Phytopathogenic bacteria are causative infectious agents of plant diseases. The word 'Phyto' refers to plant and pathogenic bacteria, fungi, molds and viruses specific to plant species are responsible for induction of disease symptoms. Generally bacteria are saprophytic but few species are parasitic which can be found within or outside plant tissues. Many bacteria are naturally rewarded with pathogenicity factors such as:
a) Presence of cell wall degrading enzymes like pectinase which can dissolve plant cell wall and release internal cytoplasmic contents.
b) Toxin production which cause gummosis, necrosis or chlorosis and damage to plant host tissue.
c) Production of effector proteins to suppress host defense mechanisms.
d) Excessive formation of phytohormones like IAA and tumor induction.
e) Exopolysaccharide and gum production to block xylem vessels and,
f) Mollicutes (bacteria lacking cell walls) like spiroplasmas and mycoplasmas cause phloem infection via sap sucking insects.
In addition to these, bacteria do possess special survival characteristics like an ability to thrive on inanimate objects like water, air, soil, insects and plant debris for long time; their resistance to desiccation, ability to enter wounds, natural orifices, invasion of host tissue, its attachment, colonization and multiplication in order to establish inside host body. Bacteria are easily disseminated by wind, water, insects, soil, humans and farm equipments and practices which facilitate bacterial infections of plants.
Bacterial species from the genera: Erwinia, Pseudomonas, Acidovorax, Ralstonia, Xanthomonas, Corynebacterium, Agrobacterium, Xylella, Arthrobacter, Clavibacter, Streptomyces. Bacillus, Clostridium, Rhodococcus, Rhizobacter, Pantoea, Rhizomonas, Serratia, Leifsonia and Sphingomonas are some of the important phytopathogens. Major bacterial diseases of plants include galls, wilts, cankers, rots, leaf spots, fruit deformation, retarded ripening, leaf discoloration, dwarfing and stunted growth. All plant types like cereals, pulses, grasses, shrubs, trees and even medicinal plants are prone to bacterial infections. Some of the bacterial diseases of plants alongwith their symptoms are described in the following paragraph.
Scabs: It is infection of underground stems and roots such as potatoes, carrots and radishes. Cork tissue is lesioned and pathogen enters host tissue via these lesions. Scabies of potato caused by Streptomyces scabies represents best example of scab disease.
Wilts: Wilting indicates infection of water conducting xylem vessels of plant. It is usually caused by slime or gum producing bacteria which plug xylem passages and interferes with translocation of nutrients and water. Cell wall of xylem tissue is also destroyed during the infection which results in collapse of vascular network within host tissue. All plant parts above the ground are prone to wilting followed by death. Some classical wilts are found in tomato and tobacco (Pseudomonas solanacearum), sweet corn (Erwinia stewartii), cucumber (Er. tracheiphila) and alfalfa (Corynebacterium insidiosum). Bacteria responsible for wilt disease are disseminated by beetles.
Soft rots: Rots are caused by opportunistic pathogenic strains which generally infect wounded tissue. Infected plant debris, humid weather and nematode infection also induce rotting disease. Underground structures and fruits fell to infection by soft rot bacteria. Pathogen enters via wounds and produce enzymes that hydrolyze middle lamellae. Lamellar cells disintegrate and die forming watery and soft rot. Infection gives out rotten smell and oozing of slimy liquid from infected tissue. Different species of Erwinia, Pseudomonas and Bacillus are causative agents of soft rot.
Spots, blight and necrosis: Spot diseases usually occur on leaves, fruits and stems. Rapid progression of spot disease is also termed as blight. Blight pathogens are disseminated by water. Spots of dicotyledonous plants have rotten, watery, angular spots while in monocotyledons, spots are like stripes or streaks and lesions can be necrotic. Necrosis appears as yellow or brown spots. Xanthomonas campestris is causative agent of blight in tomatoes and peaches. Spot disease of oats is caused by Pseudomonas syringae.
Canker and Anthracnose: The terms cankers and anthracnose are used synonymously; they are characteristic diseases of stone and citrus fruits. Canker affects trunks, stems, branches and twigs. Gummosis or gum production is typical symptom of canker disease. Hardening of exuded gum forms characteristic brown or orange brown colored sunken mass or canker. Some cankers can be soft and moist and have sour odor. Cankers are formed during hot and humid weather conditions and pathogens like P. syringae (Stone fruits) and X. campestris (Citrus) are transmitted by insects.
Galls: Gall is cancer like growth in plants and formed only by Agrobacterium strains (A. tumefaciens, rubi and vitis species). Gall is made up of disorganized mass of plant cells in rapidly dividing state. Overgrowth of tumor or gall cut off flow of nutrients and water via xylem vessels resulting in death of plants. Galls can be formed on trunk, fruits, stems and underground plant parts. Crown gall disease is typical example of gall disease caused by A. tumefaciens. It contains Ti (Tumor inducing) plasmid which transfers plant cells into cancer cells. Ti plasmid integrates into nuclear material of plant cells; transformed cells produce unusual opines and amino acid derivatives like octopine and nopaline. They are used as carbon and nitrogen source and support the growth of A. tumefaciens inside host tissue. Infection first appears as whitish round overgrowth and later develops as orange brown colored convoluted and enlarged mass. Disease is called 'crown' as it starts at crown (main root) of plant. Gall like infections has also been formed by species of Pseudomonas, Rhizococcum, Rhizobacter and Arthrobacter.
Discoloration, fruit deformation, retarded ripening and dwarfing indicates progress of bacterial infection in plant tissues. Bacterial phytopathogenesis is responsible for tremendous economic loss and induction of severe environmental issues which result from indiscriminate use of chemicals pesticides to control bacterial diseases of plants. But phytopathogenic bacteria also have one good use as they are used for effective biological weed control agents in sustainable agriculture.
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