Plant Disease Caused by Bacteria

Dr. Sanjeev Kumar

Assistant Professor/Scientist Plant Pathology JNKVV-Jabalpur

[email protected]

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BACTERIA: INTRODUCTION

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1. Bacteria is prokaryotes. 2. These are generally single-celled microorganisms . 3. Genetic material (DNA) is not bound by a membrane and therefore is not organized into a nucleus. 4. Their cells consist of cytoplasm containing DNA and small (70 S) ribosomes. 5. The cytoplasm in bacteria is surrounded by a cell membrane and a cell wall. 6. Plant pathogenic bacteria have been known since 1882. 7. It cause a variety of plant disease symptoms. 8. Some types of phytopathogenic bacteria, e.g., fastidious phloemor xylem inhabiting bacteria, which for several years were thought to be rickettsia-like organisms (RLO), were only discovered in 1972.

Classification of plant pathogenic Bacteria

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Kingdom: Procaryotae Bacteria — Have cell membrane and cell wall Division: Gracilicutes — Gram-negative bacteria Class: Proteobacteria — Mostly single-celled bacteria Family: Enterobacteriaceae Genus: 1. Erwinia, causing fire blight of pear and apple, Stewart’s wilt in corn, and soft rot of fleshy vegetables 2. Pantoea, causing wilt of corn Serratia, S. marcescens, being a phloem-inhabiting bacterium causing yellow vine disease of cucurbits 3. Sphingomonas, causing brown spot of yellow Spanish melon fruit

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Family: Pseudomonadaceae Genus: 1. Acidovorax, causing leaf spots in corn, orchids, and watermelon 2. Pseudomonas, causing numerous leaf spots, blights, vascular wilts, soft rots, cankers, and galls 3. Ralstonia, causing wilts of solanaceous crops. 4. Rhizobacter, causing the bacterial gall of carrot 5. Rhizomonas, causing the corky root rot of lettuce 6. Xanthomonas, causing numerous leaf spots, fruit spots, and blights of annual and perennial plants, vascular wilts, and citrus canker 7. Xylophilus, causing the bacterial necrosis and canker of grapevines

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Family: Rhizobiaceae Genus: 1. Agrobacterium, the cause of crown gall disease 2. Rhizobium, the cause of root nodules in legumes Family: still unnamed Genus: 1. Xylella, xylem — inhabiting, causing leaf scorch and dieback diseases on trees and vines 2. Candidatus liberobacter, phloem inhabiting, causing citrus greening disease

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Division: Firmicutes — Gram-positive bacteria Class: Firmibacteria — Mostly single-celled bacteria Genus: 1. Bacillus, causing rot of tubers, seeds, and seedlings, and white stripe of wheat 2. Clostridium, causing rot of stored tubers and leaves and wetwood of elm and poplar

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Class: Thallobacteria — Branching bacteria

Genus: Arthrobacter, causing bacterial blight of holly Clavibacter, causing bacterial wilts in alfalfa, potato, and tomato Curtobacterium, causing wilt in beans and other plants Leifsonia, causing ratoon stunting of sugarcane Rhodococcus, causing fasciation of sweet pea Streptomyces, causing the common potato scab

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Mollicutes — Have only cell membrane and lack cell wall Division:Tenericutes Class: Mollicutes Family: Spiroplasmataceae Genus: Spiroplasma, causing corn stunt, citrus stubborn disease Family(ies): still unknown Genus: Phytoplasma, causing numerous yellows, proliferation, and decline diseases in trees and some annuals

Bacterial strains

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1. A bacterial species is really a group of bacterial strains that share certain phenotypic and genotypic characteristics. 2. One of these strains serves as the type strain, with the other strains of the species differing to a lesser or greater extent from the type strain.

3. Bacterial strains may differ from one another in morphological, cultural, physiological, biochemical, or pathological characteristics. 4. When a STRAIN or group of strains infects a host plant not infected by the other strains of the species, that strain or group of strains comprise a pathovar (pv.) of the species.

PLANT PATHOGENIC BACTERIA

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1. About 100 species of bacteria cause diseases in plants. 2. Most plant pathogenic bacteria are facultative saprophytes and can be grown artificially on nutrient media;. 3. Fastidious vascular bacteria are difficult to grow in culture and some of them have yet to be grown in culture. 4. Bacteria may be rod shaped, spherical, spiral, or filamentous (threadlike). 5. Some bacteria can move through liquid media by means of flagella, whereas others have no flagella and cannot move themselves. 6. Some can transform themselves into spores, and the filamentous bacteria Streptomyces can produce spores, called conidia, at the end of the filament. 7. Other bacteria, however, do not produce any spores. 8. Vegetative stages of most types of bacteria reproduce by simple fission.

