Citrus Canker is a deteriorating bacterial disease caused by the bacterium Xanthomonas citri subsp. The threatening disease appears as lesions on the citrus plants’ leaves, stems, and fruits.
What citrus canker caused by? Let’s dig for more information related to the infection of citrus canker.
1) What is Citrus Canker?
The citrus canker infection is a disease spread across especially citrus species by a bacterium Xanthomonas citri subsp. The bacterial disease caused by the bacterium makes the infected plants dangerous to other healthy citrus plants.
Citrus canker disease is a highly contagious plant disease caused and spread by infected citrus trees. Infected Citrus fruit, citrus tree trunks, and citrus leaves are also known to spread citrus canker.
The disease causes the fruits of citrus species to drop prematurely. Water-soaked margins can be spotted on the same fruit.
The disease goes long back. It was first found in the southeastern parts of America when the seedlings were exported from Japan in the early 1910s. The citrus canker disease was detected by the agriculture and consumer services which lead to the decision to eradicate the infected plants by the state government. (Gottward, et al 2007)
The introduction of invasive plant pests into crops has increased in the United States. One such invasive species in Florida is a bacterial plant pathogen that causes Asiatic citrus canker. Its abolition has generated significant journalistic interest and legal challenges, resulting in far-reaching political and economical consequences in Florida, as well as repercussions for national and worldwide trade. (Gottwald, et al. 2018)
Citrus plants and trees are abundantly found in parts of Asia. With the growth of agriculture and consumer services, citrus varieties are now available in almost every part of the world.
A citrus tree is known for its richness of Vitamin C and fibre assisting in having healthy overall well-being. Consuming citrus fruits can help in reducing the risk of kidney stones, it may protect you against cancer-causing agents, and boost heart health.
Infected fruit is still safe to be consumed by humans as bacterial disease does not have any significant side effects on us humans. Although, the infected plants have reduced marketability as fresh fruits. The blemishes on the fruit resist many individuals from buying the infected fruit.
1.1) Citrus Canker is Caused By:-
Citrus Canker is caused by the bacteria Xanthomonas citri subsp. The bacterial plant disease is invasive rendering plant parts of citrus -covered in round, water-soaked lesions.
Bacterium Xanthomonas axonopodis is a gram-negative, rod-shaped bacterium. The bacterium spread citrus canker among citrus relatives causing huge agricultural loss across many countries.
1.1.1) Life Cycle of Citrus Canker Bacteria
The bacteria is known to enter the plant body through stomata or can pass into the citrus plant by any other natural openings or wounds present on leaves and other green plants. The bacterial disease causes severe damage to the leaves, stems, and fruit.
When the bacteria Xanthomonas citri infect the citrus trees and plants, it spreads citrus canker lesions all over the plant surface. The infected plant material produces more bacteria which oozes out of the canker lesions.
Rain splash causes the inoculum to spread and infect new potential hosts. The rain storms lead the spread to occur over distances up to 100m. During the rainy season, the congested water puddles on the leaf surface form pathways for the infection to enter into the openings of the stomata.
Usually, the bacteria can live on the surface of living citrus plants for about 10 months. For long dry periods, the bacterium loses its activity and becomes less potent. But when the dry period is overtaken by high rainfall and warm weather season, bacteria can become active to infect citrus canker disease.
Symptoms can emerge 14 days after inoculation into a susceptible host in a controlled laboratory setting. In the field, the time it takes for symptoms to show and be distinguishable from other foliar diseases varies; it could be many months after infection.
1.2) Symptoms of Citrus Canker infection on Citrus Plants and Citrus Trees:-
The infected plant material is vividly visible and recognized by the plant health instructor. The infected trees or plants form round-like blisters that are small in size. The canker lesions cause the plant to defoliation, immature fruit drop, and twig dieback.
Citrus plants like lemon, oranges, grapefruit, lime, etc. get affected by the bacterium Xanthomonas axonopodis leaving the plant parts with lesions that appear as yellow halo and have a water-soaked margin.
The Citrus canker lesions can be spotted on leaves, stems, and fruits of citrus plants as brown spots with water-soaked rings visible with keen clarity. The infected tree also bares these brown spots with an oily soaked appearance.
When the lesions are present for a long time they have a corky appearance on the plant surfaces with a yellow halo effect still evident.
Citrus canker symptoms mainly include premature fruit drop and the growth of a mature fruit into a blemished fruit. When the plant is severely infected by the bacterium, it causes the plant to lose its life.
Furthermore, Citrus Leafminer (CLM) damage caused by larvae that feed by digging shallow tunnels or mines exposes more tissue to citrus canker infection and expands the afflicted areas. Fruit rind blemishing disease (aka citrus canker) is noticeable on the mature fruits of the citrus plant.
1.3) Reasons for the Spread of the Infection
Long-distance spread occurs when the wet foliage and diseased plant tissues are carried away by the wind-driven rain and rain splash. Insects are one other major carrier responsible for the spread of the infectious disease.
Wind-driven rain and severe weather events assist the spread of citrus canker by providing a free moisture environment. Numerous contaminated pieces of equipment like mechanical hedgers, sprayers, gardening tools, etc., used by the caretaker of citrus can cause the disease to spread.
Practices of hedging, rootstock seedlings, and budded trees can allow the entry of Xanthomonas citri into the mesophyll tissues making the plant moderately susceptible to the infection.
