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Plant bacterial pathogens

 

 

Introduction

Most plant pathogenic bacteria belong to the following genera: Xanthomonas, Xylella, Erwinia, Pseudomonas, Ralstonia, Pectobacterium, Pantoea, Agrobacterium, Burkholderia, Acidovorax, Clavibacter, Streptomyces, Spiroplasma, and Phytoplasma. The first five contain species of most important plant pathogens affecting food crops.

Plant pathogenic bacteria cause many different kinds of symptoms that include galls and tumors, wilts, necrotic spots, specks and blights, soft rots, as well as scabs and cankers.

Gammaproteobacteria

cellular organisms - Bacteria - Proteobacteria - Gammaproteobacteria -

Class Gammaproteobacteria belong to phylum of Proteobacteria that contains purple bacteria and their relatives. Gammaproteobacteria comprises facultatively anaerobic and fermentative gram-negative bacteria and contains many important pathogens of animals and plants.

Organism Disease Transmission Management
Xanthomonadales - Xanthomonadaceae - Xanthomonas - Xanthomonas campestris
Synonyms: Achromobacter lunatus, Xanthomonas campestris var. armoraceae, Xanthomonas campestris var. aberrans, Phytomonas campestris, Bacterium campestre, Pseudomonas campestris, Bacillus campestris
Straight, Gram-negative, mostly yellow-pigmented (Greek xanthos - yellow) rods with a polar flagellum; strictly aerobic chemoorganotrophs; most strains are phytopathogenic. Strains are host species-specific. Infection can result in severe damage to leaves and fruits in cruciferous plants (necrosis, gummosis and/or vascular or parenchymatous diseases on leaves, stems or fruits). Some of the diseases are known as bacterial blight, bacterial wilt, bacterial leafspot, bacterial leaf streak, leafscald disease, blackrot or citrus canker. Transmitted from flowers to seeds and from seeds to seedlings. Non-host plants' seeds as an inoculum source have been suggested. Dispersed via fruit containers, transported plants or wind-driven raindrops, enter plants through pores or other openings. Breeding pathogen-resistant cultivars; genetically-engeneered plants; production of pathogen-free propagation material; eradication of all plants with or without visible symptoms; application of antibiotics during growth flushes.
Xanthomonadales - Xanthomonadaceae - Xylella - Xylella fastidiosa
Gram-negative, aerobic bacteria, phytopathogenic, found in the xylem of plant tissue. Causes leaf-scorch diseases that are typically associated with extensive colonization of xylem vessels and subsequent blockage. Symptoms usually appear many weeks after inoculation in late summer. May cause great yield losses in important crops such alfalfa, peach, plum, almond, elm, coffee, sycamore, oak, maple, pear, and many others. Most economically important diseases are Pierce's disease of grape (PD) and citrus variegated chlorosis (CVC). Obligately vector-transmitted from one plant to another by various xylem sap-feeding insects, for example, by glassy-winged sharpshooter (Homalodisca coagulata). Breeding pathogen-resistant cultivars; genetically-engeneered plants; management of vectors.
Pseudomonadales - Pseudomonadaceae - Pseudomonas - Pseudomonas syringae group - Pseudomonas syringae group genomosp. 1 - Pseudomonas syringae
A species of Gram-negative, fluorescent, phytopathogenic bacteria; approximately 50 pathovars with different pathogenicities and host specificities exist. Cause leaf blight, fruit spotting, cankers and "bleeding lesions". Cankers occur on stems or trunks of trees or woody plants appearing as irregular, dark, water-soaked lesions. The buds and leaves above the canker may turn yellow and die. On leaves, symptoms show as brown spots, sometimes with a yellow halo. Spots turn to black and spread killing the entire leaf. Infected flower clusters, such as on lilac and blueberry, may turn brown, then black, and die. Can be transmitted by aphids that ingest and become colonized by bacteria while feeding on plants. Bees also can be carriers. There is evidence that this bacterium can be dispersed with pollen. Can be transmitted vertically from seed to seedling. Pathogens enter plant through openings in the stem or leaf. Spraying bactericides such as copper compounds or other heavy metals; antibiotics or other organic bactericides are used to a lesser extent. Other strategies include (i) seed disinfection by heat or a combination of heat and bactericides, (ii) biological control by antagonistic microorganisms, and (iii) soil solarization.
Enterobacteriales - Enterobacteriaceae - Erwinia - Erwinia amylovora
A species of Gram-negative short rods, with rounded ends, motile by many peritrichous flagellae; cause necrotic disease known as fire blight in some Rosaceae like apple, pear, quince, raspberry and several ornamentals; is native to North America and was introduced into northern Europe in the 1950s to 1960s. All the above-ground parts of hosts can be affected. Most common and characteristic symptoms are: (a) wilt and death of flower clusters; (b) withering and death of shoots and twigs; (c) leaf blight: necrotic spots which start from the margin of the blade or blackening of the petiole and leaf midrib; (d) fruit blight: fruits turn brown to black and shrivel; (e) limb and trunk blight: cankers on limbs and trunk causing quick death of branches or the whole tree by girdling. Natural dispersal by insects or rain disseminates locally, or by migrating birds over longer distances. Overwinters exclusively in infected host plants. Hold-over cankers are the most important source of primary inoculum for blossom infection in the spring. Enter the plant through blossoms, natural openings (stomata, lenticels, hydathodes) or wounds. Sanitation, pruning, eradication, tree nutrition and use of resistant or tolerant cultivars is recommended; in North America streptomycin sprays during blossoming are used. The only sure method for preventing or postponing the spread is strict phytosanitary measures on imported host plant material.
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Betaproteobacteria

cellular organisms - Bacteria - Proteobacteria - Betaproteobacteria -

Betaproteobacteria is a class in the phylum Proteobacteria comprised of chemoheterotrophs and chemoautotrophs which derive nutrients from decomposition of organic material.

Organism Disease Transmission Management
Burkholderiales - Burkholderiaceae - Ralstonia - Ralstonia solanacearum
Synonyms: Pseudomonas solanacearum, Burkholderia solanacearum, Bacillus solanacearum
An aerobic non-sporing, Gram-negative and motile short rods with a polar flagellar tuft. Regarded as one of the world's most important bacterial plant pathogens because of its aggressiveness, large host range (over 200 plant species from 50 families covering both monocots and dicots from annual plants to trees and shrubs), broad geographical distribution and long persistence in soil and water environments. Causes bacterial wilt. Symptoms include wilting, stunting, and yellowing of the foliage, and eventually death. Affected plants can have glistening beads of dark gray slimy ooze from the infected xylem. Potato tubers can exhibit grayish brown discoloration, a sticky exudate may form at the eyes in advanced infection. Transmitted by contaminated soil, water, equipment, and personnel or by transplantation of infected plants, tubers, or cuttings. Transmission can occur from plant to plant or through contaminated shared irrigation water systems. Not spread aerially. Bacteria can remain viable in soil for more than a year in the presence of a host. Production of pathogen-free propagation material; phytosanitary measures (desinfecting equipment, usung gloves, etc.); planting resistant cultivars; crop rotation. There is no effective chemical control.

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