Important Genera of PP Bacteria

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Characteristics of Plant Pathogenic Bacteria

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Morphology

1. Most plant pathogenic bacteria are ROD SHAPED the only exception being Streptomyces, which is FILAMENTOUS. 2. Bacteria range from 0.6 to 3.5 micrometers in diameter. 3. Cell walls of bacteria of most species are enveloped by a viscous, gummy material, which, if thin and diffuse, is called a slime layer, but if thick, forming a definitive mass around the cell, is called a capsule. 4. Most plant pathogenic bacteria have delicate, threadlike flagella, considerably longer than the cells on which they are produced. 5. In some bacterial species, each bacterium has only one flagellum, whereas others have a tuft of flagella at one end of the cell (polar flagella); still others have peritrichous flagella, i.e., distributed over the entire surface of the cell.

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1. Streptomyces species, cells consist of branched threads, which usually have a spiral formation and produce conidia in chains on aerial hyphae 2. Single bacteria appear hyaline or yellowish-white under the compound microscope 3. When a single bacterium is allowed to grow (multiply) on the surface of or in a solid medium, its progeny soon produces a visible mass called a COLONY. 4. Colony may be circular, oval, or irregular. 5. Their edges may be smooth, wavy, or angular, and their elevation may be flat, raised, or wrinkled. 6. Colonies of most species are whitish or grayish, but some are yellow. 7. Some produce diffusible pigments into the agar that may be fluorescent with ultraviolet light.

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1. Bacteria have thin, relatively tough, rigid cell walls and an inner cytoplasmic membrane. 2. Gram-negative bacteria also have an outer membrane that appears to merge with the slime layer or capsule. 3. Cell wall allows the inward passage of nutrients and the outward passage of waste matter and digestive enzymes. 4. All the material inside the cell wall constitutes the protoplast. 5. Protoplast consists of a cytoplasmic or protoplast membrane. 6. Cytoplasm, which is the complex mixture and nuclear material, which consists of a large circular chromosome composed of DNA.

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1. Chromosome DNA makes up the main body of the genetic material of a bacterium . 2. Often, bacteria also have single or multiple copies of additional, smaller circular genetic material called PLASMIDS. 3. Each plasmid consists of several nonessential genes and can move or be moved between bacteria or even between bacteria and plants, as happens in the crown gall disease.

Most Important Genera of PP Bacteria and Kind of Symptoms they cause

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Reproduction

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1. Rod-shaped phytopathogenic bacteria reproduce by the asexual process known as binary fission, or fission. 2. This occurs by the inward growth of the cytoplasmic membrane toward the center of the cell, forming a trans-verse membranous partition dividing the cytoplasm into two approximately equal parts. 3. Plasmids also duplicate and distribute themselves equally in the two cells. 4. Bacteria reproduce at an astonishingly RAPID RATE. 5. Under favorable conditions, bacteria may divide every 20 to 50 minutes, one bacterium becoming two, two becoming four, four becoming eight, and so on. 6. At this rate, one bacterium conceivably could produce one million progeny bacteria in less than a day.

Ecology and Spread

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1. Almost all plant pathogenic bacteria develop mostly in the host plant as PARASITES, on the plant surface, and partly in plant debris or in the soil as SAPROPHYTES. 2. Wherever plant pathogenic, and other, bacteria exist, they often exist as BIOFILMS, that is, communities of identical or different microorganisms attached each other and/or to a solid surface. Erwinia amyovora, which causes fire blight of pear • Produce their populations in the plant host. • while in the soil their numbers decline rapidly and usually do not contribute to the propagation of the disease from season to season. • These pathogens have developed sustained plant-to

SOIL INHABITANTS & SOIL INVADERS

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1. Soil inhabitants. 2. Such bacteria build up their populations within the host plants, but these populations only gradually decline when they are released into the soil. 3. If susceptible hosts are grown in such soil in successive years, sufficiently high numbers of bacteria could be present to cause a net increase of bacterial populations in the soil from season to season. Examples: 1. Agrobacterium tumefaciens, which causes crown gall, 2. Ralstonia solanacearum, which causes the bacterial wilt of solanaceous crops 3. Streptomyces scabies, which causes the common scab of potato

II. Soil invaders 1. Most plant pathogenic bacteria, however, can be considered soil invaders. 2. Such bacteria enter the soil in host tissue and, because they have poor ability to compete as saprophytes, persist in the soil either as long as the host tissue resists decomposition by saprophytes or for varying durations afterward, depending on the bacterial species and on the soil temperature and moisture conditions.

