Plant Pathology – agriinfo.in https://agriinfo.in Sun, 14 Apr 2019 13:11:12 +0000 en-US hourly 1 https://wordpress.org/?v=5.1.1 Classification of Phytopathogenic Bacteria https://agriinfo.in/classification-of-phytopathogenic-bacteria-1816/ https://agriinfo.in/classification-of-phytopathogenic-bacteria-1816/#respond Sat, 19 May 2018 11:57:33 +0000 http://agriinfo.in/index.php/2018/05/19/classification-of-phytopathogenic-bacteria/ Classification of Phytopathogenic Bacteria According to the 8 th Ediction of Bergy’s Manual Phytopathogenic bacteria are classified as: Division II:  Sctobacteria (indifferent to light) Class I: The Bacteria Part: Gram negative aerobic rods and cocci. Family: Pseudomonaceae Genera: 1. Pseudomonas 1. Pseudomonas: Characters: Cells single, straight or curved rods but not helicie, generally 0.5 X […]

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Classification of Phytopathogenic Bacteria

According to the 8 th Ediction of Bergy’s Manual

Phytopathogenic bacteria are classified as:

Division II:  Sctobacteria (indifferent to light)

Class I: The Bacteria

Part: Gram negative aerobic rods and cocci.

Family: Pseudomonaceae

Genera: 1. Pseudomonas

1. Pseudomonas:

Characters:

Cells single, straight or curved rods but not helicie, generally 0.5 X 1.5 to 4 microns in size, motile by polar flagella one or more in number, non spore forming, gram negative; strict aerobes ; chemoorganotrophs; metabolism respiratory, never fermentative.

Species:

1. Pseudomonas solanacearum causes brown rot or wilt of potatoes.
2. Pseudomonas rubrillineans causes red stripe of sugarcane.

2. Xanthomonas:      

Characters:

Cells single, straight rods measuring 0.2 to 0.8 X 0.6 to 2.0 microns ( Usually 0.4X 1.0 microns); Gram –ve , motile by a single polar flagellum; non spore forming; copious extracellular slime produced; strict aerobes; metabolism respiratory, never fermentative ; oxidase negative and catalase positive; bacterial colonies yellow coloured due to production of Xanthomonadin pigment.

Species:

1. X. campestris pv. Citri causes citri cause citrus canker.

2. X. campestris pv. Malvacearum causes angular leaf sport and black arm of cotton.

3. X. campestris pv. Oryzae causes bacterial leaf blight of rice.

Family: Rhizobiaceae

Genus: Agrobacterium

Character:

The bacteria are rod shaped, 0.8 X 15. m. They are motile by means of 1.4 peritrichous flagella, when only one flagellum is present it is more often lateral than polar. When growing on carbohydrate – containing media the bacteria produce abundant polysaccharide slime. The colonies are non-pigmented and usually smooth. These bacteria are rhizosphere and soil inhabitants.

Species:

Agrobacterium tumefaction causes crown gall of pome and stone fruit trees, brambles and grapes.
A. rubi causes cane gall of raspberries and black berries.
A. rhizogenes causes hairy root of apple.

Gram Negative, Facultative Anaerobic Rods
 
Family:
Enterobacteriaceae

Genus: Erwinia

Characters:

Cells predominantly single, straight rods, measuring 0.5 to 1.0 X 1.0 to 3.0 microns; motile (Except E.stewartii and E.dissolves) by peritrichous flagella; gram negative ; grows well on artificial media aerobically as well as anaerobically and produce acid. The genus has three major groups of species.

i) Pectolytic bacteria that cause oft rot.
ii) Those do not cause soft rot but dry necrosis and wilt and
iii) Epiphytes that cause neither soft rot nor necrosis.

Species:

1. E. amylovora causes fire blight of apple.
2. E.tracheiphia causes vascular wilt of cucurbits.
3. E.carotovora pv. atroseptica causes black leg of potato.
4. E. carotovora pv. Carotovora causes soft rot numerous fleshy fruits, vegetables and ornamentals.

