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Plant Diseases

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Rural Technology

The importance of plant diseases

The study of plant diseases are important as they cause loss to the produce. The various types of losses occur in the field, in storage or any time between sowing and consumption of produce. The diseases are responsible for direct monitory loss and material loss. Further, these diseases are harmful for the society as they cause stomach disorders, paralysis and liver diseases.

Hence, the diseases are required to be prevented and controlled to avoid loss of valuable food. The pathogen infects the soil and cause devaluation of land also. Causes of diseases A pathogen is always associated with disease. When the plant is suffering its functioning and development is disturbed, we call that as a diseased plant. The pathogen may not be living thing also. Thus the causes of diseases are grouped as follows:-

(A) Abiotic Factors:-

These are the resultants of deficiencies or excess of nutrients, light, moisture, aeration, adverse soil conditions or atmospheric condition etc. These are generally referred as disorders.

(B) Mesobiotic Factors:-

The causal agent is neither living thing nor a non living thing. The diseases caused by viroids and viruses are of this category.

(C) Biotic Factors:-

This category includes diseases caused by living or cellular organization. 1) Eukaryotes: - Fungi, Protozoa, Algae, Nematodes, Parasites. 2) Prokaryotes : Mycoplasma, Rickettsia, Bacteria

Classification of Plant diseases:

1) Endemic diseases: - Endemic means prevalent in and confined to a particular locality. These diseases are more or less constantly present in a particular area. 2) Epidemic diseases: - These diseases occur incidentally and occasionally in a particular locality. The word epiphytotic is used particularly for plant diseases instead of epidemic. 3) Sporadic diseases: These occur at very irregular intervals and locations. Modes of spreading of diseases: 1) Soil borne diseases: - Inoculums of the diseases causing pathogen remains in soil and penetrate the plant resulting in diseased condition e.g. Root rot, wilt. 2) Seed borne diseases: - The micro organisms are carried along with seeds and cause diseases when congenial condition occurs. E.g. Damping off. 3) Air borne diseases: - The micro-organisms are spread through air and attack the plants causing diseases. E.g. Blight, rust, powdery mildew. 4) Diseases spread by insects: The viral diseases are spread by insects. The insets which carry the viruses are known as vectors.

General Symptoms and diagnosis of plant diseases

Evidence of disease shown by plant is called symptom. Symptoms are seen on the plant either due to character and appearance of the visible pathogen or its structure or organs or due to some effect upon or change in the host plant.

A) Symptoms due to visible pathogen

The pathogen is visible when they are in larger size or in sufficient mass, such symptoms are as follows: i) Mildew: - Pathogen is seen as a growth on the surface of the host. Downy mildew and powdery mildew. ii) Rust: - Rusty symptoms are seen on the host epidermis Red, Green, yellow and black rust. iii) Smuts: - Sooty or charcoal like powder appears on floral organs, smut symptoms also found on stem, leaves and roots. iv) White blisters : Numerous white blister, like pustules are seen. v) Scab: Crust like lesion on the diseased organs. vi) Sclerotia: A compact, often hard, mass of dormant fungus mycelium. vii) Blotch: Superficial growth giving the fruit a blotched appearance. viii) Fruiting bodies: Relatively large spore bearing structures which are either fleshy or woody. ix) Exudations: mass of bacteria oozes out to the surface of affected organ. x) Tar spots: Raised, black coated fungus bodies with the appearance of a flattened out drop of tar on leaves.

B) Symptoms due to some effect on host plant

1) Colour changes, Discoloration, Chromosis, and Chlorosis. 2) Overgrowth Galls, Curl, bladder, witches broom, hairy root, 3) Dwarfing stunted growth 4) Necrosis Spots, strips, blight, damping off, scald, scorch, rot. 5) Anthracnose: ulcer like lesions on stems, leaves, pods. 6) Dieback: Drying of plant organ from the tip backwards. 7) Wilts : Drying or wilting of entire plant 8) Miscellaneous symptoms. Change in habit, dropping of leaves, flowers, fruits, destruction of organs etc.

Management of plant diseases:-

For control and prevention of plant diseases various methods are followed. According to the pathogen involved, symptoms and crop affected the method is selected. Prevention is always better but timely control also helps in reducing the losses. 1) Use of resistant varieties: This is very useful easy and low cost method for preventing the diseases. Growing resistant varieties to particular disease is important. 2) Use of cultural methods: Proper ploughing, harrowing, timely sowing, balance fertilization and irrigation, crop rotation, mix cropping, proper drainage are the cultural method for protecting crops from diseases. ) Chemical Control: Use of fungicides as a seed treatment and for spraying is a method of controlling various diseases. Spraying insecticides to control disease spreading insects is also essential to protect the crops from viral diseases. Fungicide - Any chemical used to kill or inhibit growth of fungi that cause economic damage to crop or ornamental plants (including rust in cereals, blight in potatoes, mildew in fruits) Most are applied as sprays or dust; seed fungicides are applied as a protective coating to seeds before germination. Copper compounds, especially copper sulfate mixed with lime and water (Bordeaux mixture), and sulfur have long been used for this purpose, but now synthetic organic compounds are commonly used. Many antifungal substances occur naturally in plant tissues. 4) Biological methods: Use of natural extracts and micro-organisms to protect the crops from diseases is safest method from environmental point of view. 5) Legislative control: To control spread of diseases from other countries each country has its own legislation.


Disease or disorder: These two terms are synonymous. The malfunctioning of plant body is known as disease or disorder. The term disease includes all types of harmful physiological changes in the plant while non infectious changes due to abiotic factors are termed as disorders. Pathogen: The agent responsible for suffering plant body. Parasite: The organisms which derive the food material from the host plant. Infections: Establishment of parasitic relationship between the pathogen and the host following the entry or penetration. Symptom: Any evidence of disease or disorder shown by plant is called symptom. Syndrome: A set of symptoms characterizing a disease is collectively called as syndromes. Fungus: It is a eukaryotic protests, chlorophyll less nucleated, unicellular or multicellular filamentous micro-organism. Virus : Microscopic, simple infectious agent that can multiply only in living cells of animals, plant, or bacteria. Bacteria : Group of microscopic, single-celled organisms that inhabit virtually all environments, including soil, water, organic matter, and the bodies of multicellular animals. Viroid : Infectious particle that is smaller than any of the known viruses

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