Plant Pathology Study

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Plant Disease Classification 

Plant pathology

Definition:- Plant pathology or phytopathology consists of three Greek words, the term plant pathology or phytopathology means knowledge of plant disease.

Greek words Means
Phyton plant
pathos ailments or diseases
logus discourse or knowledge
 
 
Disease:- it is abnormal condition in the plant produced by Pathogen is called
 
Pathogen:- An organism that produces disease is called as pathogen.
   
 
Classification of plant disease
 
1. Classification Based on occurrence:-

(a) Endemic disease:- When a disease is more or less constantly present from year to year in moderate to severe form in a particular part of country or state it is classified as endemic to that area.

  • The word “endemic” means prevalent in, and confined to a particular country, district,or location. E. g. Early blight of potato. 
 

(b) Epidemic/Epiphytotic disease:- A disease which occurs widely in severe but periodically.

  • It assumes severe form only on occasions.
  • Examples: Red rot of sugarcane, Wheat rusts & late blight of potato occurred in epiphytotic form in 1913 in the district of Rangpur now in Pakistan.
 
 
(c) Sporadic diseases:- Sporadic diseases occur at very irregular intervals and locations.
Examples: Green ear disease of bajra and black rust of wheat 
 
(d) Pandemic diseases:- Diseases which occur all over the world and cause mass mortality, are called pandemic diseases.
Example:-  late blight of potato.
 
2. Classification according to causal agents:-
According to causal agents the diseases may be classified into two main groups:
(a) Parasitic or biotic diseases 
(b) Non-Parasitic diseases
 
(a) Parasitic or infectious diseases:– These diseases are caused by fungi, bacteria, viruses, nematodes and  Phanerogamic plant parasites. These diseases are usually contagious in nature and there is easy and efficient dispersal of the pathogens causing these diseases.
 
(b) Non-parasitic or non-infectious diseases:- These diseases are  infected by the nutritional deficiencies or excess, by low and high temperature, by low and extreme light and by air pollutants etc. These diseases are not infectious in nature.
 
 
Plant Diseases
Parasitic infectious or biotic diseases Non-Parasitic non-infectious or abiotic diseases
Fungal diseases Nutritional deficiency
Bacterial disease Nutrients in excess
Viral diseases Mineral toxicities
viroid and plasmids Low and high temperature
phytoplasma/mycoplasma Low and extreme sun-light
RLO’s Toxic air pollutants
Algae Soil acidity and alkalinity (pH)
Caused by nematodes Lack of oxygen
Caused by insects  High moisture
Phenerogamic plant parasites  
 
 
(3) Classification according to nature of diseases:-
 
Diseases classified according to the nature may be of the following types
 
(a) Seed borne diseases:- In such diseases the inoculum is found associated with the seed internally or externally and are called as internally seed borne or externally seed borne diseases.
Examples:- Fusarium wilt of gram
 
(b) Soil borne diseases:- These diseases are incited by the inoculum present in the soil in the form of oospores, sclerotia and mycelia etc.
Examples:- damping-off,
 
(c) Air borne diseases:- These diseases are incited by the inoculum present in the air in the form of conidia, chlamydospores and sporangia etc.
Examples:- Blight, rust, powdery mildew.
 
(4) Classification according to Symptoms:-
  • Symptoms of the diseases may also be considered as a base of classification and according to the symptoms, the diseases, may be grouped as rust diseases, leaf spot diseases, smut diseases, downy mildew diseases etc.
 
(5) Classification according to the Host:-
  • According to the host diseases may be grouped as cereal diseases and plantation diseases etc.
 
(6) Classification according to the Nature of Infection:-
  • According to the nature of infections, the diseases may be classified into two groups namely,
(1) the systemic diseases such as green ear diseases of bajra and loose smut  of wheat 
(2) Localized diseases such as leaf spot disease

Fungi

Fungi are filamentous, multi-cellular organisms, having organized nucleus with nuclear membrane, always lacking of chlorophyll, but presence of cell wall in true fungi and multiplication by production of asexual or sexual spores of vegetative cells.

vegetative structure of fungi:-

1. Thallus:- Most of the fungi have vegetative body called thallus consisting more or less filamentous

2. Hypha:- Filamentous thallus or individual branch is called as hypha.

3. Mycellium:- Group of hyphae is called Mycellium.

4. Aseptate myceilium:- When mycellium do not having cross walls (septa) are known as non septate or coenocytic or Aseptate myceilium.

5. septate myceilium:- When mycellium having cross walls are called as septate

 
Reproduction of fungi
 

(1) Asexual reproduction:- In asexual reproduction the hyphae cut off minute spores.

  • The structure and origin of these spores vary greatly and each types has been given different names ie. chlamydospores, conidiophores (conidia), in mastigomycotina, sporangium, sporangia are formed, in aquatic forms, sporangia liberate naked protoplasmic bodied, the swamspore or zoospores
    In the ascomycotina and deuteromycotina, the conidia may be produced in special structures known as pycnidia, sporodochiu, acervuli.

(2) Sexual reproduction:-

  • Involves two separate gametes.
  • The sexual reproduction by union of two morphologically dissimilar gametes is known as oogamy (leads to development of simple structure oospores er a complex fruit cleitothecium, perithecium, apothecium)
  • In most of higher form of fungi the sexual process involves the union of two similar gamatangia. This is known as isogamy (lead to formation of zygospore or zygote)

Classification of spores

(A) Asexual Spores:-
(a) Exogenous:- 1. Conidia 2. Oidia
(b) Endogenous:-
(i) Motile:- 1. Zoospores 2. Swarmspores
(ii) Non-motile:- Applanospore

(B) Sexual Spores:-

1.Zygote 2. Zygospore 3. Oospores 4. Ascospores 5. Basidiospores

(C) Vegetative Spores:-
1. Chlamydospores

Classification of spore fruits:

(A) Asexual Spore fruit:-  1. Sporangium 2. Synemmata (Coremium)  3. Sporodochium 4. Sorus 5. Pycnidium 6. Accium 7. Acervulus 8. Pycnium

(B) Sexual Spore fruit:- 
(a) Ascocarps:- 1. Cleistothecium 2. Perithecium 3. Apothecium 4. Ascostroma

(b) Basidiocarps:- 1. Puffballs 2. Bracket fungi 3. Mushroom

1. Pycnidium:- The spore fruits usually have an opening is called Ostioles

2. Cleistothecium:- It is closed without ostioles

3. Apothecium:- A cup or saucer shaped spore fruit with broad opening is called apothecium
4. Ascotroma:- It has ostiole

Common group of plant diseases fungi are

Ustilaginales Smut fungi
Uredinales Rust fungi
Erysiphaceae Powdery mildews
Peronosporaceae Downy mildews
 
 
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