Significance of Plant diseasePlant Diseases are very important to humans because they damage plants and plant products on which humans depend for food, clothing, furniture, the environment, and housing.

1. Plant diseases reduce the quantity and quality of plant produce.

Quantity: Plant disease may cause 0 to 100% yield loss.

Quality: Spots, scabs and blotches on fruits, vegetables and

ornamental plant cause inferior quality.This may reduce the market value.

What is Plant Disease?Plant Disease can be defined as any disturbance by a pathogen or an environmental factor which interferes with normal physiological processes in plant in such a way that the affected plant or its part dies or changes in appearance and/or yields less than a normal, healthy plant.Physiological processes- manufacture, translocation or utilization of food, mineral nutrients and water.

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2. Plant diseases may limit the kinds of plants that can grow in a large geographic area. e.g.- American chestnut was annihilated in North America as a timber tree by the chestnut blight disease (Cryphonectria parasitica).

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3. Plant diseases may make plant products unfit for human or animal. e.g. ergot (Claviceps purpurea) of rye and wheat. Ergots contain a number of alkaloids. The best known is lysergic acid diethylamide, LSD.

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4. Plant diseases may causes financial losses. e.g. resistant varieties are expensive and less productive, incur expenses for chemicals, machinery and labor, etc.

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5. Plant disease can change the course of history of mankind. e.g . In 1845 late blight of Potato completely destroyed the potato crop of Ireland. There was great famine (Irish famine) and 2 million people died and many more migrated to USA.

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Causes of plant diseases

Typical plant diseases are caused by

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

(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) 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. 6

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Animate or biotic or infectious agents: The animate agents are living causes of plant

diseases. These are- Fungi Bacteria Nematodes Green algae Mollicutes Parasitic higher plants mycoplasmas, etc

Mesobiotic agents:The causal agent is neither living thing nor a non living thing. The diseases caused by-• Viruses • Viroids

Inanimate or abiotic or noninfectious agents:The inanimate causes are nonliving causes of plant

diseases. Such as Adverse climatic conditions-e.g. too high or

too low temperature Adverse soil condition- e.g. lack or excess of

soil moisture or nutrients, injurious pH Chemical injuries caused by faulty application

of pesticides or fertilizers.

Symptoms of plant diseases

What is Symptom of plant disease?

All the visible and detectable changes in diseased plants are known as

symptoms of the disease. Symptoms are the expression of plant diseases.What is sign?

A visible manifestation of a causal agent of plant disease.

In many diseases, pathogen grows, or produces various structures, on the surface of the host. These structures may include mycelium, sclerotia, sporophores, fruiting bodies and spores, and are called sign. For example in mildews, one sees mostly the sign consisting of a whitish, downy or powdery growth for fungus

mycelium and spores on the plant leaves, fruit or stem.

Downy mildew diseases

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