In this article we will discuss about the classification of plant diseases.
I. Non-Parasitic or Abiotic Diseases:
These are induced by unfavourable environmental conditions of soil or air such as mineral deficiencies or excesses in the soil, low or high temperatures, improper water, oxygen and light relations. The non-parasitic diseases may also be caused by mechanical injuries.
Common examples of the non-parasitic diseases are blossom and rot of potatoes, black heart of potatoes, scald of apples, tip bum of lettuce and heat canker of flax.
II. Parasitic Diseases:
These are caused by the attack of some other living or non-living agents called the causal agents. The causal agent may be a plant or an animal or a virus. It is called the parasite. In the case of a disease caused by a parasitic organism the diseased plant is called a host.
The parasite may subsist in whole or in part upon the living tissue of the host. The host is more appropriately called the suspect. It is the term applied to the plant which is prone to a given disease caused by a given causal agency.
The parasitic diseases of plants may also be caused by the animal parasites such as the nematodes, molluscs (snails and snugs) and insects.
Mostly the plant diseases are caused by the following pathogens:
1. Parasitic fungi:
They produce a great majority of the parasitic plant diseases called the fungal diseases. The fungal diseases are of first importance because they sometimes cause much damage and destruction.
2. Parasitic bacteria:
They are also the causative agents of many plant diseases. The bacterial diseases of plants are second in importance to the fungal diseases.
3. Parasitic seed plants:
There are a very few phanerogamic parasites which cause plant diseases. The most common parasitic forms are the mistletoes (Viscum), Dodders (Cuscuta) Cystanche, Orobanche, and a few others.
4. Parasitic algae:
There are a very few parasitic algae that cause plant diseases.
The most important ones are:-
(i) Cephaleuros virescens which causes red rust of tea.
(ii) C. coffeae is parasitic on coffee.
(iii) C. parasitica incites ‘Orange rust of tea’.
5. Viruses:
The viruses are pathogens all of which are the most extreme parasites being almost completely dependent upon their host for their growth and replication. They cannot be seen with the light microscope and are best known from their effects on the host plants or animals.
None of them has ever been cultivated outside the bodies of plants or animals. They cause serious plant and animal (including man) diseases. All diseases caused by the viruses are of systemic type.
The common examples of virus diseases of plants are tomato and tobacco mosaic, potato and cucumber mosaic, curly top of sugar beet, spotted wilt of various economic plants such as pea, potato, tomato, tobacco, and spinach, and yellow diseases of peach and aster.
The parasitic diseases are often subdivided in relation to their occurrence into three categories, namely, endemic, epiphytotic (or epidemic) and sporadic.
(a) Endemic diseases:
A disease which is regularly present (from year to year) in a certain region (country) or a part of a region (district) in a moderate to severe form is referred to as endemic.
This is possible only when the causal agent is well established in that locality and can persist by virtue of its ability to survive from one crop season to the next in soil, crop debris or in wild hosts and the environmental conditions are favourable for its development.
(b) Epiphytotic (epidemic) diseases:
Epiphytotic is a term applied to the infectious plant diseases which spread widely but occur periodically. The causal agent may be regularly present in the locality but the environment favourable for its rapid development occurs only periodically. The epiphytotic diseases are thus very responsive to variations in the environment.
(c) Sporadic diseases:
These are plant diseases which occur only here or there at irregular intervals or occasionally.