Plant diseases can be classified as either infectious or non-infectious. Non-infectious diseases are caused by non-living factors like temperature, soil moisture, or pollution and result in physiological disorders. Infectious diseases are caused by living pathogenic organisms like viruses, bacteria, fungi, mycoplasma, or algae that can be transmitted from plant to plant. These organisms use different pathogenicity factors like toxins, cell-degrading enzymes, or phytohormones to infect plants and cause disease.

1. CLASSIFICATION OF PLANT
DISEASES BASED ON
CAUSATIVE ORGANISM

2. Classification of plant diseases based
on causative organism
Based on causative organism the plant diseases are classified into two.They are Non-infectious and
Infectious diseases.
.
Non-Infectious diseases
• Non-infectious diseases are caused by non living factors or agents.
• These diseases are caused by disturbance in the plant body due to the lack of some inherent
qualities and improper environmental conditions of soil and hair.
• The physiological disorders in plants are
1)unfavourable temperatrure
2)undesirable soil moisture and Ph
3)presence of toxic gases in the atmosphere.

3. Non-Infectious diseases
 4)lightning injury
5)mineral excess and deficiencies in the soil.
6)absence or excess of light
7)pesticide toxicity
8)improper agricultural practices

4. 1)unfavourable temperatrure
 High temperature may induce the formation of cankers at the soil surface of
cotton and peanut plants.
 Frost injury is relatively common,but temperature just above freezing also may
cause damage such as necrosis(localized tissue death) in potato tubers and
silvering of corn leaves.
 The drought and dry winds that often accompany high temperatures cause
stunting,wilting,blasting,marginal scorching of leaves and dieback of shoots.

5. 2)undesirable soil moisture and pH
*Blossom rot of tomato and pepper is prevalent when soil moisture
and temperature levels fluctuate widely and calcium is low.
*The pH of a soil has a dramatic impact on nutrient availability to
the plants.
*In acidic soils some nutrients are far more available and may reach
concentrations that are toxic or that inhibit absorption of some other
nutrients.
*A similar situation exist in alkaline soils.oats planted in alkaline soils
that actually contain a sufficient amount of manganese and
manganese deficiency disease is formed.
*Because an elevated soil pH causes manganese to react with oxygen to
produce manganese dioxide.So the nutrient is insoluble to plants.

6. 3)presence of toxic gases in the atmosphere
 Serious damage may results when fertilizers,herbicides,fumigants,insecticides
etc. are applied at excessive rates under hot,cold or slow-drying conditions.
 The major pollutants toxic to plants are sulfur dioxide,fluorine,ozone and
peroxy acetyl nitrate.
 Sulphur dioxide formed from the burning of large amounts of soft coal and
high sulphur oil.Flourides are more toxic to the plants and it will accumulate
on the leaves.
 Ozone and peroxyacetyl nitrate injury are more prevalent in near cities with
heavy traffic problems.They are formed whensunlight reacts with nitrogen
oxides and hydrocarbons.Hydrocarbons are formed by combustion of fossil
fuels.

7. INFECTIOUS DISEASES
 Infectious diseases are caused by pathogenic living organism and they are
transmitted from one plant to another.
 The common pathogenic living organism are
virus,bacteria,mycoplasma,algae,fungi.
 Bacteria:
Most bacteria that are associated with plants are
actually saprotrophic and do no harm to the plant
Itself. Bacterial diseases are much more prevalent in subtropical and tropical
regions of the world.
*Most plant pathogenic bacteria are rod-shaped(bacilli). In order to be able to
colonize they have specific pathogenicity factors.

8.  Five main types of bacterial pathogenicity factors are known:
 1)uses of cell wall degrading enzymes.
 2)toxins
 3)effector proteins
 4)phytohormones
 5)exopolysaccharides.
virus
 They are able to multiply or replicate only within a living cell of a particular
host.
 Diseases are generally most serious in plants that are propagated
vegetatively or asexually.
 Mycoplasma
 Phytoplasma and spiroplasma are a genre of bacteria that lack cell walls and
are related to mycoplasmas,which are human pathogens.

9. Fungi
*most phytopathogenic fungi belonging to the ascomycetus and basidiomycetes.
*The fungi reproduce both sexually and asexually. Spores may be spread long distances by air or water
or they may be soilborne. Many soil inhabiting are capable of living saprotrophically, carrying out the
part of their lifecycle in the soil. These are known as facultative saprotrophs.
*Fungal diseases may be controlled through the use of fungicides and other agriculture practices.
Rice blast caused by necrotrophic fungus

10. Algae
 Some colourless parasitic algae(cephaleuros) also cause plant diseases.