Case Study #3: Citrus Canker

Bacterial canker of citrus is a contagious bacterial illness caused by a bacterium called Xanthomonas citri.

This sickness can cause significant financial losses for the citrus industry worldwide. It damages trees, leading to decreased fruit production, limited access to export markets, and expenses for prevention and control.

The disease creates lesions on leaves, twigs, and fruit. These lesions result in leaf loss, twig decay, premature fruit falling, and spoiled fruit. In severe cases, it can even cause the death of the tree.

The disease spreads to new areas when infected citrus fruits and seedlings are moved. Despite quarantine measures in many countries, unintentional reintroduction of the disease is highly likely.

Infected trees suffer from weakened growth and a decrease in the quality and quantity of their fruit.

Host Range

This illness can affect all types of citrus, including oranges, lemons, limes, grapefruits, mandarins, tangelos, and citrus rootstock. However, some citrus varieties are more prone to developing canker lesions than others.

Symptoms

Development of Lesions

When the bacterium enters the plant tissue, it triggers the formation of blister-like structures known as lesions. These lesions gradually grow larger over several months and vary in size depending on their age.

Infection in Various Plant Tissues

The bacterium can infect the leaves, fruit, and stems/twigs of the plant.

Prevalence of Lesions on Leaves

Leaf tissue provides more opportunities for infection, so it typically displays the highest number of lesions over time. It is uncommon to see multiple lesions on fruit or stems if there are no lesions present on the leaves.

Entry Points for the Bacterium

The bacterium enters the plant tissues through wounds and stomata (tiny openings on leaves).

Symptoms on Leaves

On leaves, symptoms initially manifest as small, raised lesions resembling blisters. These lesions eventually turn tan and then brown, with water-soaked edges and a yellow halo. As the infection progresses, the lesions become raised and corky, and the center of old lesions may eventually fall out.

Symptoms on Fruit

On fruit, symptoms start as dark brown raised lesions, similar to those on stems and twigs, but often with a yellow halo like the ones on leaves. Infections on fruit can cause premature fruit drop, but the remaining fruit on the tree is still edible, although unattractive in appearance. However, the lesions usually do not penetrate the fruit’s skin.

Symptoms on Stems and Twigs

On stems and twigs, citrus canker symptoms appear as raised corky lesions, dark brown in color. Initially, these lesions may appear water-soaked or oily at the edges, but they lack the yellow halo seen on leaves and fruit. As the lesions age, they become dry and scabby.

Inoculum Storage in Stems and Twigs

Stem lesions can store the bacterium, which can reinfect new plant tissue as the citrus tree produces it.

DISEASES CYCLE

Citrus canker is a bacterial disease that affects citrus plants. The bacteria, known as Xanthomonas citri, multiply in the canker lesions found on citrus leaves, stems, and fruit. When these lesions come into contact with moisture, the bacteria seep out onto the surface of the plant. They can then be carried away by rain or wind.

The main way the bacteria spread is through strong winds and rain, which help them enter the plant through tiny openings called stomata. However, if there are any wounds on the plant, such as from thorns or pruning, the bacteria can infect the tissues even with lower wind speeds. Only a few bacterial cells are needed to cause an infection, and this starts the cycle of infection and lesion formation all over again.

The bacteria can survive on the edges of the lesions until the affected plant parts fall off and begin to decompose. At this point, other microorganisms in the soil compete with the bacteria for resources.

Direct sunlight speeds up the death of the bacteria, and when they dry out on plant surfaces, they also die. However, the canker bacteria can survive for years in lesions on woody tissue, serving as a source of infection for future growing seasons.

Normally, the bacteria are spread by rain over short distances. But in extreme tropical weather, the pathogen can be carried over long distances. Additionally, humans can unknowingly spread the bacteria through contaminated clothing, tools, or by moving infected plants.

Rainwater flowing over the lesions and splashing onto uninfected shoots is a common way the bacteria are rapidly spread. The greatest spread occurs when there are high temperatures, heavy rainfall, and strong winds.

DISEASE MANAGEMENT

Citrus canker is a significant problem that can cause major losses in citrus production. It is crucial to control this disease in order to minimize the impact it has on the industry.

There are several methods available to manage and control bacterial canker in citrus:

Chemical control

Prevention is key when it comes to managing citrus canker. Once a lesion appears on a citrus tree, it cannot be cured. However, it is possible to prevent the bacteria from spreading beyond the lesion. The bacteria are confined to the lesions and do not move within the plant unless they are carried by physical means like water.

To effectively control the disease, it is recommended to use fungicides that contain copper. Copper has been found to effectively suppress and prevent the activity of bacterial pathogens.

Some examples of these fungicides are:

  • GREENCOP 500WP
  • TRINITY GOLD 452 WP
  • COLONIZER 440WP

Spraying the plants with BIODISTINCTION XTRA provides protection against bacterial infection.

PYRAMID 700WP helps to suppress the activity of the bacteria within the plant.

OPTIMIZER boosts the plant’s immunity and helps manage stress.

Non-chemical control methods

  • Using disease-free and healthy planting materials is important.
  • If citrus canker is detected, it is essential to isolate suspected nursery stock from healthy plants.
  • Practicing good sanitation and hygiene is crucial.
  • Minimizing movement within the farm is important, as workers, visitors, vehicles, and animals can spread the disease.
  • Planting resistant or tolerant varieties can help.
  • Tools and equipment should be sterilized between uses to prevent the spread of citrus canker to other trees.
  • Severely infected trees should be removed and destroyed to prevent the infection from spreading to nearby healthy trees.

Note:

When spraying, it is advisable to mix the fungicides with INTEGRA at a rate of 3ml per 20 liters. INTEGRA acts as a sticker, spreader, and penetrant, which enhances the effectiveness of the fungicide for better control.
Timely disease management and control are crucial to prevent or reduce yield losses.
Providing proper nutrition to plants increases their resistance to infections.

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