ka3: crop protection

• A weed is any kind of plant that grows where it is not wanted.

  In agriculture, weeds cause a significant economic impact.
• • 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.
• Annual plants are plants which complete their entire life cycle (from seed to death) in one year.
• Properties of annual weeds include:

  • Rapid growth.
  • Short life cycle.
  • High seed output.
  • Long-term seed viability.
• Perennial plants are plants which live for several years, becoming dormant in winter time and growing again in the spring.
• Properties of perennial weeds include:

  • Have storage organs to provide food when conditions are poor.
  • Reproduce vegetatively (asexually).
• Pests which attack crops broadly fall into three main groups:

  • Nematode worms.
  • Molluscs e.g. snails.
  • Invertebrate Pests e.g. aphids and butterflies.
• Invertebrate pests destroy the leaves. This reduces the plants ability to carry out photosynthesis, and therefore their ability to produce sugar. This causes a reduction in vigour and yield.
• Some pests can also be a vector (carrier) for other diseases which can damage the crop. E.g. aphids which transmit a virus which causes the disease potato leaf roll.
• Plant diseases are caused by pathogens. These can be either:

  • Bacteria.
  • Viruses.
  • Fungi.
• Plant diseases can affect the yield of the crop. They can also make crops less marketable if they are blemished or because they degrade too quickly in storage.
• 1. 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. The crop seed can then be planted and become established before the weeds return.
• 1. 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.
• 1. Crop Rotation

  A series of different crops are grown one after the other on the same piece of ground – often over 4 growing seasons.

  Pathogens in the soil from the first crop cannot then grow on the second crop.

  Growing leguminous (nitrogen fixing) plants can add nutrients to the soil.
• Pesticides include:

  Herbicides – kill weeds

  Selective herbicides have a greater effect on certain plant species (broad leaved weeds).

   Systemic herbicides spread to all areas of the plant through the vascular system and prevents regrowth.
• 1. Systemic insecticides (kill insect pests), molluscicides (kill mollusc pests) and nematicides (kill nematodes) spread through the vascular system of the plants and kills pests feeding on the plants.  Without these pesticides about a third of crops would be lost due to invertebrate “attack”.
• 1. Fungicides – control fungal disease

  Systemic fungicides are absorbed by the plant and therefore not   washed away by the rain. They only kill fungi that are affecting the crop plant.
• Applications of fungicide based on disease forecasts are often more effective than treating diseased crops, e.g. in preventing potato blight.
• Ideal characteristics of a pesticide

  • 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.

  Unfortunately not all pesticides have these characteristics.
• Plant protection chemicals may be toxic to non-target species e.g. animals. Chemical insecticides are generally intended for particular insect pests. Nevertheless, problems often arise because these chemicals are usually toxic to a broader range of organisms.
• Some plant protection chemicals may be persistent in the environment.  This means they are not biodegradable and remain in the environment for long periods of time after their application.
• Many pesticides 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. Bioaccumulation is a build-up of a chemical in an organism. 
• DDT. As a result of biomagnification the concentration of DDT is large enough to cause thinning of egg shells, reducing successful reproduction rates in birds of prey
• Biomagnification is an increase in the concentration of a chemical moving between trophic levels.
• The use of pesticides may result in a population selection pressure producing a resistant population of pests.
• The Colorado potato beetle has a legendary ability to develop resistance to a wide range of pesticides used for its control.The Colorado potato beetle has a legendary ability to develop resistance to a wide range of pesticides used for its control.
• Biological control describes the control of a pest population through the introduction of a control agent.  This is of one of its natural “enemies”.
• Either:

  1. A predator e.g. using ladybirds to control aphids.
  2. A parasite e.g. using Encarsia – a parasitic wasp which lays its eggs inside whitefly and destroys it.
  3. A pathogen e.g. using Bacillus thuringiensis – a bacterium which infects caterpillars with a toxin called Bt toxin.
• Biological control works best in “closed systems” such as greenhouses so that the control agent cannot “escape” into the wider environment.
• If escape occurs into an environment which is free from predators, parasites or disease the control organism may become an invasive species, parasite, prey on or become a pathogen of other species, e.g. Cane toads in Australia.
• Integrated pest management (IPM) combines chemical, biological and cultural control.  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.
• Weeds compete with crop plants, while other pests and diseases damage crop plants, all of which reduce productivity.
• Properties of annual weeds — rapid growth, short life cycle, high seed output and long-term seed viability.
• Properties of perennial weeds with competitive adaptations — storage organs and vegetative reproduction.
• Most of the pests of crop plants are invertebrate animals such as insects, nematode worms and molluscs.
• Plant diseases can be caused by fungi, bacteria or viruses, which are often carried by invertebrates
• Control of weeds, other pests and diseases by cultural methods.

  • Ploughing, weeding and crop rotation.
• • Pesticides include herbicides to kill weeds, fungicides to control fungal diseases, insecticides to kill insect pests, molluscicides to kill mollusc pests and nematicides to kill nematode pests.
• Selective herbicides have a greater effect on certain plant species (broad leaved weeds)
• Systemic herbicide spreads through vascular system of plant and prevents regrowth.