Bacterial diseases (Citrus Canker causing organisms,Bacterial blight of bean, Rickettsia )

Bacterial diseases:

The plant disease is caused by various bacteria is commonly known as bacterial diseases. Some of the major bacterial plant diseases are as follow:

Fig: Pathogenesis of Bacteria

1. Citrus Canker causing organisms
2. Bacterial blight of bean
3. Rickettsia
4. Mycoplasma
5. Spiroplasma
6. Mycoplasma-like organisms (MLO) / Phytoplasma

• Citrus Canker causing organisms:

This disease is believed to be originated in Southeast Asia. Citrus species are affected by Citrus Canker and disease is caused by the bacterium Xanthomonas axonopodis. The infections are causes lesions on leaves, stems and fruit of citrus trees, including grapes, lime, and oranges. As Canker is not harmful to humans but simultaneously affects the citrus trees, causing leaves and fruit to drop prematurely.

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Fig: Citrus Canker on fruit.

Characteristics of pathogen:

1. Xanthomonas axonopodisis a rod-shaped. gram negative, which produces a yellow pigment with polar flagella.
2. The optimum temperature for its growth is 28-30*C and it produces yellow pigment.

Host of the pathogen:

The numerous species like cultivars and hybrid of citrus,citrus relatives including grapes, lime, sour oranges, oranges, etc are its host.

Diversity and distribution of disease:

Different pathovars and variants of the bacterium are the causative agents of citrus canker diseases . Diversity and its distribution are described as :

• Xanthomonas axonopodispv. citri causes the Asiatic type of canker (canker A) which is originally found in Asia, is the most widespread and severe form of the disease.
• Xanthomonas axonopodispv. aurantifolii causes the Cancrosis-B which is originally found in South America and is a disease of lime, bitter orange, lemons, etc in Argentina, Paraguay, Uruguay.
• Xanthomonas axonopodispv. aurantifolii causes theCancrosis-C which is originally found in Brazil and is a disease of Mexican lime.

Symptoms of Citrus Canker causing organisms:

This is disease can be a serious disease where rainfall and warm temperature are frequent during the period of early fruit development or shoot emergence. The disease can be characterized by :

• The appearance of the lesion on leaves stem about (1-10mm).
• Those lesions are raised ,brown in color, have water-soaked margin and sunken centers.
• Those lesions are surrounded by a yellow halo.

Pathology:

This bacteria can enter through the plant's stomata or through wounds or leaves and from other green parts of the plant body. Bacteria propagates in lesions in leaves, stem, and fruit. Those lesions ooze bacterial cells which are dispersed by windblown rain, to the other plants in the area and infection can also be spread by hurricanes. The disease can also be spread by transport of infected plants, by contaminated equipment etc.

The wind blew rain carries inoculums to unaffected plants.

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The wind drove rain aids penetration through stomatal pores or wounds.

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The death of the tissue giving rise to lesions.

Fig: The flow chart of Pathogenesis

Detection and disease management:

The disease can be detected in groves and on fruit by the appearance of lesions. Additional diagnostic tests like antigen-antibody tests, polymerase chain reaction, etc. can also be conducted.

The following procedures can be followed for disease management:

1. The proper sanitation around the planting area should be maintained.
2. The eradication process like removal and destruction if infected and exposed parts can be done.
3. Exclusion can be done.

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Fig: Lifecycle of Citrus Canker

• Bacterial blight of bean:

This bacterial blight of bean occurs in two forms, which are:

1. Common bacterial blight.
2. Halo blight of bean.

fig: Diseases caused by the bacterial blight of bean

• Common bacterial blight:

Systematic classification:

Kingdom : Bacteria

Phylum :Proteobacteria

Class :Y - Proteobacteria

Order :Xanthomonadales

Family :Xanthomonadaceae

Genus :Xanthomonas

Binomial name:

Xanthomonas campestris

Pathogen:

Xanthomonas campestris(axanopodis) pv. Phaseoli is the pathogen which is Gram-negative, aerobic, polar flagellated and non-spore forming bacteria. It produces a yellow pigment xanthomonadin.

fig: Common bacterial blight

Host:

Bean (Phaseolus vulgaris ) is its host.