Survival of Bacteria

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1. In soil 2. Bacteria live mostly on plant material. 3. Less often they live freely or saprophytically, or in their natural bacterial ooze, which protects them from various adverse factors. II. On plants 1. Bacteria often survive epiphytically, in buds, on wounds, in their exudate, or inside the various tissues or organs that they infect. 2. Bacteria may also survive in or on seeds, other plant parts, or insects found in the soil.

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Dissemination of Plant Pathogenic Bacteria

1. Dissemination of plant pathogenic bacteria carried out primarily by water, insects, other animals, and humans . 2. Even bacteria possessing flagella can move only very short distances on their own power. 3. Rain, by its washing or spattering effect, carries and distributes bacteria from one plant to another and from the soil to the lower parts of plants. 4. Water also separates and carries bacteria on or in the soil to other areas where host plants may be present. 5. Insect, birds, rabbits, and other animals

Identification of Bacteria Plant pathogenic genera of bacteria

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Agrobacterium 1. Rod shaped, 0.8 by 1.5–3 micrometers. 2. Motile by means of one to four peritrichous flagella 3. When growing on carbohydrate-containing media, bacteria produce abundant polysaccharide slime. 4. Colonies are nonpigmented and usually smooth. 5. These bacteria are rhizosphere and soil inhabitants. Clavibacter (Corynebacterium) 1. Cells have the shape of straight to slightly curved rods, 0.5–0.9 by 1.5–4 micrometers. 2. Sometimes they have irregularly stained segments or granules and clubshaped swellings. 3. Bacteria are generally nonmotile, but some species are motile by means of one or two polar flagella. 4. They are gram positive.

Erwinia

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1. Bacteria are straight rods, 0.5–1.0 by 1.0– 3.0 micrometers,. 2. Motile by means of several to many peritrichous flagella. 3. Erwinia are the only plant pathogenic bacteria that are facultative anaerobes. 4. Some Erwinia do not produce pectic enzymes and cause necrotic or wilt diseases (the ―amylovora‖ group),. 5. Erwinia have strong pectolytic activity and cause soft rots in plants (the ―carotovora‖ group).

Pseudomonas

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1. Pseudomonads are straight to curved rods, 0.5– 1 by 1.5–4 micrometers. 2. Motile by means of one or many polar flagella. 3. Many species are common inhabitants of soil or of freshwater and marine environments. 4. Plant pathogenic Pseudomonas species (e.g., P. syringae), when grown on a medium of low iron

content, produce yellow-green, diffusible, fluorescent pigments.

Ralstonia

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1.Until very recently classified as Pseudomonas. 2.These resemble the latter in most respects with the important difference that its cells do not produce fluorescent pigments.

Xanthomonas

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1. Cells are straight rods, 0.4–1.0 by 1.2–3 micrometers,. 2. Motile by means of a polar flagellum. 3. Growth on agar media is usually yellow,. 4. Most are slow growing. 5. All species are plant pathogens and are found only in association with plants or plant materials.

Streptomyces

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1. Soil inhabitants 2. Gram positive 3. Shape of slender, branched hyphae without cross walls, 0.5–2 micrometers in diameter. 4. Aerial mycelium forms chains of three to many spores. At maturity. 5. On nutrient media, colonies are small (1–10 millimeters in diameter) at first with a rather smooth surface but later with a weft of aerial mycelium that may appear granular, powdery, or velvety. 6. Streptomyces produce a wide variety of pigments that color the mycelium and the substrate. 7. They also produce one or more antibiotics active against bacteria, fungi, algae, viruses, protozoa, or tumor tissues.

Xylella

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1. Cells are mostly single, straight rods, 0.3 by 1–4 micrometers, producing long filamentous strands under some cultural conditions. 2. Colonies are small, with smooth or finely undulated margins. 3. Nutritionally fastidious, Xylella require specialized media; their habitat is xylem of plant tissue. 4. Gram negative, nonmotile, aflagellate, strictly aerobic, and nonpigmented.

Identification of bacteria belonging to the rod-shaped genera

1.

