and it can make a tree unsafe. Call an arborist.
Treatment summary
| Problem | Signs | Treatment |
|---|---|---|
| Willow scab | Dark leaf blotches in spring, olive-brown spores, twig dieback | Prune, rake, air flow; spray small trees only |
| Black canker | Brown-black lesions late spring, sunken twig cankers | Prune and disinfect tools, keep leaves dry |
| Branch or trunk canker | Sunken, cracked bark | Prune branch; arborist for trunk |
| Rust | Yellow-orange spots under leaves | Rake and destroy leaves |
| Leaf spot, tar spot | Brown or black spots | Rake leaves in fall |
| Powdery mildew | White coating | Usually none needed |
| Root rot, decay | Conks, mushrooms, decline | Arborist assessment |
A good pruning routine
Prune infected twigs back into clearly healthy wood below the visible damage. Disinfect blades between cuts, as Montana State University advises. Dispose of the prunings rather than composting them, so spores are not left near the tree. For timing and technique, see when to prune a willow tree. If your willow is losing large branches or the whole crown is thinning, read why is my willow tree dying next.
Sources
- Scab and Black Canker of Willow, University of Illinois Home, Yard and Garden Newsletter
- Willow: Twig Blight (Venturia), Washington State University Hortsense
- Black Canker of Willow, Montana State University Extension
- Salix babylonica: Weeping Willow (ENH-734), University of Florida IFAS Extension
- Willow, UC Statewide IPM Program
Top photo: AfroBrazilian, CC BY-SA 4.0, via Wikimedia Commons