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General Characters of Plant Pathogenic Bacteria https://agriinfo.in/general-characters-of-plant-pathogenic-bacteria-1815/ https://agriinfo.in/general-characters-of-plant-pathogenic-bacteria-1815/#respond Wed, 16 May 2018 18:37:56 +0000 http://agriinfo.in/index.php/2018/05/16/general-characters-of-plant-pathogenic-bacteria/ General Characters of Plant Pathogenic Bacteria 1. Almost all plant pathogenic bacteria are rod shaped, the only exception being Streptomyces, which is filamentous. 2. The rod shaped bacteria are more or less short and cylindrical and in young cultures, they range from 0.6 to 3.5 m in length and from 0.5 to 1.0 m in […]

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General Characters of Plant Pathogenic Bacteria

1. Almost all plant pathogenic bacteria are rod shaped, the only exception being Streptomyces, which is filamentous.
2. The rod shaped bacteria are more or less short and cylindrical and in young cultures, they range from 0.6 to 3.5 m in length and from 0.5 to 1.0 m in diameter.
3. In older cultures or at high temperatures, the rods of some species are much longer and they may even appear filamentous.
4. Sometimes deviations from the rod shape in the form of a club , a Y or V shaped, and other branched forms occur, and some bacteria may occasionally occur in pairs or in short chains.
5. The cell walls of bacteria of most species are enveloped by a viscous, gummy material, which may be thin (Slime layer) or may be thick, forming a relatively large mass around the cell (Capsule).
6. Most plant pathogenic bacteria are equipped with delicate, thread like flagella.
7. In some bacterial species each bacterium has only the flagellum, others , have a tuff of flagella at one end of the cell (Polar flagella); some have a single flagellum or a tuft of flagella at each end, and till others have peritrichous flagella, that is, distributed over the entire surface of the cell.
8. In the filamentous Streptomyces species, the cells consist of non septate branched threads, which usually have a spiral formation and produce conidia in chains on aerial hyphae.
9. Single bacterium appears hyaline or yellowish white under the compound microscope.
10. Bacteria grow and produce colonies on solid medium.
11. Colonies of different species may vary in size, shape, form of edges, elevation and colour, and are sometimes characteristics of a given species.
12. Bacterial cells have thin, relatively tough, and somewhat rigid cell walls.
13. All the material inside the cell wall constitutes the protoplast.
14. The nuclear material consist of a large circular chromosome composed DNA and appear as spherical, ellipsoidal or dumbbell shaped body within the cytoplasm.
15. Often bacteria also have single or multiple copies of addition smaller circular chromosomes called ‘Plasmids’ that can move or be moved between bacteria or between bacteria and plants as for example in the crown gall disease.
16. Rod shaped Phytopathogenic bacteria reproduce by the asexual process known as binary fission or fission. Under favourable conditions bacteria may divide every 20 minutes.
17. Almost all plant pathogenic bacteria develop mostly in the host plant as parasites and partly in plant debris or in the soil as saprophytes.

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Factors affecting microbial flora of the Rhizosphere / Rhizosphere Effect https://agriinfo.in/factors-affecting-microbial-flora-of-the-rhizosphere-rhizosphere-effect-152/ https://agriinfo.in/factors-affecting-microbial-flora-of-the-rhizosphere-rhizosphere-effect-152/#respond Tue, 15 May 2018 22:01:22 +0000 http://agriinfo.in/index.php/2017/04/13/factors-affecting-microbial-flora-of-the-rhizosphere-rhizosphere-effect/ Factors affecting microbial flora of the Rhizosphere / Rhizosphere Effect The most important factors which affect / influence the microbial flora of the rhizosphere or rhizosphere effect are: soil type & its moisture, soil amendments, soil PH, proximity of root with soil, plant species, and age of plant and root exudates. A. Soil type and […]

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Factors affecting microbial flora of the Rhizosphere / Rhizosphere Effect

The most important factors which affect / influence the microbial flora of the rhizosphere or rhizosphere effect are: soil type & its moisture, soil amendments, soil PH, proximity of root with soil, plant species, and age of plant and root exudates.

A. Soil type and its moisture: In general, microbial activity and population is high in the rhizosphere region of the plants grown in sandy soils and least in the high humus soils, and rhizosphere organisms are more when the soil moisture is low. Thus, the rhizosphere effect is more in the sandy soils with low moisture content.

B. Soil amendments and fertilizers: Crop residues, animal manure and chemical fertilizers applied to the soil cause no appreciable effect on the quantitative or qualitative differences in the microflora of rhizosphere. In general, the character of vegetation is more important than the fertility level of the soil.

C. Soil PH/ Rhizosphere PH: Respiration by the rhizosphere microflora may lead to the change in soil rhizosphere PH. If the activity and population of the rhizosphere microflora is more, then the PH of rhizosphere region is lower than that of surrounding soil or non-rhizosphere soil. Rhizosphere effect for bacteria and protozoa is more in slightly alkaline soil and for that of fungi is more in acidic soils.

D. Proximity of root with Soil: Soil samples taken progressively closer to the root system have increasingly greater population of bacteria, and actinomycetes and decreases with the distance and depth from the root system. Rhizosphere effect decline sharply with increasing distance between plant root and soil.

