Key area 3: Crop protection

1. Key area 3: Crop protection

2. Weeds, pests and diseases LI: 1. Describe the main features of weeds and what problems they cause. 2. Describe the main pest species that attack crops. 3. State the causes of the main diseases which affect crops

3. Monoculture In a natural ecosystem a balance exists between the producers and consumers. There is also a diverse variety of plant species that co-exist with their fellow plants, insects and micro-organisms. The diversity and genetic variety amongst species make them more resilient to weeds, pests and fungal infections.

5. Background Information • In monoculture only one species of crop plant is grown, usually in a large area. • The members of the crop are often genetically identical. • This makes them very susceptible to pests, disease and competitive weeds. Weeds, pests and fungal infections have to be tightly controlled to ensure the highest yield of the crop.

7. Weeds Think about what plants in the same habitat compete for: Competition in Plants

8. Competition in monoculture • Plants compete for light, space, water and soil nutrients. Weeds will compete for these with the crop plants. • Competition can be reduced in monocultures by growing the plants in rows. • However, the gaps fill in with weeds. A weed is any kind of plant that grows where it not wanted.

9. Economic impact of weeds In agriculture weeds cause a significant economic impact due. They may: • Cause a significant reduction in the productivity of the crop due to competition. • Release chemical inhibitors into the soil which further reduces the crop. • Contaminate grain crops with their seeds and reduce the crop’s value. • Act as hosts for crop pests and diseases.

10. Properties of annual weeds Annual plants are plants which complete their entire life cycle (from seed to death) in one year. Grow very quickly Produce vast numbers of seeds Annual Weeds Short life cycle Seeds are viable for long periods of time

11. Properties of Perennial weeds Perennial plants are plants which live for several years, becoming dormant in winter time and growing again in the spring. Already established in the habitat Reproduce vegetatively (asexually) Perennial Weeds Have storage organs to provide food when conditions are poor

12. Invertebrate pests Pests which attack crops broadly fall into three main groups: Insect Pests Nemotode worms Molluscs e.g. snails or slugs e.g. Potato cyst nemotode e.g. aphids, caterpillars

13. Effect of invertebrate pests Invertebrate pests such as caterpillars damage the leaves. This reduces photosynthesis, and therefore sugar production. Loss of cell sap in phloem to aphids reduces energy supply to the tissues. This causes a reduction in vigour and yield. Some pests can also be a vector (carrier) for other diseases which can damage the crop.

14. Plant diseases Plant diseases are caused by pathogens. These can be either: • Bacteria • Fungi • Viruses These can be spread through the air, the soil, or by invertebrate vectors. This is another reason for the use of pesticides.

15. Economic effect of plant diseases Plant diseases can greatly reduce the yield of the crop. They can also make crops less marketable if they are blemished or because they degrade too quickly in storage.

16. KEY AREA 1b – Weeds, pests and diseases • Testing Your Knowledge 1 Page 269 Q2-4 • What you should know page 291 Q 1-3

17. Control of weeds, pests and diseases LI: • Explain what is meant by control of weeds, pests and diseases by cultural means. • Explain the advantages of plant protection chemicals which are selective or systemic. • Explain how disease forecasts can used help successful treatment of crops.

18. Control of weeds, pests and diseases Weeds, pests and diseases can be controlled into two ways: • By cultural means • By chemical means

19. Cultural methods of crop protection These are techniques for controlling weeds, pests and diseases which are non-chemical. They have developed over a long period of time, some by trial and error.

20. Ploughing – by turning over the top 20cm of soil every time a field is ploughed many weeds are buried deep enough for them to die and decompose.

21. Ploughing (cont.) Time of sowing • Sow when • Annual weeds have died (exist only as dormant seeds) • Perennial weeds have been ploughed

22. 2. Weeding – removal of weeds should be done early in the life of the crop to reduce competition. Weeds should also be removed from the edges of fields as they provide a breeding ground for pathogens.

23. Field 1 Potatoes Field 2 Cabbage 3. Crop rotation A series of different crops are grown one after the other on the same piece of ground – over 4 growing seasons. Field 4 Onions Field 3 Peas (nitrogen fixing) Pathogens in the soil from the first crop cannot then grow on the second crop. Growing leguminous (nitrogen fixing) plants such as clover and beans can add nutrients to the soil.

