Anthracnose 🦠
Fungal — several genera (commonly Apiognomonia on sycamore; Colletotrichum on many plants)
Photo to source (Wikimedia Commons — sycamore anthracnose: blackened new leaves / shoot dieback).
At a glance
- Cause: Fungal; flares in cool, wet spring weather
- Tell-tale sign: Dark, sunken lesions; on sycamores, blackened, curling new leaves and shoot dieback
- Severity: Moderate — disfiguring but rarely kills an established tree
How to identify
A group of fungal diseases that cause dark brown-to-black, often sunken lesions on leaves, stems, and fruit. On Western Sycamore — the classic local example — it shows as blackened, curled, distorted new leaves in spring, dead shoot tips, and early leaf drop, sometimes defoliating a tree only for it to releaf later. Lesions often follow leaf veins. It’s worst after cool, wet springs.
What causes it
Host-specific fungi overwinter in fallen leaves and infected twigs, then spread by rain-splash onto tender new growth in spring. Wet weather during leaf-out is the key driver — which is why severity swings year to year.
Treatment & management
Following Integrated Pest Management:
- Sanitation (most important): Rake up and remove fallen leaves and twigs, which carry the fungus to next year. Prune out dead/cankered twigs.
- Improve airflow: Thin dense canopies; avoid overhead irrigation that keeps foliage wet.
- Support tree vigor: Appropriate water and mulch; a healthy tree shrugs it off and re-leafs.
- Fungicides are rarely warranted on large established trees; for high-value specimens, a professional may treat at bud break in bad years.
Prevention
Choose resistant species/cultivars for new plantings; keep up leaf-litter sanitation; don’t wet the foliage.
Affects (in this guide)
Western Sycamore (the prominent local case); many trees, shrubs, and vegetables have their own anthracnose strains.