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Plant disease: 1. Introduction to plant diseases - types of pathogen, attack by insect infestation

Doc Brown's Biology exam study revision notes: There are various sections to work through, after 1 they can be read and studied in any order. They will help you answer questions such as ...   How do plants protect themselves from pathogens?   What chemical defences can plants employ?   What physical defences do adapted plants have?   What methods are used to detect plant diseases?

INDEX of Plant Disease Notes


(1) Introduction to plant diseases

Plants, like any other living organisms, are susceptible to attack by pathogens causing diseases.

The study of plant diseases, pests and nutrient deficiency and their effects are important for two reasons:

(i) Plants are usually the primary producers in food chains - all part of the world's ecological systems.

(ii) We rely on plants for our food directly or indirectly and these affect crop yields or destroy crops.

Its not difficult using fertilisers or compost/manure to correct nutrient deficiency in the soil, but its often quite difficult to control plant diseases and insect pests.

The attack by pathogens often causes leaf damage that reduces photosynthesis on which plants depend for their own food and energy - so any means of defence is useful and examples are described on this page.

The pathogens can be viral, bacterial or fungal.

A virus, bacteria or fungus can have harmful effects on a plant with serious consequences if it is unable to defend itself against such attacks.

Plants can also be attacked and infested by insects

e.g. aphids cause considerable damage to plants, and are not difficult to spot..

Aphids (greenfly) are very destructive common sap-sucking insects that reproduce rather rapidly and can cause:

a lack of plant vigour - decreased growth rate, distorted growth, mottle or yellow eaves (suggests decreased photosynthesis), and often excrete a sticky substance (honeydew) on foliage which allows the growth of sooty moulds leading to infection.

Unfortunately, aphids can also act as disease vectors conveying pathogens from plant to plant.

Aphids have specially adapted mouthparts that pierce phloem tubes to feed on sap - in doing so they can transfer viruses to the plant from their saliva.

The feeding punctures in the phloem cells has the effect of the lowering turgor pressure in the plant cells causing wilting.

Aphids can be controlled by using their natural predators like ladybirds (eat them) and parasitic wasps (lay eggs in them), these are biological methods.

Spraying plants with insecticides is a chemical method, but potentially harmful to harmless organisms including poisoning pollinating bees.

Plants have also developed physical defences against pathogens and also to deter animals from eating them.

Plants have evolved means of fighting against pathogens including the use of chemical defences and some of these compounds have been of great interest to pharmaceutical companies developing and producing drugs-medicines.

Plants are the start of most food chains, so they are of obvious importance to any subsequent source of food for animals.

Therefore, the ability of plants to defend themselves against infection by pathogens is not only important to the plant species itself, it is also important for the survival of other organisms, including ourselves!

Damage to crops lowers yields and endangers the ability of human populations (or any animal population) to feed themselves properly with a nutritious diet, both in quantity and quality.


Keywords, phrases and learning objectives for this part on plant diseases and nutritional deficiency

Know the various ways in which plants become diseased e.g. types of pathogen like viruses, bacteria, fungi and attacks from insect infestation.


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