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Size, shape, structure, color, number and arrangement of flagella on the bacterial cell 2. Chemical composition. 3. Staining. 4. Serological reactions. 5. Ability to use certain nutrients. 6. Enzymatic action. 7. Pathogen city to plants. 8. Growth on selective media. 9. Substances that they can or cannot use for food 10. kinds of enzymes produced on certain media.

Gram’s staining reaction

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1. Chemical compositions of certain substances in bacterial cells can be detected with specific staining techniques. 2. Information about the presence or absence of such substances is used for the identification of bacteria. 3. Gram’s staining reaction differentiates bacteria into grampositive and gram-negative types. 4. In this reaction,bacteria fixed on a glass slide are treated with a CRYSTAL VIOLET solution for 30 seconds, rinsed gently, treated with IODINE SOLUTION, and rinsed again with water and then alcohol. 5. Gram-positive bacteria retain the violet-iodine stain combination because it forms a complex with certain components of their cell wall and cytoplasm. 6. Gram-negative bacteria have no affinity for the stain combination, which is therefore removed by the alcohol rinse, and bacteria remain as nearly invisible as before.

Phytopathogenic bacteria Gram +ve Vs Gram -ve

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1.Gram positive: Clavibacter and Curtobacterium, Arthrobacter, Bacillus, and Rhodococcus, are 2.Gram Negative : Agrobacterium, Erwinia, Pseudomonas, Ralstonia, Xanthomonas, and Xylella .

Symptomatology

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1. External symptom: • •

A plant appears wilted Spots on the leaves are surrounded by a halo.

II. Internal symptoms: 1. Wilted plant shows discoloration of the vascular system. 2. So the wilt is caused by a pathogen and not by drought. 3. Further examination of the wilted plant can be done by placing a freshly cut wilted stem in a tube or dish of water and looking for appearance or lack of a cloudy diffusate from the stem which, if present, indicates that the wilt is cause by BACTERIA rather than a fungus or anything else. 4. If further work is needed, then one cultures the bacteria and observes the shape, size, color, and so on of its culture . 5. To make sure that the isolated bacterium is the pathogen rather than a saprophyte, a series of dilutions of the bacteria is injected into the leaves of a nonhost, such as tobacco. 6. If nonpathogenic, the leaves show no change at the points of injection. If the bacterium is pathogenic, however, it produces a hypersensitive response (dead tissues around the points of injection) .

Use of Selective Nutrient Media

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1. An excellent method of isolation and identification of bacteria obtained from plant tissues or soil is through the use of selective nutrient media. 2. Selective media contain nutrients that promote the growth of a particular type of bacterium while at the same time contain substances that inhibit the growth of other types of bacteria.

3. Positive identification usually requires more than one subculturing on selective media because seldom does only one bacterium grow on a selective medium.

Serological methods

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1.Serological methods, especially those employing antibodies labeled with a fluorescent compound (immunofluorescent staining), are used for the quick and fairly accurate identification of bacteria and have gained popularity in recent years. 2. Use of serological methods is becoming widespread in plant pathology as the availability of species-specific and pathovar-specific antisera increases.

Quick distinction and identification of bacteria

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Fatty acid profile analysis 1. In the past 10 years, fairly quick distinction and identification of bacterial genera, species, and, in some cases, lower subdivisions have been made by extraction and comparison of the FATTY ACIDS present in the bacterial cell membranes 2. Same bacteria grown under identical conditions also produce identical membrane proteins and identical enzymes and isoenzymes. Isolation and comparison of such structural proteins or enzymes are also used to identify bacteria

Profiles of DNA bands

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1. Bacteria are detected, identified, and their genetic relatedness measured by comparison of the profiles of DNA bands obtained on a separation gel following digestion (cutting up) of the bacterial chromosomal DNA with certain restriction endonucleases. 2. Enzymes cut the DNA only at certain nucleotide sequences and release defined sets of DNA fragments called restriction fragment length polymorphisms (RFLPs). 3. RFLP profiles may be characteristic of the bacterium and, therefore, can be used to identify the bacterium.

DNA probes

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1. DNA probes are used to detect and identify bacteria. 2. The probe consists of a comple-mentary segment of a part of the DNA of the bacterium that exists only or primarily in that kind of bacterium, e.g., DNA of a specific toxin gene or a virulence gene of the bacterium. 3. A radioactive element or a colorproducing substance is attached to the DNA probe. 4. Bacteria to be tested with the probe are treated so that their DNA is released onto a nylon membrane or filter and the DNA is then treated with the probe. If the probe finds its complementary DNA on the filter, it reacts (hybridizes) with it and stays on the filter even after washing. 5. The presence of the probe is detected by its radioactive element or the color-producing (chromogenic) compound attached to it; a positive hybridization signal, of course, indicates the identity of the bacterium tested.

