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.

The advantages of pesticides which are either selective or systemic.

• 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.

• Systemic insecticides, molluscicides and nematicides spread through the vascular system of plants and kill pests feeding on plants.

Problems with pesticides: toxicity to non-target species, persistence in the environment, bioaccumulation or biomagnification in food chains, producing resistant populations of pests.

• Applications of fungicide based on disease forecasts are more effective than treating diseased crops.

• Bioaccumulation is a build-up of a chemical in an organism. 

• Biomagnification is an increase in the concentration of a chemical moving between trophic levels.

Control of weeds, other pests and diseases by biological control and integrated pest management.

• In biological control the control agent is a natural predator, parasite or pathogen of the pest.

• Integrated pest management is a combination of chemical, biological and cultural control.

Risks with biological control.

• The control organism may become an invasive species, parasitise, prey on or be a pathogen of other species.