Citrus canker on lemon — symptoms and treatment

Severity: high

What is Citrus canker

Citrus canker is caused by Xanthomonas citri, which attacks leaves, fruit and stems and causes raised corky lesions on all three. Lemon is among the most susceptible of the citrus, and the disease is rated high severity: it rarely kills an established tree outright, but it defoliates it, blemishes every fruit it touches and causes premature fruit drop.

Symptoms

The lesions appear on leaves, fruit and stems alike. They begin as small raised spots and develop into rough, tan, corky eruptions that stand proud of the surface — run a finger over a leaf and you can feel them. Each is surrounded by a yellow halo, and around the raised centre sits a water-soaked margin, a greasy ring where the bacterium is still spreading into fresh tissue. On fruit the same lesions form in the rind, ruining it for sale, and heavily infected fruit drops early.

Key signs:

• Early: small raised spots with a water-soaked margin and yellow halo
• Early: lesions on both sides of the leaf, rough to the touch
• Advanced: raised tan corky lesions on leaves, fruit and stems
• Advanced: defoliation and premature fruit drop

A raised corky centre, a water-soaked margin and a yellow halo separate canker from citrus scab and other leaf spots.

Causes and conditions

Wind-driven rain is the engine of this disease, and that single fact explains its whole management. The bacterium oozes from lesions in wet weather and is blown onto healthy leaves and fruit; the harder the wind drives the rain, the further it travels and the more forcefully it is pushed into the tissue. It enters through natural pores and, far more readily, through wounds — which is why storms and hail are so dangerous, delivering the inoculum and creating the injuries at once. Young leaves and small fruit are the most susceptible tissue. It also travels on tools, hands and clothing, and long distances in infected planting material.

Treatment

There is no cure — copper protects healthy tissue but does not clear the bacterium from lesions already formed.

Windbreaks and sanitation — cultural

Throughout season. Use windbreaks to reduce wind-driven rain. Remove and destroy infected material. Quarantine new plantings. Windbreaks are the most effective single measure available, because they attack the mechanism the bacterium depends on: slow the wind and the rain no longer drives bacteria into the leaves. Prune out and destroy infected twigs and leaves.

Copper hydroxide — chemical

Preventive after wounding events. Apply copper sprays preventively, especially after storms or hail that create leaf wounds. Copper forms a protective film that kills bacteria before they enter, so it must be on the tree ahead of infection; a spray after lesions appear is too late for that tissue. Pre-harvest interval: 21 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Plant certified, disease-free stock, and quarantine new plantings.
• Establish windbreaks — reducing wind-driven rain does more than any spray.
• Do not work in the grove while the foliage is wet; you will carry the bacterium on hands and tools.
• Disinfect pruning tools between trees.

Frequently asked questions

Is it contagious to my other citrus trees? Yes, and it moves easily. Wind-driven rain carries it tree to tree, and people spread it faster than the weather when they prune or pick wet foliage. All citrus is at risk; lemon especially.

Can I eat the fruit? The lesions are on the rind and the fruit is not harmful to eat — the damage is cosmetic, plus early drop and lost market value. After copper, respect the 21-day pre-harvest interval.

When should I spray? Preventively, and above all after storms or hail — wounded tissue in wet weather is exactly the opening the bacterium needs. Spraying a tree already covered in lesions will not clear them.