General Classification of Plant Diseases PDF

# General Classification of Plant Diseases Plant diseases may be classified in various ways on the basis of: - Causal agencies. - Spread and severity of infection. - Perpetuation and transmission. - Part of host affected. - Symptoms produced on host plants, etc. ## Classification Based on Causes...

# General Classification of Plant Diseases Plant diseases may be classified in various ways on the basis of: - Causal agencies. - Spread and severity of infection. - Perpetuation and transmission. - Part of host affected. - Symptoms produced on host plants, etc. ## Classification Based on Causes of Diseases On the basis of various causes of plant diseases, they can be classified into two main categories: - Non-infectious diseases. - Infectious diseases. ### Non-infectious (or inanimate) diseases These diseases are not caused by living organisms or viruses. They are usually caused by a lack of proper environmental conditions, nutritional deficiencies and other physiological irregularities. These diseases are non-infectious and cannot be transmitted from one diseased plant to another healthy plant. These diseases are sometimes called physiological diseases because they are induced by physiological disorders in the host. However, the term physiological diseases is not suitable because physiological disorders occur both in non-infectious as well as in infectious diseases. Examples of non-infectious diseases: - Freezing injury due to low temperature. - Heat canker of flax caused due to high temperature (i.e heat injury). - Black heart of potato caused due to unfavorable oxygen. - Flooding injury caused due to unfavorable soil moisture. - Apple scald and black tip of mango caused due to injurious atmosphere. - Lighting injury. - Internal bark necrosis of delicious apple due to excessive minerals. - Whiptail disease of cauliflower caused due to deficiency of molybdenum. - Khaira disease of rice caused due to deficiency of zinc in the soil. - Tip-rot or necrosis of mango fruits caused due to boron deficiency. ### Infectious (or animate & viral) diseases These diseases are caused by foreign organisms or viruses which are transmitted from one diseased plant to another healthy plant. The pathogens causing such diseases are usually microbial in nature or of animal or plant origin. However, it is not necessary that the parasite should always be present in the host plant. Sometimes the pathogens produce toxic substances from outside which cause pathological effects. The infectious diseases can further be classified into two groups: - **Animate diseases**: These diseases are caused by living pathogenic organisms such as the diseases caused by mycoplasma like bodies, bacteria, fungi, parasitic algae, nematodes, protozoans, etc. - **Viral diseases**: These are still not regarded as living organisms. They are regarded as links between living and non-living entities. Therefore, the diseases caused by viruses are treated separately. The diseases caused by viruses are highly infectious. ## Differentiation between non-infectious (physiological) and infectious (parasitic) diseases | Physiological diseases | Parasitic diseases | |---|---| | The plant diseases caused by environmental and physiological disorders such as, low or high temperature, lack or excess of soil moisture, lack or excess of light, lack of oxygen, air pollution, nutritional deficiencies, mineral toxicity, soil acidity, alkalinity or toxicity, etc. | The diseases caused by parasitic pathogens such as mycoplasmas, bacteria, fungi, viruses, nematodes, parasitic algae, protozoans, etc. | | These diseases do not show the presence of sign (i.e. the actual presence of pathogen). | These diseases show presence of sign. | | The physiological diseases cannot be transmitted from one diseased plant to another healthy plant. | These diseases can be easily transmitted from diseased plant to healthy plants. | ## Classification of Plant Diseases Based on Spread and Severity of Infection On the basis of geographical distribution and severity of infection, the plant diseases are classified into the following three types: - **Endemic diseases**: These diseases are confined to a particular country or district or part of the Earth and occur more or less constantly year after year in moderate or severe form. The causal agents of such diseases are well established in that particular locality where they are endemic. The pathogens of endemic diseases usually persist in a particular locality through their survival on alternate hosts. - **Epidemic or epiphytotic diseases**: The term 'epidemic' is derived from a Greek word meaning 'widespread among the people'. In strict sense the term epidemic is applied for diseases occurring in large human population. The term 'epiphytotic' is used in case of such diseases occurring in a plant population. These diseases cover a large area and population of plants resulting in severe outbreak but occur periodically. They are very responsive to the changing environmental conditions and follow two distinct phases: - During early destructive phase an epiphytotic disease spreads vigorously causing diseases in new hosts. - The decline phase, due to the biological equilibrium reached between host, parasite and the environment. - **Sporadic diseases**: These diseases occur occasionally at irregular intervals and in irregular localities, infecting only a few plants. ## Classification Based on the Part of Host Infected On the basis of the portion of host plant affected, the diseases may be divided into two types: - **Localized diseases**: The diseases which are restricted to a particular part or organ of host plant are called localized diseases. These can be further classified as: - Root diseases. - Stem diseases. - Foliage diseases. - Flower diseases. - Spike diseases. - Fruit diseases. - **Systemic diseases**: The diseases that spread throughout the entire plant body are called systemic diseases. ## Classification Based on the Perpetuation and Dispersal of Diseases On the basis of mode of perennation the plant diseases are classified into four types: - **Seed borne diseases**: The pathogen of seed borne diseases remains and perpetuates inside the seed or other propagative structure in the form of resting spores, oospores, mycelium, etc. or on the surface of seed in the form of spores, conidia, bacterial cells, sclerotia, etc. The inoculum carried through seeds becomes active or germinates along with the germination of seeds, and infects the young seedlings. These diseases are usually systemic and infect the whole plant. - **Soil borne diseases**: The primary inocula of these diseases are perpetuated and transmitted in the soil. These pathogens survive in the soil either as resting spores, mycelial strands or rhizomorphs. They usually infect roots or other underground parts of host plants. The pathogens of soil borne diseases are usually facultative saprophytes or facultative parasites. - **Water borne diseases**: This category of plant diseases includes those in which the primary inoculum is perpetuated and transmitted through water. - **Air borne diseases**: The primary inocula and the secondary inocula of air borne diseases are disseminated by wind. The pathogen of these diseases discharge their propagules in the air which can be blown to long distances and cause infection. ## Classification Based on Disease Symptoms Plant diseases are classified on the basis of specific symptoms such as leaf spots, blights, rusts, smuts, wilts, powdery mildews, downy mildews, damping off, rots, galls, anthracnose, cankers, mosaic, etc. A detailed account of general symptoms of plant diseases is given in the next chapter.