Classification of plant diseases
Plant diseases may be classified according to fallowing bases.
i) Classification of plant diseases on the bases of plant organ they affect:
Plant diseases are sometimes grouped according to the plant organ they affect such as root diseases, stem diseases, foliage diseases, vascular diseases etc.
ii) Classification of diseases on the basis of type of host plant:
On the basis of the host plant, diseases may be classified as a) Field crop diseases b) vegetable diseases c) fruit diseases and d) Ornamental plant diseases.
Classification of the diseases on the basis of the extent to which they are associated with host plant when the effect of the pathogen is localized near the place of infection, it is known as local disease, but if the entire plant is invaded, the disease is said to be systemic.
iii) Classification of diseases on the basis of their method of Perpetuation:
a) Soil born diseases: Such diseases are caused by soil borne pathogens which survive in soil or on plant debris as resting spores or mycelium stands. They all attack the root system of host plants. example: colletotrichum.
b) Air borne diseases: In these diseases, the primary inocula are disseminated by wind example smut rusts and powdery mildews.
c) Seed borne diseases: Many plant pathogens survive as dormant mycelium in the seeds or other perennating structures of host plants. When seeds germinate, the resting spores and mycelia become active and cause infection example: smut.
iv) Classification of diseases on the basis of their occurrence:
According to their occurance, the diseases are classified as a) Endemic b) Epidemic and c) Sporadic.
a) Endemic diseases: The disease occurring more or less constantly in moderate to severe form in a particular region is an endemic disease, example: Wart disease of potato caused by Synchytrium endobiotichum.
b) Epidemic or Epiphytotic diseases: The disease occuring widely and periodically. affecting many individual plants is termed as epidemic disease. Irish potato famine was caused by Phytophthora late blight of potato.
c) Sporadic diseases: Sporadic diseases are epidemics that occur occasionally and at irregular intervals, example: Angular leaf spot in blotch disease in cucumber.
v) Classification of diseases on the basis of symptoms:
Diseases can be classified into three categories based on symptoms.
a) Disease showing necrotic symptoms: Necrotic symptoms results due to the death of cells, tissues, organs, and even whole plants.
b) Disease showing hypoplastic symptoms: Stunting of plant organs or a general dwarfing of the entire plant is a common symptom of such diseases.
c) Diseases showing hyperplastic symptoms: In some diseases, there occurs excessive growth of some plant parts or even the entire plant. This increase in size is brought about by either or both of the two processes; a) increase in cell number(hyperplasia) and b) increase in the size of the component cells(hypertrophy).
vi) Classification of diseases on the basis of nature of causal organism:
The most useful criteria for grouping diseases, however, is the type of pathogen that causes the disease. The advantage of such a grouping is that it indicates the cause of disease, which immediately suggests the possible control measures.
On this basis, plant diseases are classified as fallows:
a) Infectious or Biotic, Plant Diseases:
i) Disease caused by fungi
ii) Disease caused by bacteria
iii) Disease caused by parasitic higher plants
iv) Disease caused by viruses and viroids
v) Disease caused by nematodes
vi) Disease caused by Protozoa
b) Non-infectious or Abiotic, plant diseases:
i) Disease caused by too low or too high temperature
ii) Disease caused by lack or excess of soil moisture
iii) Disease caused by lack or excess of light
iv) Disease caused by lack of oxygen
v) Disease caused by air pollution
vi) Disease caused by nutrient deficiencies
vii) Disease caused by mineral toxicities
viii) Disease caused by Soil acidity or alkalinity
ix) Disease caused by toxicity of pesticide
x) Disease caused by improper cultural practices.