E. Plant Species: Different plant species inhabit often some what variable microflora in the rhizosphere region. The qualitative and quantitative differences are attributed to variations in the rooting habits, tissue composition and excretion products. In general, legumes show / produce a more pronounced rhizosphere effect than grasses or cereals. Biennials, due to their long growth period exert more prolonged stimulation on rhizosphere effect than annuals.

F. Age of Plant: The age of plant also alter the rhizosphere microflora and the stage of plant maturity controls the magnitude of rhizosphere effect and degree of response to specific microorganisms. The rhizosphere microflora increases in number with the age of the plant and reaching at peak during flowering which is the most active period of plant growth and metabolism. Hence, the rhizosphere effect was found to be more at the time of flowering than in the seedling or full maturity stage of the plants. The fungal flora (especially, Cellulolytic and Amylolytic) of the rhizosphere usually increases even after fruiting and the onset of senescence due to accumulation of moribund tissue and sloughed off root parts / tissues: whereas, bacterial flora of the rhizosphere decreases after the flowering period and fruit setting.
 
G. Root / exudates /excretion: One of the most important factors responsible for rhizosphere effect is the availability of a great variety of organic substances at the root region by way of root exudates/excretions. The quantitative and qualitative differences in the microflora of the rhizosphere from that of general soil are mainly due to influences of root exudates. The spectrum of chemical composition root exudates varies widely, and hence their influence on the microflora also varies widely.

Root exudates are composed of the chemical substances like:

Sr. No

Root Executes

Chemical Substances

1

Amino Acids

All naturally occurring amino acids.

2

Organic acids

Acetic, butyric, citric, fumaric, lactic, malic, propionic, succinic etc.

3

Carbohydrates / sugars

Arabinose, fructose, galactose, glucose, maltose, mannose, oligosaccharides, raffinose, ribose, sucrose, xylose etc.

4

Nucleic acid derivatives

Adenine, cystidine, guanine, undine

5

Growth factors (phytohormones)

Biotin, choline, inositol, pyridoxine etc

6

Vitamins

Thiamine, nicotinic acid, biotin etc

7

Enzymes

Amylase, invertase, protease, phosphatase etc.

8

Other compounds

Auxins, glutamine, glycosides, hydrocyanic acid peptides, Uv-absorbing compounds, nematode attracting factors, spore germination stimulators, spore inhibitors etc.

The nature and amount of chemical substances thus exuded are dependent on the species of plant, plant age, inorganic nutrients, and temperature, light intercity, O2 / CO2 level, root injury etc. Another source of nutrients for the microorganisms in the rhizosphere region is the sloughed off root epidermis which exert selective stimulation effect on some specific groups of microorganisms. For instance, glucose and amino acids in the exudates readily attract Gram-negative rods which predominantly colonize the roots. Sugars and amino acids in the root exudates stimulate the germination of chlamydospores and other resting spores of fungi; stimulation effect of root exudates on plant pathogenic fungi, nematodes is also well known.

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Gram Positive, Irregular Rods and Filamentous Bacteria https://agriinfo.in/gram-positive-irregular-rods-and-filamentous-bacteria-1817/ https://agriinfo.in/gram-positive-irregular-rods-and-filamentous-bacteria-1817/#respond Tue, 08 May 2018 00:39:17 +0000 http://agriinfo.in/index.php/2018/05/08/gram-positive-irregular-rods-and-filamentous-bacteria/ Gram Positive, Irregular Rods and Filamentous Bacteria a) Irregular Rods: Corynebacterium (Plant Pathogenic) Genera: 1. Curtobacterium 2. Clavibacter 1. Characters of Curtobacterium Cells small short rods, coccoid cells found in old cultures, weakly gram negative, frequently old cell lose Gram Positivity generally motile by lateral flagella; cell multiplication by bending type of cell division; pleomorphism […]

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Gram Positive, Irregular Rods and Filamentous Bacteria

a) Irregular Rods: Corynebacterium (Plant Pathogenic)

Genera:

1. Curtobacterium
2. Clavibacter

1. Characters of Curtobacterium

Cells small short rods, coccoid cells found in old cultures, weakly gram negative, frequently old cell lose Gram Positivity generally motile by lateral flagella; cell multiplication by bending type of cell division; pleomorphism only slight; major cell wall amino acid is ornithine; G + C content of the DNA ranges form 66 to 71 moles per unit.

Species:

Curtobacterium flaccumfaciens pv. Flaccumfaciens (Hedges) Collins and Jones (Corynebacterium flaccumfaciens Dowson) causes vascular wilt of beans.