24. Clearance of crop residue • E.g. stubble or straw (may harbour fungal spores if left) • Cover crop • Planting of crop e.g. clover, that prevents growth on fallow ground. The clover can then be ploughed into

25. Chemical means of crop protection Herbicides, fungicides, insecticides/pesticides, molluscicides and nematicides can be used to control pests when cultural means of control fail.

26. 1. Herbicides Used to kill weeds. These can be either: (a) selective (b) Systemic

27. (a) Selective herbicides Selective herbicides mimic the action of plant growth hormones. This speeds up the metabolism of broad leaved plants to the extent that they use up their food reserves and die. Narrow leaved plants e.g. cereal crops, are not affected. Selective herbicides are usually biodegradable though some residues may enter the food chain.

28. (b) Systemic herbicides Systemic herbicides are absorbed by the plant and quickly transported to all areas of the plant through the vascular system and prevents regrowth. This has a lethal effect on the leaves and the roots of the plant. They are usually effective as they reach storage organs and root systems

29. 2. Pesticides Are essential to protect crops. With out them about a third of crops would be lost due to invertebrate “attack”. These can be either: • Contact- kills the invertebrate when it comes into contact with the pesticide • Systemic

30. (b) Systemic pesticides These are absorbed by the plant and spread through the vascular system of plants. It kills invertebrates when they feed on the plants.

31. 3. Fungicides Kill fungal parasites which cause diseases in crop plants. Applications of fungicides based on disease forecasts are more effective than treating diseased crops.

32. Weeds removed early to prevent competition with crop plant. Weeds in top 20 cm of soil and buried and die. Weeds in top 20 cm of soil and buried and die. Ploughing Weeding Crop rotation Cultural Control of pests Chemical Fungicide Herbicide Systemic Insecticides, Molluscicides and nematicides Selective Based on disease forecasts Affects broad leafed plants only. Mimics plant growth hormones. Increases metabolic rate – growth outstrips demand and plant starves. Systemic Absorbed by the plant. Only kills the invertebrate if it eats the plant. Quickly transported through entire plant. Kills roots and shoots.

33. Problems with plant protection chemicals LI: • Describe the ideal characteristics of a pesticide • Explain what is meant by bioaccumulation. • Describe the effects of bioaccumulation on biodiversity.

34. Ideal characteristics of a plant protection chemical Ideally a plant protection chemicals (herbicide, pesticide and fungicides) should be: • Specific to the pest concerned. • Have a short life (i.e. should not persist in the environment – but be broken down into harmless by-products). • Be safe for human users and animals.

35. Bioaccumulation Many plant protection chemicals have been found to persist in the environment. Whilst they are found at low concentrations in the environment they accumulate along food chains – this is called bioaccumulation.

36. An example of bioaccumulation: DDT DDT is a plant protection chemical which is now banned. Watch this clip http://youtu.be/DxqDaTUh08o and http://youtu.be/Ipbc-6IvMQI on the “Silent Spring” Research and make notes on what DDT was used for and what happened to the plants and animals at each stage in the food chain.

37. Biological control and integrated pest management LI: • Describe what is meant by “biological control” • Explain the possible risks associated with biological control. • Explain what is meant by “integrated pest management”.

38. Biological control Biological control describes the control of a pest population through the introduction of one of its natural “enemies”. Either, • A predator • A parasite • A pathogen

40. Risks associated with biological control Biological control works best in “closed systems” such as greenhouses so that the control agent cannot “escape” into the wider environment.

41. If escape occurs into an environment which is free from predators, parasites or disease then their numbers could increase rapidly and infect the local populations. e.g. Cane toads in Australia Watch this clip on Biological control gone wrong: http://youtu.be/IFIsz4szc_A

42. Integrated pest management IPM uses a combination of chemical, biological, cultural methods and plant resistance to improve yield. The main aim of IPM is to reduce chemical use and only use chemicals which do not persist and reduce pests to levels which allow biological control methods to take over.

43. KEY AREA 1b – Weeds, pests and diseases • Testing Your Knowledge 1 Pages 282-283 Q1-4 • What you should know Page 291 Q 4-7