Bacterial canker of citrus is a contagious bacterial disease caused by the bacterium Xanthomonas citri.
It is a disease which results in heavy economic losses to the citrus industry worldwide either in terms of damage to trees, particularly reduced fruit production, reduced access to export markets, or the costs of its prevention and control.
Lesions appear on leaves, twigs and fruit which cause defoliation, twig dieback, premature fruit abscission and blemished fruit, and can eventually kill the tree.
The disease is introduced to new areas through the movement of infected citrus fruits and seedlings, and inadvertent re-introduction is highly likely despite the quarantine restrictions that are in place in many countries.
Trees infected with the disease suffer from low vigour and a reduction in fruit quality and quantity.
HOST RANGE
All citrus cultivars can be affected, e.g. orange, lemon, lime, grapefruit, mandarin, tangelo, and citrus rootstock. However, some cultivars of citrus more readily develop canker lesions than others.
SYMPTOMS
The bacterium enters the plant tissue and causes the development of blister-like formations, commonly known as lesions. The lesions continuously enlarge from pin-point size over several months and can be of many different sizes based on the age of the lesion.
Leaf, fruit and stem/twig tissues may be infected.
Leaf tissue offers more opportunity for infection and as such typically displays the most numerous lesions over time. It is unusual to see multiple lesions on fruit or stems if lesions are not present on leaves.
The bacterium enters the plant tissues through wounds and stomata.
Leaves; symptoms on leaves first appear as small, raised lesions that resemble blisters. These lesions eventually turn tan and then brown and the edges appear water-soaked and develop a yellow halo. As infection progresses, the lesions take on a raised, corky appearance, and the center of old lesions may eventually drop out.
Fruit; symptoms begin as dark brown raised lesions which are similar to those on stems and twigs, but they often have a yellow halo like those that appear on the leaves.
Infections on fruit can lead to early fruit drop, but the fruit that remains on the tree is still edible, although unappealing in appearance, but the lesions typically do not penetrate the skin.
Stems & Twigs; symptoms of citrus canker on twigs and stems appear as raised corky lesions that are dark brown in color.
Initially, these lesions appear water-soaked or oily at the edges, but they do not have the yellow halo that shows on the leaves and fruit. As the lesions age, they become dry and scabby.
Stem lesions may also store inoculum that re-infects new tissue as it is produced by the citrus tree.
DISEASE CYCLE
Citrus canker is a polycyclic bacterial disease.
Xanthomonas citri bacterium reproduces by binary fission in canker lesions in citrus leaves, stems and fruit. When moisture comes into contact with these lesions, the bacteria ooze out onto the surface of the plant tissues and can be dispersed by rain splash or wind-driven rain.
The primary dispersal agent for the pathogen is strong wind-driven rain, which helps the inoculum enter through the stomata. However, if there are wounds in the plant tissues, from thorns, pruning, other mechanical damage, leafminer feeding galleries, etc., the bacteria can inoculate plant tissues at much lower wind speeds. As few bacterial cells get into the plant tissues, they lead to infection and lesion formation, and the infection cycle begins again.
The bacterium can survive in the margins of lesions on plant tissues until they fall and begin to decompose, at which point the bacteria face competition from saprophytic microorganisms in the soil
Exposure to direct sunlight accelerates the death of bacteria, and bacteria that oozes onto plant surfaces die when dried out. However, the canker bacteria can survive in lesions on woody tissue surfaces for years, and serve as inoculum across growing seasons.
Although the dispersal of bacteria by normal wind-driven rain is generally limited to relatively short distances, extreme tropical weather has been demonstrated to disperse the pathogen many miles. Also, long-distance movement of the pathogen can occur through human vectors, i.e., through contaminated clothing, equipment, tools, etc., and by the movement of diseased plant propagating material.
- citri is rapidly disseminated by rainwater running over the surfaces of lesions and splashing onto uninfected shoots.
Spread is greatest under conditions of high temperature, heavy rainfall and strong winds.
DISEASE MANAGEMENT
Citrus canker is a serious disease impacting on citrus production and should therefore be controlled to reduce/prevent the losses attributed to it.
The following methods are used in the management/control of bacterial canker of citrus;
Chemical control
Control for citrus canker is better achieved through prevention. Once a lesion develops on citrus tree, the lesion can’t be cured. However, the bacteria can be prevented from spreading beyond it because the bacteria are limited to the lesions and do not move within the plant unless moved by physical means such as water.
To effectively control the disease, copper-based fungicides are recommended. Copper is known to highly suppress and prevent activity of bacterial pathogens.
These fungicides include the following;
- GREENCOP 500WP
- TRINITY GOLD 452 WP
- COLONIZER 440WP
Spraying the plants with BIODISTINCTION XTRA gives protection against the bacterial infection.
PYRAMID 700WP suppresses the activity of the bacterium within the plant.
OPTIMIZER boosts the plant’s immunity and manages stress
Non- chemical control methods
- Use disease free and healthy planting materials
- If citrus canker is detected, isolate suspect nursery stock from healthy plants
- Use good sanitation and hygiene practices.
- Minimize movements within the farm. Remember that workers, visitors, vehicles and animals can spread the disease.
- Plant resistant/tolerant varieties.
- Tools & equipment should be sterilized between uses to prevent the spread of citrus canker to other trees.
- Severely infected trees should be destroyed to prevent infecting healthy trees nearby.
Note;
- Mix the fungicides with INTEGRA 3ml/20l whenever spraying. It is a sticker, spreader and penetrant which increases the efficacy of the fungicide for an effective control.
- Timely disease management/control is very critical as it prevents and/or reduces yield losses.
- A proper nutrition increases plant’s resistance to infections.