2. Characters of Clavibacter:

The cells are Gram positive; non avoid fast plemorphic rods, often arranged at an angle to give V-formation as a result of snapping or bending type of cells division, no coccoid cells are seen; non endospore forming; non motile; strict aerobes; nutritionally exacting; nitrate not reduced; cell wall peptidoglycan contains diaminobutyric acid; G+C content of the DNA is 70 +-5 moles per cent.
 
Species:

Clavibactor michinganese subspecies michiganese (Smith) Davis et al. (Corynebacterium michiganese or C. michiganese pv. michiganese or C. michiganese sub species michiganese causes canker of tomato and chilli.

b) Filamentous Bacteria:

Order: Actinomycetales

Family: Streptomycetaceae

Genus: Streptomyces

Characters:

Slender, coenocytic filaments, 0.5 to 2.0 microns in diameter; aerial mycelium at maturity forms chains of three to many spores; cell walls contain diaminopimelic acid; Gram Positive; aerobic; heterotrophs; generally reduce nitrates, on isolation colonies are small, 1 to 10 mm in diameter, descrete and lichenoid , leathery or butyrous, initially relatively smooth but later develop a weft of aerial mycelium that may appear granular, powdery, velvety or floccosed.

Species:

S. scabis causes common scab of potato.

 

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Flowering Plant Parasites (Phanerogams) https://agriinfo.in/flowering-plant-parasites-phanerogams-1949/ https://agriinfo.in/flowering-plant-parasites-phanerogams-1949/#respond Mon, 07 May 2018 09:45:57 +0000 http://agriinfo.in/index.php/2018/05/07/flowering-plant-parasites-phanerogams/ Flowering Plant Parasites (Phanerogams) Most of the diseases are caused by fungi bacteria and viruses. There are few seeds plants called flowering parasites (Phanerogams) which are parasitic on living plants. Some of these attack roots of the host, while some parasites on stem. Some are devoid of chlorophyll and entirely dependent on their host for […]

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Flowering Plant Parasites (Phanerogams)

Most of the diseases are caused by fungi bacteria and viruses. There are few seeds plants called flowering parasites (Phanerogams) which are parasitic on living plants. Some of these attack roots of the host, while some parasites on stem. Some are devoid of chlorophyll and entirely dependent on their host for food supply, while other have chlorophyll and obtain only mineral constituents of food from host by drawing nutrition and water they are called as Holoparasites or complete or total parasite. They have haustoria as absorbing organs, which are sent deep into the vascular bundle of the host to draw nutrients, water and minerals.

Flowering Plant Parasites: There are two types of parasites.

1) Root Parasites:

i) Striga (Partial root parasite)

ii) Orobanche (Complete root parasite) 

2) Stem Parasites:

i) Dodder (Cuscuta) (Complete stem parasite)                                

ii) Loranthus (Partial Stem parasite)

1. Root Parasites:

1. Total or Complete or Holoparasite:

Orobanche (Broom rape or Tokra)
 
It is annual flashy flowering plant growing to height of about 15-50 cm long, yellow or brownish colour and covered by small thin scaly leaves. Flowers appears in the axil of leaves are white or tubular. Fruits appears in the axil of leaves are white or tubular. Fruits are capsule containing and seeds are very small, black in colour remain viable for several years. The hausteria of parasite penetrates into the roots of hosts and draw its nourishment. The growth of the plant is retarded, may die some times. It attacks tobacco, tomato, brinjal, cabbage, cauliflower.

2. Hemi Partial or Semi Root Parasite:

Striga (Witch Weed or Turfula or Talop)
Family Scrophulariaceae

It is a small plant with bright green leaves grows upto height 20-60 cm leaves beers chlorophylls and developed in clusters of 10-20 % host plant. They are obligate parasites therefore, do not obtain all their nutrient from their host root. Flowers are pink in colour, seed are very minute and produce in grate number 5000 to 100000 seeds plant per years. One flower contain 1200-1500 seeds and remains viable upto 12-40 years. Dissemination takes place with rain water, flood, wind and irrigation water. It cause yellowing and wilting of host leaves. It attacks sugarcane jowar, Maize, cereals and millets.

b. Stem Parasites:

1. Total or Complete or Holoparasite:

Cuscuta or dodder (Amarvel, Lovevine) Family cuscutaceae.
Genus – Cuscuta     
It is non chlorophyllous, leaf less parasitic seed plant.

It is yellow pink or orange in colour and attached to the host. They do not bear leaves but bear minute function less scale leaves is produces flower and fruits. Flower are white, pink or yellowish in colour and found in clusters. Seed are form in capsules. A single plant may be produce 3000 seeds.

