Why Does My Citrus Tree Have Corky Bark Lesions?

Raised, corky, or crater-like lesions appearing on the bark, leaves, and fruit of citrus trees are the hallmark of citrus canker, a serious bacterial disease caused by Xanthomonas citri subsp. citri. The lesions start as small water-soaked spots that enlarge and develop a characteristic rough, spongy texture surrounded by a yellow halo. Citrus canker is one of the most economically damaging citrus diseases globally and is a notifiable disease in many countries, meaning that confirmed cases may need to be reported to plant health authorities. Understanding what you are dealing with is the critical first step.

Identifying True Citrus Canker

Genuine citrus canker (Xanthomonas) produces circular lesions with a raised, spongy, tan to brown centre. These are visible on both sides of the leaf and the yellow halo around each lesion is a distinctive feature. On fruit, lesions are raised and corky. On stems, the bacteria cause elongated lesions that can girdle young shoots. A key diagnostic test: true canker lesions appear equally on both sides of the leaf. Compare your lesions with images from a reputable plant pathology source or submit a sample to your local agricultural advisory service if you suspect true citrus canker.

Look-Alikes — Melanose and Scab

Two fungal diseases produce superficially similar lesions and are far more common in hobby growing situations in temperate climates. Citrus melanose (Diaporthe citri) causes small, dark, slightly raised spots that may coalesce into larger patches with a rough "mudcake" texture. Citrus scab (Elsinoe fawcettii) produces corky, wart-like, raised lesions but tends to cause leaf distortion. Both fungi colonise dead wood in the canopy and spread during wet weather. Removing dead twigs and improving airflow through the canopy significantly reduces their impact. Copper-based fungicide sprays applied at the start of new growth flushes provide good preventative control.

Cultural Management for Bark Diseases

Whether dealing with true canker or fungal diseases, the cultural response is similar: remove and destroy all infected material by cutting back to clean wood at least 10 cm beyond visible lesions, sterilising secateurs between cuts with 70% alcohol or a 10% bleach solution. Bag and dispose of all cuttings — do not compost infected material. Avoid working with the tree during or after rain when bacterial and fungal spores spread most readily via splashing water. Move container trees under cover during prolonged wet periods.

Copper-Based Preventative Sprays

Copper fungicide/bactericide sprays (copper oxychloride, copper hydroxide, Bordeaux mixture) applied at the start of each new growth flush provide protection against both fungal bark diseases and, to a degree, bacterial canker. Cover the entire canopy including the undersides of leaves and stem surfaces. Repeat after heavy rain. Do not apply copper excessively as it accumulates in the soil and can become phytotoxic. For a single container tree, a modest preventative programme of two to three applications per growing season is usually sufficient for melanose and scab control.

Regulatory Obligations

If you suspect your tree has true citrus canker (Xanthomonas) rather than a fungal look-alike, contact your national or regional plant health authority. In many countries citrus canker is a notifiable disease and there are legal obligations around containing and reporting it. The disease can spread rapidly via contaminated tools, clothing, and plant material, and local authorities may have eradication programmes in place. In hobby growing contexts in temperate northern Europe, confirmed Xanthomonas canker is rare — most lesions turn out to be melanose, scab, or mechanical damage.