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Plant disease (Plant defences (Physical defences (Many plants are covered…
Plant disease
Plant defences
Physical defences
Many plants are covered with thick bark. This is an external layer of dead cells which forms a physical barrier against infection. In this way, it is very much like our skin. This can fall off the plant, taking the pathogen with it.
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Mechanical defences
Thorns and hairs
Plants like roses have evolved large thorns to avoid being eaten, while plants like cacti have evolved thin spines. These adaptations protect plants from damage and keep their vital water stores safe.
Drooping leaves
The Mimosa pudica plant is very special. It has evolved to close its leaves and then point its stems towards the ground when touched by an insect as it lands on it, making it difficult for the insect to feed on the leaves.
Mimicry
Some plants have evolved the ability to mimic what is around them. The passionflower vine has evolved small yellow spots on its leaves. These look like the eggs left behind by butterflies. Female butterflies are unable to tell the difference between these and real eggs, so they lay eggs on other leaves to avoid competition when they 'might' turn into caterpillars. Therefore, the passionflower vine suffers less damage to its leaves from feeding caterpillars.
Chemical defences
Some plants such as mint and witch hazel produce antibacterial chemicals. These limit the spread of bacteria that were not stopped by physical defences.
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Other plants, like stinging nettles, have developed poisons to stop themselves being eaten by herbivores. These do not defend plants from infection by pathogens.
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Deficiency diseases
Nitrate deficiency
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Plants use nitrates as a supply of nitrogen, which is needed to make proteins for healthy growth. Plants absorb nitrates in water through their roots.
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Fungal disease
Rose black spots
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It infects leaves and causes black or purple spots on the leaves. The rest of the leaves often turn yellow and can drop off the plant.
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It is treatable using fungicides and by removing and destroying infected leaves. (Burning all infected leaves and stem)
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Viral diseases
Tobacco mosaic virus
It is transmitted by contact between plants, either naturally or on the hands of farmers.
It infects the chloroplasts of plant leaves and changes their colour from green to yellow or white in a mosaic pattern. It can also make leaves crinkled or curled up.
This reduces the plant's ability to photosynthesise and grow properly, which can reduce farmers' crop yields.
The tobacco mosaic virus infects tobacco and lots of other closely related species like tomatoes and peppers.
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