The first appearances of parasites is noticed as thread like leaf less stem which devoid of green pigment and twine around the stem or leaves of the host. When stem of parasitic plant comes in contact with host, the minute root like organs. i.e. hausteria penetrates into the host and absorbs. When the relation ship of the host is firmly established, the dodder plant looses the contact from soil.

These affect plant get weakened and yield poorly the seeds spread by animals, water and implements and remain viable when condition are unfavorable.

It attacks berseem alfalfa, clover, flax, onion, potato, ornamental and hedge plants.

2. Partial, Semi or Hemi Stem Parasites:

Loranthus
Family- Loranthaceae.
It is a partial parasite of tree trunks and branches with brown stem, dark green leaves but no roots.

1. Stem is thick and flattened of the node, appear in clusters at the point of attack which can be easily spotted on the trees.

2. At the point of attachment with the tree, it shows swellings or tumourous growth where the haustoria are produced. It produces flowers which are long, tabular, greenish, white or red colour and found in clusters. It produces fleshy berries with single seed.

3. The affected host plant becomes stunted in growth and dispersal of seed is mostly through the birds and animals. It attacks mango, citrus, apple, guava.

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Nitrogen Cycle: Nitrification & Nitrate Reduction https://agriinfo.in/nitrogen-cycle-nitrification-nitrate-reduction-159/ https://agriinfo.in/nitrogen-cycle-nitrification-nitrate-reduction-159/#respond Fri, 04 May 2018 14:37:11 +0000 http://agriinfo.in/index.php/2017/10/30/nitrogen-cycle-nitrification-nitrate-reduction/ Nitrogen Cycle: Nitrification & Nitrate Reduction Several biochemical steps involved in the nitrogen cycle are: 1. Proteolysis 2. Ammonification 3. Nitrification 4. Nitrate reduction and 5. Denitrification. 3. Nitrification: Ammonical nitrogen / ammonia released during ammonification are oxidized to nitrates and the process is called “nitrification”. Soil conditions such as well aerated soils rich in […]

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Nitrogen Cycle: Nitrification & Nitrate Reduction

Several biochemical steps involved in the nitrogen cycle are:
1. Proteolysis
2. Ammonification
3. Nitrification
4. Nitrate reduction and
5. Denitrification.

3. Nitrification:
Ammonical nitrogen / ammonia released during ammonification are oxidized to nitrates and the process is called “nitrification”. Soil conditions such as well aerated soils rich in calcium carbonate, a temperature below 30 ° C, neutral PH and less organic matter are favorable for nitrification in soil.

Nitrification is a two stage process and each stage is performed by a different group of bacteria as follows.

Stage I: Oxidation of ammonia of nitrite is brought about by ammonia oxidizing bacteria viz. Nitrosomnonas europaea, Nitrosococcus nitrosus, Nitrosospira briensis, Nitrosovibrio and Nitrocystis and the process is known as nitrosification. The reaction is presented as follows.

2 NH3 + 1/2O2 ——————-> NO2 + 2 H + H2 O
 Ammonia                                         Nitrite    

Stage II: In the second step nitrite is oxidized to nitrate by nitrite-oxidizing bacteria such as Nitrobacter winogradsky .Nitrospira gracilis, Nirosococcus mobiiis etc, and several fungi (eg. Penicillium, Aspergillus) and actinomycetes (eg. Streptomyces, Nocardia).

NO2 (-)   +   ½ O2 ———————->   NO3
Nitrite ions                                             Nitrate ions 

The nitrate thus, formed may be utilized by the microorganisms, assimilated by plants, reduced to nitrite and ammonia or nitrogen gas or lost through leaching depending on soil conditions. The nitrifying bacteria (ammonia oxidizer and nitrite oxidizer) are aerobic gram-negative and chemoautotrophic and are the common inhabitants of soil, sewage and aquatic environment.

4. Nitrate Reduction:
Several heterotrophic bacteria (E. coli, Azospirillum) are capable of converting nitrates to nitrites and nitrites to ammonia. Thus, the process of nitrification is reversed completely which is known as nitrate reduction. Nitrate reduction normally occurs under anaerobic soil conditions (water logged soils) and the overall process is as follows:
                               Nitrate      
HNO3 + 4 H2 ——————–> NH4 + 3 H20
 Nitrate                  Reductase         ammonium

Nitrate reduction leading to production of ammonia is called "dissimilatory nitrate reduction" as some of the microorganisms assimilate ammonium for synthesis of proteins and amino acid.

