Troubleshooting
American mountain ash problems — and how to fix them
American mountain ash (Sorbus americana) is generally forgiving once you match its basics, but a few issues come up again and again. Here is what each one looks like, why it happens, and the fix.
Fire blight (Erwinia amylovora)
Bacterial infection causes shoot tips to blacken and curl ('shepherd's crook'). Prune infected wood 30 cm below visible symptoms with sterilised tools; avoid excess nitrogen.
Borers (Agrilus sp.)
Flatheaded apple tree borer and similar species tunnel under bark, causing dieback. Maintain tree vigour through proper watering; remove and destroy infested branches promptly.
Powdery mildew
White powdery coating on leaves in warm, humid conditions. Improve air circulation, avoid overhead watering, and apply a sulfur-based fungicide if severe.
Prevent american mountain ash problems before they start
Most american mountain ash issues are care-mismatch, not bad luck. Dial in the basics:
American mountain ash problems — FAQ
Why is my american mountain ash fire blight (erwinia amylovora)?
Bacterial infection causes shoot tips to blacken and curl ('shepherd's crook'). Prune infected wood 30 cm below visible symptoms with sterilised tools; avoid excess nitrogen.
Why is my american mountain ash borers (agrilus sp.)?
Flatheaded apple tree borer and similar species tunnel under bark, causing dieback. Maintain tree vigour through proper watering; remove and destroy infested branches promptly.
Why is my american mountain ash powdery mildew?
White powdery coating on leaves in warm, humid conditions. Improve air circulation, avoid overhead watering, and apply a sulfur-based fungicide if severe.