Plant Disease Causal Organisms
Fungi, Bacteria, Actinomycets Protozoa & Virus
Subject:- Plant Pathology
Plant Disease Classification & Fungi:- Click
Crop disease & Causal Organisms:- Click
Soil Bacteria
Bacteria are unicellular microorganisms, the name comes from the Greek word for “rod” designating the usual bacterial shape. The numbers of bacteria in the soil generally exceeded all other microorganisms, although fungi may exceed bacteria in weight.
| Bacteria | Suitable Temperature |
| Psychrophilic (cryophilic) | <15°C |
| Mesophilic | 15-45°C |
| Thermophillic | 45-65°C |
| Thermoduric | >65°C |
Shapes of Bacterial cell
1. Ellipsoidal or spherical:- These are called as Cocci (Single: Cocous)
2. Cylindrical or rod shaped:- These are called as Bacili (Single:Bacilius)
3. Spiral or helical:- These are callad as spirila (Single: spirillum)
Classification of Bacteria on the basis of Nutritional requirement
(1) Autotrophs bacteria
(2) Heterotrophs bacteria
(1) Autotrophs bacteria
They use carbon from atmosphere. These are two types
(A) Photoautotrops (Photosynthetic):- Derive energy from Sunlight. these are also divided into two parts
(a) Photoorganotrophs:- use organic substances as nutrition.
(b) Photolithotrophs:- use light as energy source and inorganic substances as their nutrition. these is classified into three parts.
(i) Purple Non sulfuric bacteria:- e.g. Rhodospirillum rubum.
(ii) Green sulphur bacteria (Chlorobiaceae):- e.g. Chlorobium limicola.
(iii) Purple sulphur Bacteria (Thirohodaceae):- e.g. Chromatium spp
(B) Chemoautotrophs (Chemosynthetic):- Derive energy from chemicals. these are two types
(a) Chemoorganotrophs:- use organic chemicals as nutrition.
Examples:- Azotobacter spp.
(b) Chemoautotrophs:- use inorganic chemicals as nutrition.
Examples:- Nitrobactor Spp.
(2) Heterotrophs bacteria
They do not use carbon from atmosphere. These are two types
(1) Nitrogen fixing group:- This bacteria nitrogen fix from atmosphere. These are two types
(A) Symbiotic N2 fixing bacteria:- associated with leguminous plants
Examples:- Rhizobium, Azospirillum
2. Non-symbiotic N2 fixing bacteria:- free living bacteria and do not require a host plant
Examples:- Azotobacter, Clostridium.
(2) Non-nitrogen fixing group:- most dominant soil bacteria and actively associated with the decomposition of organic materials
Classification based on Arrangement of Cocci
(1) Diplococci:- The cocci are arranged in pairs.
Examples:- Streptococcus pneumoniae, Moraxella catarrhalis.
(2) Streptococci:- The cocci are arranged in chains, as the cells divide in one plane.
Examples:-Streptococcus pyogenes, Streptococcus agalactiae
(3) Tetra cocci:- The cocci are arranged in packets of four cells, as the cells divide in two plains.
Examples:- Aerococcus, Pediococcus and Tetragenococcus
(4) Sarcinae:- Devide in three plane but occur in regular form cuboidal group.
Examples:-Sarcina ventriculi, Sarcina ureae, etc.
(5) Staphylococci:- The cocci are arranged in grape-like clusters formed by irregular cell divisions in three plains.
Examples:-:Staphylococcus aureus
Classification based on Arrangement of Bacilli
The cylindrical or rod-shaped bacteria are called ‘bacillus’ (plural: bacilli).
(1) Diplobacilli:- Most bacilli appear as single rods. Diplobacilli appear in pairs after division.
Example of Single Rod: Bacillus cereus
Examples of Diplobacilli: Coxiella burnetii, Moraxella bovis.
(2) Streptobacilli:- The bacilli are arranged in chains, as the cells divide in one plane.
Example:- Streptobacillus moniliformis
(3) Coccobacilli:- These are so short and stumpy that they appear ovoid. They look like coccus and bacillus.
Example:- Haemophilus influenzae, Gardnerella vaginalis, and Chlamydia trachomatis
(4) Palisades:- The bacilli bend at the points of division following the cell divisions, resulting in a palisade arrangement resembling a picket fence and angular patterns that look like Chinese letters.
Example:- Corynebacterium diphtheriae
Actinomycets
- Actinomycets are also called as Ray Fungi. Width of Mycellium is 1 micron.
- Optimum temp range of soil actinomycets is 28-37°C .
- In actinomyoets genus Frankia fix atm nitrogen (N), symbolically in non leguninous plant like species of Casurina by producing root nodules.
Protozoa
- The protozoa are microscopic, Primitive, unicellular, organism which may or may not contain chlorophyll pigments reproduce either sexually or asexulay and belongs to phylum protozoa.
- Protozoa grow up to 30°C and death occurs at 35-40°C.
Virus
- A virus is a chain of nucleic acids (DNA or RNA) which lives in a host cell.
- The nucleic acid makes up 5-40% of the virus and protein making up the remaining 60-95%.
Classification of Virus in respect of insect vector
(1) Non persistent virus:- Also called as stylet born viruses. Viruses are not retained in insect body more than one hour.
Examples:-Papaya mosaic, cow pea mosaic
(2) Persistent virus:- Virus are retained for longer period or whole life of vector.
Examples:-Ground nut rosette
(3) Semi-persistent viruses:-They are non persistent and persistent viruses. Viruses retained in shorter period or slightly longer period. Examples:-Tristeza of citrus. Caulifiower mosaic.
Another Way of Grouping of Viruses
(1) Non circulative virus:- This includes both non persistent (Stylet borne) and semi-persistent viruses. In this group viruses do not circulate in the body of insect. Not required incubation period.
(2) Circulative viruses:-These includes persistent viruses. viruses are circulated from the body of insect. They are required incubation period.