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Rhizosphere Concept and It’s Historical Background https://agriinfo.in/rhizosphere-concept-and-its-historical-background-150/ https://agriinfo.in/rhizosphere-concept-and-its-historical-background-150/#respond Thu, 26 Apr 2018 21:05:51 +0000 http://agriinfo.in/index.php/2017/01/25/rhizosphere-concept-and-its-historical-background/ Rhizosphere Concept and It’s Historical Background The root system of higher plants is associated not only with soil environment composed of inorganic and organic matter, but also with a vast community of metabolically active microorganisms. As living plants create a unique habitat around the roots, the microbial population on and around the roots is considerably […]

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Rhizosphere Concept and It’s Historical Background

The root system of higher plants is associated not only with soil environment composed of inorganic and organic matter, but also with a vast community of metabolically active microorganisms. As living plants create a unique habitat around the roots, the microbial population on and around the roots is considerably higher than that of root free soil environment and the differences may be both quantitative and qualitative.

1. Rhizosphere: It is the zone/region of soil immediately surrounding the plant roots together with root surfaces, or it is the region where soil and plant roots make contact, or it is the soil region subjected to influence of plant roots and characterized by increased microbial.

2. Rhizoplane: Root surface along with the closely adhering soil particles is termed as rhizoplane.

Historical Background:

Term "Rhizosphere" was introduced for the first time by the German scientist Hiltner (1904) to denote that region of soil which is subjected to the influence of plant roots. The concept of "Rhizosphere Phenomenon" which shows the mutual interaction of roots and microorganisms was came into existence with the work of Starkey et al (1929), Clark (1939) and Rauath and Katznelson (1957).

N. V. Krassinikov (1934) found that free living nitrogen-fixing bacteria, Azotobacter were unable to grow in the wheat rhizosphere.

Starkey (1938) examined the rhizosphere region of some plant species and demonstrated the effect of root exudates on the predominance of bacterial population in particular and other soil microorganisms in general in the rhizosphere region. Thus, he put forth the concept of "Rhizosphere effect / phenomenon" for the first time.

F E Clark (1949) introduced / coined the term "Rhizoplane” to denote the root surface together with the closely adhering soil particles.

R. I. Perotti (1925) suggested the boundaries of the rhizosphere region and showed that it was bounded on one side by the general soil region (called as Edaphosphere) and on the other side by the root tissues (called Histosphere).

G. Graf and S. Poschenrieder (1930) divided the rhizosphere region into two general areas i.e. outer rhizosphere and inner rhizosphere for the purpose of describing the same site of microbial action.

H. Katznelson (1946) suggested the R:S ratio i.e. the ratio between the microbial population in the rhizosphere (R) and in the soil (S) to find out the degree or extent of plant roots effect on soil microorganisms. R: S ratio gives a good picture of the relative stimulation of the microorganisms in the rhizosphere of different plant species.

R: S ratio is defined as the ratio of microbial population per unit weight of rhizosphere soil (R), to the microbial population per unit weight of the adjacent non-rhizosphere soil (S)

A. G. Lochhead and H. Katznelson (1940) examined in detail the qualitative differences between the microflora of the rhizosphere and microflora of the non-rhizosphere region and reported that gram-negative, rod shaped and non-spore forming bacteria are abundant in the rhizosphere than in the non-rhizosphere soil

C. Thom and H. Humfeld (1932) found that corn roots in acidic soils yielded predominantly Trichoderma while roots from alkaline soils mainly contained Penicillium.

M J. Timonin (1940) reported some differences in the fungal types and population in the rhizosphere of cereals and legumes. R: S ratio of fungal population was believed to be narrow in most of the plant species, usually not exceeding 10.

E. A. Peterson and others (1958) reported that the plant age and soil type influence the nature of fungal flora in the rhizosphere, and the number of fungal population gradually increases with the age of plant.

M. Adati (1932) studied many crops and found that though actinomycetes were relatively less stimulated than bacteria, but in some cases the R: S ratio of actinomycetes was as high as 62.

R. Venkatesan and G. Rangaswami (1965) studied the rhizosphere effect in rice plant on bacteria, actinomycetes and fungi and reported that (i) for actinornycetes R: S was more (ranging from 0 to 25) depending on the age of plant roots and the dominant genera reported were Nocardia, (ii) R:S ratio reduced with the depth of soil.

