Plant Pathology : Plant pathology is the branch of botany which deals with the nature of plant diseases, their diagnosis, treatment, prevention and control.
Another name of plant pathology is phytopathology.
Classification of plant diseases
A disease is a harmful deviation from a normal functioning of physiological process. Following are the basis of the classification of plant diseases.
Pathogen : A pathogen is an agent or organism capable of producing a disease, e.g. Fungi, Bacteria, virus, etc.
Pathogenecity : The ability of a pathogen to cause a disease is known as pathogenecity.
Symptoms : The pathological effects on the host, expressed as abnormal changes in its structure or function, are called symptoms, e.g. Rusts, smuts, downy etc.
Etiology : The study of the pathogens of diseases is called ‘etiology’.
Animate Pathogens : The living pathogens are called animate pathogens which are parasitic and the diseases they cause are infectious. The animate pathogens are, e.g. fungi, bacteria, etc.
Viral Pathogens : Viruses are ultra-microscopic bodies capable of causing diseases. They are considered as the connection link between living and non-living things, e.g. Tobacco mosaic virus (TMV), Cucumber mosaic virus (CMV), Tomato aucuba virus (TAV).
Inanimate Pathogens : The inanimate pathogens are non-living entities. They are non-parasitic and non-infectious, e.g. Mineral deficiencies in soil, excess nutrition, pollutants, chemicals, adverse climate, etc.
Classification of Diseases : Wheller 1969, plant diseases caused by fungi and bacteria based on symptoms. Some of the important symptoms are as follows :
1. Damping Off : The fungal pathogens cause the diseases of germinating seeds. Damping off disease occurs at two stages. They are, (a) Pre-emergence stage and
(b) Post-emergence stage, e.g. Damping off of cotton and Damping off of cereals.
2. Root Rot : The root rot is the progressive rotting of the root system. Fungal pathogens cause foot rots on crops.
3. Vascular Wilts : Vascular wilting is the permanent, dropping of the shoot system due to decay of roots. Due to vascular wilting the plant shrivels and finally dies.
Fungal wilts
1. Wilt of red gram : Fusarium oxysporium.
2. Wilt of cotton : Fusarium vasinfecti.
Bacterial Wilts
1. Wilt of tomato : Pseudomonas tobacco, potato : Solancearum.
2. Wilt of sugarcane : Xanthomonas vasculorum.
3. Downy Mildews : Develop-ment of clusters of white or grey downy sporangiophores on the lower side of infected leaves is called downy mildew.
4. Powdery Mildews : Powdery mildew diseases are characterized by the fungal mycelium on conidiophores and conidia as white powdery patches on the host.
5. Rusts : The rust symptoms appear as small pustules of rusty spores formed by breaking of host epidermis. The rust diseases are caused by fungi.
6. Smuts : Smuts are soot like spore masses of the pathogen produced on their hosts. Usually the smut symptoms are found in the reproductive organs of the host.
Smut diseases are produced in cereals, millets, sugarcane, onion etc.
7. Blight : The blight is the sudden burnt appearance of infected plant parts, like leaves and flowers. The blighted plant parts turn yellow and gradually disintegrate, e.g. Late blight of potato.
8. Anthracnose : Charcoal coloured spots produced on plant parts like leaves and fruits are called anthracnose. This disease is caused by a fungi. Anthracnose disease occurs on bean, coffee, neem, respberry, etc.
9. Leaf Spots : A localized self limiting areas of discoloured dead tissue on a leaf spots. The leaf spots are caused by bacteria.
10. Leaf Curl : Leaf curls are the formation of irregular wrinklings of a leaf. The leaf curls are formed due to abnormal growth of cells on either side of the midrib and veins.
11. Witches Broom : Excessive lateral branching and aggregation of short and swollen shoots in a diseased plant appears as a broom. This is called Witches broom.
12. Galls : The localized swellings induced by the pathogens in plant organs are called galls. These are caused by bacteria, fungi, nematodes, insects, etc, e.g. Crown gall of apple.
