Healthy and unhealthy plants

1. Healthy and Unhealthy Plants Learning Objectives: What are the signs of a healthy plant? Why are aphids and slugs pests? What is damping off and potato blight? How can pests, viruses and fungi be controlled in glasshouses?

2. Describe three features of a healthy plant.

3. Poor answers: It has a good colour It has flowers It looks good Better answers: Leaves are a dark green colour It has lots of brightly coloured flowers There are no signs of pests of diseases

4. Wilting Plants lose water all the time through their stomata during transpiration . Transpiration is quickest if: it is warm it is dry it is windy If the plant loses too much water then the leaves will become flaccid and wilt . Wilted leaves cannot absorb light and so cannot photosynthesise or grow.

5. Pest Damage Some pests such as slugs and caterpillars eat leaves so there is less area for photosynthesis.

6. Aphids Aphids have long, tubular mouthparts which they use to suck sucrose from the phloem. This reduces the amount of sugar available to the plant for respiration and therefore energy. Aphids can also transmit viruses from one plant to another. Aphids can be controlled by spraying with insecticide or using biological agents.

7. Poor Growth Lack of nutrients can lead to leaves becoming discoloured and reduced photosynthesis.

8. If soil is too damp or the air is too humid, seedlings can get a fungal disease called damping off. The seedlings fall over and white mould can be seen at the base of the stems. To prevent this, do not overwater seedlings and ensure adequate ventilation. Damping Off Disease

9. Potato Blight This is caused by a fungi which can also infect tomatoes. Fungal spores are spread through the air and it is most likely to occur when the weather is warm and humid. The leaves will start to develop brown patches. Tubers will have brown patches on their skin and the inside becomes brown and soft. To prevent the diesase spreading remove and burn infected leaves and spray with a fungicide. Resistant varieties can be grown.

10. Only grow good quality seed potatoes to prevent spreading the disease.