E. A. Gonsalves and V. S. Yalavigi (1960) reported the presence of greater number of algae in the rhizosphere

J. W. Rouatt et al reported positive rhizosphere effect on protozoa, but a negative effect on algae in wheat plants.

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Composition of Organic Matter https://agriinfo.in/composition-of-organic-matter-164/ https://agriinfo.in/composition-of-organic-matter-164/#respond Thu, 26 Apr 2018 06:09:12 +0000 http://agriinfo.in/index.php/2016/09/30/composition-of-organic-matter/ Composition of Organic Matter Soil organic matter plays important role in the maintenance and improvement of soil properties. It is a dynamic material and is one of the major sources of nutrient elements for plants. Soil organic matter is derived to a large extent from residues and remains of the plants together with the small […]

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Composition of Organic Matter

Soil organic matter plays important role in the maintenance and improvement of soil properties. It is a dynamic material and is one of the major sources of nutrient elements for plants. Soil organic matter is derived to a large extent from residues and remains of the plants together with the small quantities of animal remains, excreta, and microbial tissues. Soil organic matter is composed of three major components i.e. plants residues, animal remain and dead remains of microorganisms. Various organic compounds are made up of complex carbohydrates, ( Cellulose, hemicellulose, starch) simple sugars, lignins, pectins, gums, mucilages, proteins, fats, oils, waxes, resins, alcohols, organic acids, phenols etc. and other products. All these compounds constituting the soil organic matter can be categorized in the following way.

Organic Matter (Undecomposed)

  1. Organic:

  • Nitrogenous:

    1. Water Soluble eg. Nitrates, ammonical compounds, amides, amino acids etc.

    2. Insoluble eg. Proteins nucleoproteins, peptides, alkaloids purines, pyridines chitin etc.

  • Non Nitrogenous:

    • Carbohydrates eg. Sugars, starch, hemicellulose, gums, mucilage, pectins, etc.

    • Micellaneous: eg. Lignin, tannins, organic acid, etc.

    • Ether Solube: eg. Fats, oils, wax etc.

  1. Inorganic  

The organic complex / matter in the soil is, therefore made up of a large number of substances of widely different chemical composition and the amount of each substance varies with the type, nature and age of plants. For example cellulose in a young plant is only half of the mature plants; water-soluble organic substances in young plants are nearly double to that of older plants. Among the plant residues, leguminous plants are rich in proteins than the non-leguminous plants. Grasses and cereal straws contain greater amount of cellulose, lignin, hemicelluloses than the legumes and as the plant gets older the proportion of cellulose, hemicelluloses and lignin gets increased. Plant residues contain 15-60% cellulose, 10-30 % hemicelluslose, 5-30% lignin, 2-15 % protein and 10% sugars, amino acids and organic acids. These differences in composition of various plant and animal residues have great significance on the rate of organic matter decomposition in general and of nitrification and humification (humus formation) in particular. The end products of decomposition are CO2, H2O, NO3, SO4, CH4, NH4, and H2S depending on the availability of air.

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Rhizosphere in relation to Plant Pathogens https://agriinfo.in/rhizosphere-in-relation-to-plant-pathogens-155/ https://agriinfo.in/rhizosphere-in-relation-to-plant-pathogens-155/#respond Mon, 09 Apr 2018 15:05:00 +0000 http://agriinfo.in/index.php/2016/07/19/rhizosphere-in-relation-to-plant-pathogens/ Rhizosphere in relation to Plant Pathogens Plant root exudates influence pathogenic fungi, bacteria and nematodes in various ways. The effect may be in the form of attraction of fungal zoospores, or bacterial cells towards the roots; stimulation of germination of dormant spores and hatching of cysts of nematodes. Root exudates may contain inhibitory substances preventing […]

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Rhizosphere in relation to Plant Pathogens

Plant root exudates influence pathogenic fungi, bacteria and nematodes in various ways. The effect may be in the form of attraction of fungal zoospores, or bacterial cells towards the roots; stimulation of germination of dormant spores and hatching of cysts of nematodes. Root exudates may contain inhibitory substances preventing the establishment of pathogens. The balance between the rhizosphere microflora and plant pathogens and soil microflora and plant pathogens is important in host-pathogenic relationship. In this context, the biochemical qualities of root exudates and the presence of antagonistic micro-organisms plays an important role in the proliferation and survival of root infecting pathogens in soil either through soil fungi stasis, inhibition or antibiosis of pathogens in the rhizosphere.

Some of the most common interactions between plant roots and plant pathogenic microorganisms in the rhizosphere are discussed herewith.

A. Zoospore attraction: Amino acids, organic acids and sugars in the root exudates stimulate the movement and attraction of zoospores towards root of the plants. For example attraction of zoospores has been reported in Phytophthora citrophthora (Citrus roots), P. parasitica (tobacco roots) and Pythium aphanidermatum (pea root).