13. Roof Knot : The root knots are the tumours that develop on root system by nematodes.
14. Cankers : The necrotic lesions on plant parts are called cankers. Due to infection, certain necrotic areas are produced on mature stems, leaves, fruits of the host. These necrotic areas are surrounded by successive layers of callus tissue. Some dead cork cells are produced near the killed tissue. The entire necrotic area with raised margins of cork cells is the canker.
15. Sclerotia : A hard mass of fungal hyphae brown or black in colour is called sclerotium.
Plant diseases
Disease : Blast of Rice
Pathogen : Pyricularia oryzae.
Symptoms : 1. Bluish leaf spots of 1.3 mm in diameter appear on leaves.
2. In young leaves the leaf spots enlarge and appear spindle shaped.
3. Leaf spots appear water soaked. The central part remains pale green.
4. Leaf sheaths, rachis also show leaf spots.
5. The neck of the ear becomes shrivelled and finally becomes rotten.
6. Grain formation poor and the entire panicle may break.
Control Measures : 1. Seed treatment with Agrosan GN has been effective.
2. Use of balanced proportion of N-P.K. fertilizer is desirable.
3. Fungicides like Copper oxychloride, Blitox, Agrason GN, Cerasan should be sprayed.
4. Field sanitary methods should be adopted.
Disease : Red rot of sugar-cane
Pathogen : Colletotrichum fulcatum.
Symptoms : 1. Symptoms appear prominently on the stem and leaves.
2. Discolouration and dropping of leaves is the earliest symptom.
3. Elongated dark red lesions appear on midrib and leaf sheath.
4. The severely infected canes show yellowing and withering. The rind shrivels.
5. Cavities filled with greyish mycelium appear in the pith.
6. Diseased canes are lighter in weight and easily broken.
7. Juice of infected canes emit bad odour.
Control Measures : 1. Use of disease resistant varieties is the best method.
2. Use of healthy and disease free seed is an important measure.
3. Use of fungcides like Agrasan GN, Granosan-M have been recommended.
4. Crop rotation of 2-3 years minimises the soil born inoculum.
5. Field sanitary methods should be followed.
Disease : Tikka disease of ground-nut
Pathogen : Cercospora personata Cercospora arachidicola.
Symptoms : 1. Leaf spots of variable sizes appear. They are usually surrounded by yellow ‘halo’.
2. The spots appear in black or brown in colour.
3. In advanced stage leave defoliation occurs.
4. Reduced yield and poorly developed pods.
Control Measures : 1. Disease resistant varieties should be used.
2. Field sanitary methods should be adopted.
3. Crop rotation is good for reduction of disease.
4. 0.5% CuSO4 is required for seed treatment.
5. Fungicides like Bordeaux mixture, Blitox 50 reduces secondary infections.
Disease : Grain smut of Sorghum
Pathogen : Sphacelotheca sorghii.
Symptoms : 1. The disease symptoms are mostly confined to reproductive parts.
2. The infected grain are transformed into spore sacs which vary in size and shape.
3. The spore sacs are grey in colour, larger in size.
4. Each sorus consists of large number of minute chlamydospores.
5. Spore sac is covered by a thick or thin wall.
6. Diseased plants show stunted growth.
Control Measures : 1. The seeds should be sterilized with organomercurials or non-metallic carbon compounds.
2. Seeds should be treated with 0.5% CuSO4 solution.
3. Seed treatment can also be done with 0.5% formaldehyde.
4. Solar treatment of seeds is also effective.
5. Dusting the seeds with sulphur powder after harvesting is the cheapest method.
6. Seed varieties should be disease resistant.
General Methods of controlling plant diseases.
1. Fungicides : Bordeaux Mixture, sulphur dust, dithane M-45 dithane Z-78, vitavex.
Antibiotics : Agrimycin acts as antibacterial.
Antifungal : Blasticidin cycloheximide etc. are antifungal.
Selection of disease resistant varieties is highly useful.
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