Canker on citrus — symptoms and treatment

Severity: high

What is Canker

Citrus canker is caused by Xanthomonas citri. It causes raised, corky lesions on leaves, stems, and fruit. Unlike most citrus diseases this one is bacterial, not fungal, and that changes everything about how it is handled: there is no curative spray, copper works only as a protectant on tissue that is not yet infected, and once a tree is badly affected removal is the realistic option. It is a serious, regulated disease in many citrus-growing regions.

Symptoms

The classic lesion is easy to recognise once you have seen it. On leaves you get raised, corky brown spots — rough and blister-like, standing proud of the leaf surface rather than sunken into it — surrounded by a water-soaked margin and a distinct yellow halo. The same lesions turn up on fruit and stems, so unlike many leaf diseases this one marks the whole plant. Affected trees drop fruit prematurely.

• Early: small raised spots with water-soaked margins on leaves
• Early: yellow halo developing around each lesion
• Advanced: raised corky brown lesions on leaves, fruit and stems
• Advanced: premature fruit drop, defoliation on badly affected trees

The raised, corky texture plus the yellow halo separates canker from citrus black spot, whose lesions are hard and sunken, confined largely to the fruit rind, and carry no yellow halo.

Causes and conditions

The bacterium spreads in water. Rain splash moves it short distances, but the serious dispersal happens when wind drives that rain — storms are the events that carry canker across a planting, which is precisely why windbreaks are a control measure rather than a nicety. Wind-blown grit also wounds leaves and gives the bacterium an entry point, compounding the damage. It infects young, expanding tissue most readily, making flush growth the vulnerable stage. It also travels on contaminated tools, hands and clothing, and on infected nursery stock — the most common way it reaches a clean orchard.

Treatment

Windbreaks and sanitation — cultural

Timing: at planting and ongoing. Install windbreaks to reduce rain-splash dispersal. Remove and destroy severely infected trees. Use disease-free nursery stock.

Copper hydroxide — chemical

Timing: during flush growth. Apply copper sprays preventively during flush growth and after storms. Copper reduces bacterial spread. Pre-harvest interval: 21 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Buy only disease-free nursery stock — infected young trees are the usual route into a clean site.
• Install windbreaks so wind-driven rain cannot carry the bacterium through the planting.
• Disinfect pruning tools between trees, and stay out of the grove while foliage is wet.
• Remove and destroy severely infected trees rather than nursing them along as a reservoir.
• Time protective copper to the flush, when the young tissue the bacterium exploits is present.

Frequently asked questions

Is it contagious to my other plants? It spreads readily to other citrus, and vigorously in storms. It is not a threat to unrelated garden plants. Because it is regulated in many citrus-growing regions, check what your local authority requires if you suspect canker — reporting obligations and movement restrictions may apply.

Can I eat fruit from an infected tree? Citrus canker does not affect people; the lesions damage the rind and the fruit's marketability. Observe the pre-harvest interval of 21 days after any copper hydroxide application.

Will spraying copper cure my tree? No. Copper reduces bacterial spread and protects tissue that is not yet infected — it does not eliminate an established infection. That is why it is applied preventively during flush growth and after storms, and why severely infected trees are removed and destroyed rather than treated.