Polymerase Chain Reaction (PCR) Technique

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1. Availability of the polymerase chain reaction (PCR) technique, by which one or a few strands of DNA can be multiplied indefinitely to millions of copies. 2. It has made possible the detection, through a DNA probe, of the presence of one or a few bacteria in or on a seed or transplant or in a mixture of bacteria obtained from a plant or from the soil

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Symptoms Caused by Bacteria

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1. Plant pathogenic bacteria cause leaf spots and blights, soft rots of fruits, roots, and storage organs, wilts, overgrowths, scabs, and cankers. 2. Any given type of symptom can be caused by bacterial pathogens belonging to several genera, and each genus may contain pathogens capable of causing different types of diseases. 3. Species of Agrobacterium, however, can cause only overgrowths or proliferation of organs. However, overgrowths can also be caused by certain species of Rhodococcus and Pseudomonas. 4. Plant pathogenic species of Streptomyces cause only scabs or lesions of belowground crops. 5. Species of Rhizobium and the related genera Azorhizobium and Bradyrhizobium are gram-negative, soil-inhabiting bacteria that induce the formation of nodules on the roots of legume plants, but these bacteria are beneficial rather than pathogenic to the plant because they fix nitrogen that is used by the plants. Parts of the DNA of the three latter genera are nearly identical to parts of the DNA of Agrobacterium bacteria.

Control of Bacterial Diseases of Plants

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1. Bacterial diseases of plants are usually very difficult to control. 2. Frequently, a combination of control measures is required to combat a given bacterial disease. 3. Using only healthy seeds or transplants. 4. Sanitation practices aiming at reducing the inoculums in a field . 5. Adjusting fertilizing and watering . 6. Crop rotation 7. Use of crop varieties resistant . 8. Resistant varieties, supplemented with proper cultural practices and chemical applications, are the most effective means of controlling bacterial diseases.

Soil and Seed Management

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1. Soil infested with phytopathogenic bacteria can be sterilized with steam or electric heat and with chemicals such as FORMALDEHYDE, but this is practical only in greenhouses and in small beds or frames. 2. Seed, when infested superficially, can be disinfested with sodium hypochlorite or HCl solutions or by soaking it for several days in a weak solution of acetic acid. 3. If seeds can remain for 2 to 3 days in fermenting juices of fruit in which they are borne, bacterial pathogens can be eliminated. 4. When the pathogen is inside the seed coat and in the embryo, such treatments are ineffective. 5. Treating seed with hot water does not usually control bacterial diseases because of the relatively high thermal death point of the bacteria, but treatment at 52°C for 20 minutes often considerably reduces the number of infected seeds.

Chemicals control

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1. Use of chemicals to control bacterial diseases has been, generally, much less successful than the chemical control of fungal diseases. 2. Of the chemicals used as foliar sprays, copper compounds give the best results. 3. Bordeaux mixture, fixed coppers, and cupric hydroxide are used most frequently for the control of bacterial leaf spots and blights. 4. Bacterial strains resistant to copper fungicides, however, are quite common. 5. Zineb, Maneb, Or Mancozeb mixed with copper compounds is used for the same purpose, especially on young plants that may be injured by the copper compounds.

Antibiotics

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1. Antibiotics have been used against certain bacterial diseases with mixed results. 2. Some antibiotics are absorbed by the plant and are distributed systemically. 3. They can be applied as sprays or as dips for transplants. 4. The most important antibacterial antibiotics in agriculture are formulations of STREPTOMYCIN or of STREPTOMYCIN AND OXYTETRACYCLINE. 5. Unfortunately, bacterial races resistant to antibiotics develop soon after widespread application of antibiotics; in addition, no antibiotics are permitted on edible plant produce.

Biological control

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1. Successful practical biological control of the bacterial plant disease crown gall has been obtained by treating seeds or nursery stock with bacteriocinproducing antagonistic strains of Agrobacterium. 2. Treatment of tubers, seeds, and so on with antagonistic bacteria and spraying of aerial plant parts with bacteria antagonistic to the pathogen have given control of various diseases under experimental conditions but have been less successful in practice.

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THANKS