Once citrus trees are affected by citrus canker, the infected trees cannot recover. Citrus canker bacteria cause damage to the citrus tree to such an extent that they become a potent, serious disease barrier. Living citrus plants become a source of disease spread.
The critical period is 90 days after the petal fall when fruits are most vulnerable. The most susceptible plants are the young, growing, and developing plants.
Citrus canker has not been tested on all citrus species and types. It affects the majority of common citrus species and cultivars. Some species are more susceptible to infection than others, while others are resistant.
Grapefruit (C. paradise), Mexican limes (C. aurantiifolia), Lemon (C. Limon), Kaffir lime (C. hystrix), and Sweet Orange (such as Pineapple, Marrs, and Hamlin, etc.) are the most susceptible citrus plant among the citrus relatives to have been affected by the citrus canker symptoms.
Early-maturing orange cultivars are more vulnerable than mid-season cultivars, which are more susceptible than late-season cultivars. Internal leaf tissues (mesophyll) are visible when plant tissues are damaged by wounds or the feeding galleries of the Asian leafminer.
Citrus species that are mildly susceptible to citrus canker infection are Limes (C. latifolia) including Tahiti lime, Palestine sweet lime, bitter oranges (C. Aurantium), lime berry (Micromelum minutum) Valencia oranges (C. Sinensis Valencia), trifoliate orange (Poncirus trifoliata) tangerines, desert lime (C. glauca) tangors, tangelos (C. reticulata hybrids), and citranges/citrumelo (P. trifoliata hybrids).
There exist some species of citrus that are resistant to citrus canker contamination. These include Mandarins (C. reticulata) like Satsuma, Ponkan, Cleopatra, Sun Chu Sha, Sunki, and Poznan. Citron (C. medical) is also known to be highly resistant to citrus canker infection.
Some citrus is entirely resistant to the infection of citrus canker. These include kumquat (Fortunella spp.) and Calamondin (X Citrofortunella).
The canker lesions are easily recognizable. The lesions of citrus canker can be observed by the local farmer further they can inform the plant pathology department in their area to isolate and prevent the long-distance spread.
The department is meant to direct farmers to protect their crops and trees from contaminating other plants and trees.
In a few cases where the infection is severe, it becomes tough to save the plant’s life altogether. After researching and exploring the disease, the following ways can be applied to supervise or prevent the further expansion of the infection.
Citrus canker can be managed by excluding the involvement of any potential element that can lead to severe infection across the citrus fields of a state or country. Nurseries can be built in such a manner that will not promote the infected plant material from proliferating.
Nurturing nursery trees alienated from probably infected plant material can assist in the proper prevention of disease spread.
Strict restrictions can be maintained to avoid the entrance of deadly plant diseases into the area of healthy citrus trees. After removing all the citrus canker-causing plant parts from the healthy fields, new, and fresh citrus trees can be planted.
Long-distance spread occurs when diseased plant tissues are carried to other parts of the field by rains, strong winds, vectors, etc. To avoid such infectious diseases from travelling across healthy fields of citrus, the plants can be grown in areas where wind-driven rain and strong breeze are few
Cultural practices such as pruning, burning infected twigs, and defoliation of diseased plant tissues are executed to avoid the spread of citrus canker lesions.
When a citrus canker is carried into a field, the sick trees are removed to prevent the bacteria from spreading further. In Florida, for example, between 2000 and 2006, all citrus trees within 1,900 feet (580 m) of sick trees had to be removed.
The afflicted trees are uprooted and burned throughout the operation. In cities, trees are cut down and chipped before being disposed of in landfills. (Gottwald, et al, 2002)
It is critical in an eradication program to identify all potential hosts of the target pest or pathogen.
Mechanical means, such as humans and machinery, can transmit X. axonopodis site. Workers in citrus orchards are expected to decontaminate individuals and equipment thoroughly as a sanitary technique to avoid the spread of bacteria from sick areas.
Aerosol inoculum can infect wetted leaves in the zone of bacterial distribution. Vehicles can also become infected if they come into contact with moist foliage. Bactericides can be used to disinfect contaminated equipment and machines.
1.5.4) Development of Canker-Resistant Varieties
New, innovative methods can be applied for developing a unique variety of citrus that can prove to be resistant to citrus canker bacteria. The scientists in the labs can innovate varieties of citrus that can have automatic resistance to citrus canker infection.
Florida department of agriculture has several sub-departments that attempt towards understanding and researching those specific plants and trees. One such department is the Florida Department of Citrus wherein the scientists and agriculturists regulate, research, and market the state’s citrus industry. The assigned department develops new and latest solutions for diseases like citrus canker.
The need for such a department is to be able to evaluate the various conditions, and diseases and maintain a healthy and sustainable life for the citrus plant. The department deals with infected plants of citrus maximising the production of the citrus plants and keeping the nutritional value of citrus fruits intact.
The Florida department of agriculture promotes research on citrus plants and trees. It maintains the state’s citrus industry. The department constantly undergoes the process of trying to develop new canker-resistant varieties of citrus plants.
Citrus canker is caused by the bacteria Xanthomonas citri subsp. Important measures can be taken to prevent the death of many healthy plants of citrus. Along with the prevention of the lives of citrus, it can be made sure that the productivity and industry of citrus are not affected by the contamination of citrus canker lesions.