B. Spore germination: The spores or conidia of many pathogenic fungi such as Rhizoctonia, Fusarium, Sclerotium, Pythium, Phytophthora etc. have been stimulated to germinate by the root exudates of susceptible cultivars of the host plants. There are some reports on the selective stimulation of Fusarium, Pseudomonas and root infecting nematodes in the rhizosphere region of the respective susceptible hosts. This stimulus to germination is especially important to those plant pathogens which are not vigorous competitors and remain in resting stage due to shortage of nutrients or fungistasis. As a rule, germination and subsequent hyphal development are promoted by non host species and also by both susceptible and resistant cultivars of the host plants. The quantity and quality of microorganisms present in the rhizosphere of disease resistant crop varieties are significantly different from those of susceptible varieties.

C. Changes in morphology and physiology of host plant: Changes in the physiology and morphology of host plant influence the rhizosphere microflora through root exudations. Hence, significant changes in the rhizosphere microflora of diseased plants were reported which are attributed to the nature and severity of the disease. Systemic virus diseases cause marked changes in the plant morphology and physiology to drastically alter the rhizosphere microflora.

D. Increase in antagonists activity: Root exudates provide a food base for the growth of antagonistic organisms which plays an important role in controlling / suppressing some of the soil borne plant pathogens. Generally, rhizosphere of the resistant plant varieties harboure moer number of Streptomyces and Trichoderma than that of susceptible varieties. For example in the rhizosphere of pigeon pea varieties resistant to Fusarium udum, the population of Streptomyces was found more which inhibited the growth of the pathogen. High density of Trichoderma viride in the rhizosphere of Tomato varieties resistant to Verticillium wilt has been reported with its ability to reduce the severity of wilt in susceptible plants.

E. Inhibition of pathogen: Root exudates containing toxic substances such as glycosides and hydrocyanic acid may inhibit the growth of pathogens in the rhizosphere. It has been reported that root exudates from resistant varieties of Flax (eg. Bison) excrete a glucoside which on hydrolysis produces hydrocyanic acid that inhibits Fusarium oxysporum, the flax root pathogen. Exudates of resistant pea reduce the germination of spores of Fusarium oxysporum.

In this light, the rhizosphere may be considered as a microbiological buffer zone in which the microflora serves to protect the plants against the attack of the pathogens.

F. Attraction of bacteria and nematodes: Root exudates attracts phytopathogenic bacteria and fungi in the rhizosphere for example Agrobacterium tumefaciens have been reported to be attracted to the roots of the host plants like peas, maize, onion, tobacco, tomato and cucumber.

Host root exudates also influence phytopathogenic nematodes in two ways: (i) though stimulation of egg-hatching process and (ii) attraction of larvae towards plant roots.

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Classification of Fungi – up to Genus https://agriinfo.in/classification-of-fungi-up-to-genus-1969/ https://agriinfo.in/classification-of-fungi-up-to-genus-1969/#respond Wed, 04 Apr 2018 10:59:19 +0000 http://agriinfo.in/index.php/2018/04/04/classification-of-fungi-up-to-genus/ Classification of Fungi – up to Genus G.C. Ainsworth ( 1973) and J.Webster (1980) Division: 1) Myxomycota ( Plasmodium or pseudoplasmodium are present): i) Acrasiomycetes ii) Myxomycetes iii) Myxomycetes iv) Plasmodiophoromycetes 2) Eumycota (Absence of Plasmodium or Pseudoplasmodium) : i) Mastigomycotina ii) Zygomycotina iii) Ascomycotina iv) Basidiomycotina v) Deuteromycotina Classes of Sub Division of Mastigomycotina: […]

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Classification of Fungi – up to Genus

G.C. Ainsworth ( 1973) and J.Webster (1980)

Division:

1) Myxomycota ( Plasmodium or pseudoplasmodium are present):

i) Acrasiomycetes
ii) Myxomycetes
iii) Myxomycetes
iv) Plasmodiophoromycetes

2) Eumycota (Absence of Plasmodium or Pseudoplasmodium) :

i) Mastigomycotina
ii) Zygomycotina
iii) Ascomycotina
iv) Basidiomycotina
v) Deuteromycotina

Classes of Sub Division of Mastigomycotina:

i) Chytridiomycetes
Zoospores posteriorly uniflagellate i.e. Whiplash type flagella

ii) Hyphochytridomycetes
Zoosores anteriorly uniflagellate i.e. tinsel type flagella.

iii) Oomycetes:
Zoospores biflagellate flagella i.e. posterior whiplash and anterior tinsel type.

Classification of Class of Oomycetes:

Orders:

i) Saprolegniales
ii) Leptomitales

iii) Lagenidiales

iv) Peronosporales :

There are two types of parasites:
a. Non obligate parasites or Facultative parasites
Family- Pythiaceae
b. Obligate Parasites (Biotrophs)
Family: Albuginaceae
Genus: Albigo

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