Plant Diseases

Just like animals, plants can become infected by pathogens and develop disease. Instead of having an immune system, plants have their own ways of evading pathogens.

Causes of plant disease and their impact

• Pathogens are microorganisms which cause disease. They include bacteria, viruses, protists and fungi.

• Insects (pests) can also infect plants and reduce their growth.

• Certain pathogens infect plants and cause plant diseases. These diseases don’t only impact plants but affect the whole food chain. For example, diseases which affect food crops can have a detrimental effect to humans, as seen in the Irish potato famine of 1850.

Examples of plant diseases

Rose black spot is a fungal disease which affects plants. It causes reddish-black spots to appear on the leaves of plants. The leaves also begin to turn yellow and fall off. This discolouration means that the plant cannot carry out as much photosynthesis so cannot grow very well. Rose black spot is spread between plants when the fungus lands on a plant after being blown by the wind or by floating along rivers or streams. The spread of rose black spot can be prevented through the use of fungicides (chemicals which kill fungi). Gardeners will also cut off diseased leaves from a plant to reduce its spread.

Tobacco mosaic virus is a disease which affects plants. It causes discolouring of leaves, reducing the amount of pigment in the leaves which means the plant cannot photosynthesise as efficiently. The plant does not produce as much glucose so will display stunted growth. It is spread when the leaves of a healthy plant physically touches a plant with TMV.

Plants can also be infected by insects. Aphids feed on the sap within plant stems and are commonly found on tomato plants and roses. Sucking the sap from the plant’s stem reduces the amount of glucose available to the plant, reducing plant growth and can eventually lead to death of the plant.

Plants can suffer from deficiency diseases if they lack certain mineral ions:

• Nitrate deficiency — without sufficient nitrates, plants will have stunted growth and yellowing leaves. This is because nitrates are used to build proteins including chlorophyll. Without being able to make chlorophyll, the plant will not be able to photosynthesise and this contributes to their lack of growth.

• Magnesium deficiency — without enough magnesium, plant leaves turn yellow. Magnesium ions are needed to make chlorophyll. Plants which lack magnesium will therefore carry out less photosynthesis and will show stunted growth.

Detecting disease

Plant diseases can be detected by looking for the following symptoms:

• Stunted growth

• Abnormal stems or leaves

• Growths

• Decay

• Spots on leaves

• Discolouration

• Presence of pests

Gardeners can then identify plant diseases by:

1. Looking up the set of symptoms online or in a gardening manual

2. Taking infected plants to a laboratory to identify the pathogen

3. Using testing kits that contain monoclonal antibodies

Plant defence responses

Plants have ways of defending themselves from being infected with pathogens or eaten by animals. These defences can be grouped into physical, chemical and mechanical defences.

Physical defences

Physical defences form barriers to prevent pathogens from entering the plant. The following are all examples of physical defences:

• Thick layer of bark on trees

• Cell walls made of cellulose

• Tough waxy cuticle on leaves

Chemical defences

Some plants produce antibacterial chemicals which kill any bacteria that enter the plant. Plants like mint and witch hazel produce antibacterial chemicals which can be extracted and used in antiseptics. Other plants, like stinging nettles, produce poisons to deter being eaten by herbivores but this doesn’t prevent plants from infection by pathogens.

Mechanical defences

Plants may also have structural features which prevent them from being eaten. These include:

• Thorns and hairs – e.g. cacti have spines to protect them from being eaten

• Leaves that droop or curl when touched – e.g. the Mimosa pudica plant closes its leaves and droops whenever an insect lands on it. This makes it difficult for the insect to feed on the leaves.

• Mimicry – some plants can mimic other organisms to confuse animals. For example, the passion flower vine has yellow spots on its leaves which resemble butterfly eggs. Female butterflies think that eggs have already been laid on the leaf and choose another plant to lay their eggs on. This prevents the passion flower vine leaves from being eaten by the hatched caterpillars.