Survival and transmission of pathogen:

The bacteria can survive overwinter in the bean debris and as the saprophyte on bean tissue. The surviving bacteria can multiplies on contaminated beans and cause disease. The bacteria can survive on the leaves without causing disease or may produce lesions under favourable environmental conditions. They can be transmitted from plant debris by wind, irrigation water or rain resulting in outbreaks.

Disease:

Common bacterial flight is characterised by foliar (on leaves) and pod lesion. The disease is favoured by moderate and warm temperature, high moisture and plant wounds during flowering. Wounds caused by storms, hail, high winds and farm machinery enable the pathogen to enter and infect plant tissue. The mid-aged to old leaves are mostly affected.

Symptoms:

Its symptoms can be seen as :

1. Leaf of plants gets scalded in appearance and water soaked irregular lesions that are dark brown. A narrow lemon yellow margin often surrounds these lesions.
2. Initially, pods show yellow circular water-soaked lesions that often produce yellow masses of bacterial ooze. The lesions, later on, become dry and appear as red-brown.
3. Pod infection leads to discoloration of seed.

Disease management:

1. The application of various bactericides as the preventive measure in required amount.
2. The planting of the high quality of seeds and treating seed with antibiotics.
3. Crop rotation can be done.

• Halo Blight of Bean :

Systemic Classification:

Kingdom :Bacteria

Phylum : Proteobacteria

Class :Y - Proteobacteria

Order : Pseudomonadales

Family : Pseudomonadaceae

Genus :Pseudomonas

Species :syringae

Pathogen:

Pseudomonas syringaepv. Phaseolicola Halo blight's pathogen is a Gram-negative organism which are polar- flagellated aerobic and non-spore forming bacteria.

Disease :

Halo blight is favoured by cool and moderate temperature, high moisture and plant wounds before flowering. The young leaves are mostly affected.

Symptoms of B-Halo blight of bean:

Its symptoms are described as follow:

• Leaves:

1. Initial water soaked greasy lesions spots appeared which are later surrounded by the green-yellow halo.
2. Under severe infection, the disease may become systematic and cause yellowing of new leaves and death

• Pods:

1. Small circular water soaked pods or streaks.
2. Pods are associated with a light cream/ silver coloured bacterial ooze.

• Seeds:

1. The discolouration of the contaminated seeds.

• Rickettsia:

Rickettsia is a genus of non-motile, Gram-negative, non-spore forming, highly pleomorphic bacteria that can present as cocci (about 0.1µm in diameter), rods (1-4µm long) or thread-like (10µm long). They are obligate,intracellular parasites, and their survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells.Rickettsia cannot live in the artificial nutrient environment and is grown either in tissue or embryo cultures .UnlikeChlamydia, Mycoplasma, and Ureoplasma,Rickettsial organisms possess true cell walls similar to other Gram-negative bacteria. Many of them are transmitted by arthropod vectors.

Diseases caused by Rickettsia:

The disease associated with Rickettsia are listed below:

1. Beet latent Rosette
2. Citrus Greening.
3. Cloverleaf.
4. Grapevine infectious necrosis.
5. Grapevine yellows.
6. Witch's broom disease
7. Peach phony.
8. Strawberry lethal yellows.

Symptoms of the diseases caused by Rickettsia:

1. Fruit abnormalities can be seen.
2. The leaves become chlorotic, pale green to yellow (Chlorosis/Yellowing).
3. There will be under development of root systems.
4. Severe reduction in leaf size and the dwarf leaflet with chlorotic margins occurs.
5. Phyllody (Abnormal development of floral parts into leafy structures generally caused by phytoplasma.)
6. The twisted and clubbed leaflets are seen.
7. An abnormal brush like the cluster of dwarfed weak shoots arising at or near the same points.

Reference:

1. Alexander, M. Introduction to Soil Microbiology. Academic Press, 1961.
2. Rangaswami, G and Bagyaraja PT. Agricultural Microbiology. 2nd. Prentice Hall of India, 1993.
3. RM, Atlas and R Bartha. Microbial Ecology : Fundamental and Applications. The Benjamin Cummins Publication Co. Inc., 1998.