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Plant Disease
In Hindi; पादप रोग
In our Ecosystem, plants are one of the major components and we all are surrounded by it. But this Ecosystem also contains Some microscopic Organisms that can't make food themselves. These microscopic organisms rely on other living and non-living organisms of the ecosystem for food and other needs to survive. Due to this heterotrophic phenomenon, Some microscopic organisms cause harmful effects or abnormalities (like the yellowing of leaves) in the host. This abnormality is the result of Disease and the casual organism known as Pathogen.
What is Plant Disease?
In Hindi;पादप रोग क्या है?
When the plant is suffering (not developing and functioning normally) and showing some abnormalities, we call it a diseased plant. However, this does not define the term 'disease'. Because abnormality is not a disease rather it's the result of the disease. Many scholars define the term "disease". However an analytical approach to the definition of the term 'disease' was made by Horsfall & Dimond (1959) who clarified many misconceptions. According to them disease (i) is not a pathogen, (ii) is caused by a pathogen, (iii) is not a condition as the condition results from disease, (iv) is not any injury which results from disease as well as from any traumatic cause, and (v) cannot catching or infectious, it's the pathogen which is catching or infectious or transmitted.
They defined
Disease is a malfunctioning process that is caused by continuous irritation which results in some suffering. [Horsfall & Dimond (1959)]
The results of diseases (i.e. Abnormalities) produce some external and internal alterations or changes in the host, which are called Symptoms. Eg.: wilting, spots, yellowing of leaves, lesions, and abnormal growth.
If the pathogen and its part or products, which are created after infection, seem on a host then it's known as Sign.
Eg.: powdery mildew
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Symptoms & Signs of Plant Disease
Plant diseases manifest through various symptoms. Common symptoms are the following:
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Atrophy:
Underdeveloped of a plant part due to infection And Reduction in Size and change in colour of plant leaves.
Blast:
Sudden Blighting and death of young buds, flowers or young fruits and failure to produce fruit or seeds. Blast symptoms appear on leaves as elliptical spots with light-coloured centres and reddish edges it also appears on nodes, glumes, and rachis.
Blotch:
Large and irregularly shaped spots or blots on leaves, shoots, and stems. leaf blotch appears as tiny purplish to reddish spots that occur on leaf blades and leaf sheath.
Canker:
Dead areas of bark and cortex of twigs or stems and the tissue often become discoloured and raised or sunken.
Chlorosis:
Yellowing or whitening of normal green tissue of plant fir to partial or complete failure of chlorophyll to develop.
Conks:
Fungal fruiting structures formed on rotting woody plants (shell or bracket fungi).
Damping off:
Destruction of seeds under the soil, or seedlings near the soil line, resulting in toppling over the ground and poor and uneven crop stand.
Decline:
progressive, gradual weakening and death of a plant and population of the plant.
Dieback:
Progressive gradual weakening and death of individual branches of a plant, often leading to and death of an infected plant.
Distortion:
Malformed plant tissue.
Enation:
Malformation or overgrowth of tissue induced by infection of some virus.
Etiolation:
Yellowing plant cased by Lake of light.
Flagging:
The loss of rigidity and drooping of leaves and tender shoots preceding the wilting of a plant.
Fleck:
a small, white to translucent spot or lesion visible through a leaf.
Galls:
Abnormal, localised swelling or tumour, on leaf, stem or root tissue.
Gum:
The complex of sugary substances formed by cells in reaction to wounding or infections.
Gummosis:
Production of a gun by or in plant tissue.
Hypertrophy & Hyperplasia:
An overdeveloped or overgrowth of plant cells, tissue or organs, hypertrophy means an increase in the cell size and hyperplasia means an increase in the cell numbers.
Leaf spot:
A self-limiting lesion on a leaf.
Lesion:
A localised area of discoloured, diseased tissue.
Masked symptoms:
Virus-induced plant symptoms are normal but appear when the host is exposed to certain environmental conditions of light and temperature.
Mildew:
Mildew are plant disease in which the pathogen is seen as a growth on the green surface of a plant. The growth appears as white, grey, brownish or purplish patches of varying size. Eg.- Downy mildew, Powdery mildew.
In downy mildew, the superficial growth is a tangled, cottony or downy layer.
In powdery enormous number of spores are formed on the superficial growth of fungus giving a dusty or powdery appearance. Black minute fruiting bodies of the fungus may also develop in the powdery mass.
Mosaic:
Symptoms of certain viral diseases of plants are characterized by intermingling patches of normal green and light green or yellowish colours.
Mottle:
An irregular pattern of indistinct light and dark green areas.
Mummy:
A mummy is a dried, shrivelled fruit, plant part or organ that is partially or completely replaced with fungal structures, usually as the result of fungal infection and colonization of the fruit.
Mycelium:
Masses of fungal threads( hyphae) which compose the vegetative body of the fungus.
Necrosis:
Death of plant tissue.
Ooze:
A mass of bacterial cells usually embedded in a slimy matrix appearing on the diseased plant surface, often as a droplet; or, a viscid mass of juices composed of host and parasite. Simply substances occasionally found exuding from a diseased plant is the ooze.
Pycnidia:
Minute, usually globose and black, asexual fungal fruiting bodies formed on plant surfaces.
Rhizomorphs:
String-like strands of fungal mycelia are sometimes found under the bark of trees.
Ring spot:
A circular area of chlorosis with a green centre is a symptom of many virus diseases.
Rot:
The Softening, discoloration and disintegration of succulent plant tissue as the results of cork formation.
Rust:
A type of disease caused by a specific group of fungi, often producing orange or red rust-coloured spores. It mainly affects the aerial part of the plant.
Scab:
A roughened crust-like diseased area on the surface of a plant organ, a disease in which such areas form.
Smut:
The word smut means a sooty or charcoal-like powder. In plant diseases known as smut, the affected part of the plant shows a black or purplish black dusty mass composed of the fungus spores. These symptoms appear in floral organs(ovariculous smuts), Leaves, stems and even roots(culmiculous smuts).
Sclerotia:
Tough structure produced by fungi for long-term survival.
Scorch:
Burning of leaf margins as a result of infection or unfavourable environmental conditions.
Shot-hole:
A symptom in which small diseased fragments of leaves fall off and as a result small holes are produced in their place.
Spots:
Circular or irregular lesions develop on the plant tissue as a result of infection.
Streaks:
A narrow, elongated, somewhat superficial necrotic lesion with irregular margins developed on the stem or leaf veins.
Tumor:
A malignant overgrowth of tissue due to hypertrophy or hyperplasia occurs due to infection.
Vein banding:
A process where the veins retain bands of green tissue while the tissue between veins has become chlorotic due to infection by the pathogen, more particularly by a virus or phytoplasma.
Vein clearing:
An event of destruction of chlorophyll in the vein or adjacent tissue as a result of infection more particularly by a virus or other pathogen.
Water soaking(Hydrosis):
It is a condition of the tissue which becomes translucent caused by water moving from the host cells into the intercellular spaces due to damage of the cell wall by the enzyme or toxin of the pathogen.
White blisters:
Development of numerous white pustules resembling rust pustules on the leaf surface which break open on maturity and expose white spores.
Wilt:
Loss of turgidity and drooping of plant parts due to insufficient water in the plant caused by plugging of the xylem of the plant by plugging of the xylem of the plant by the pathogen or due to rotting of roots.
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Singh, R.S.2012. Plant Disease,9th Edition. Oxford & IBH Publication Co. Pvt. Ltd.
Kalita,M.K. 2018. 6. Diseases of Field & Horticultural Crops and their Management-1, 1st Ed. Kaliyani Publishers
