Crops can be driven out by weeds which take up available resources and repopulate the crop land area. Pests interfere with optimal crop growth by feeding on them, damaging them or transmitting disease. Microorganisms including viruses, bacteria and fungi can be agents of disease and compromise crops.
Weeds compete with crop plants, and can be categorised into annual and perennial.
Annual weeds get a head start on other plants by growing earlier, and focus on creating seeds that can last for long periods of time in the soil, awaiting good sprouting conditions. They include annual nettles and the opium poppy, and are characterised by fast growth with a short life cycle, high seed output and long-term seed viability.
Perennial weeds such as dandelions and stinging nettles are more difficult to deal with because they are long-lived and can grow back using an efficient root nutrient store. Their storage organs enable their survival over winter, and unless their whole root system is removed, they are able to grow the full plant again through vegetative reproduction.
Invertebrate species make up the variety of pests that crop plants must be protected against. They are often the carriers of disease agents. Insects, nematode worms and molluscs (slugs and snails) are prolific pests. For example, the whitefly is a pest of the tomato plant, while slugs feed on the leaves, flowers and stems of a wide range of plants, using their toothed tongue (creepy).
Disease agents include fungi, bacteria and viruses. For example, infection of cereal crops (including wheat) by fungi, specifically Puccinia graminis, has been an issue in agriculture since time immemorial, although fungus-resistant wheat strains became available since the 1950s. Can you see why it’s called stem rust? The fungus colonises the parts of the crop above ground level, and compromises the harvest very rapidly by disrupting seed development, as well as other symptoms of a diseased plant such as dead stems by breakdown of the surface barrier used for transpiration. This leads to uncontrollable water loss by evaporation.
The infection starts when the fungal spores carried by air land on the crop and establish themselves at the site through uredinia which are those reddish pustules. The infection affects the outer layers of the stalk and decreases the number of tillers produced by the plant. Fungicides can be used to prevent this infection.
Applying fungicides according to a disease forecast can be much more efficient than applying it to diseased crops.
Pesticides such as weedkillers (herbicides) and insecticides can be selective (or non-selective) and systemic (or contact). Selective plant protection chemicals only affect certain species, commonly certain weeds. Non-selective chemicals are useful in a large breakout, but risk contaminating wider areas, and weeds as well as other plants.
Systemic chemicals spread through the whole system of an organism, so if the leaves are sprayed, the chemical will reach the roots and other parts. Contact chemicals require application directly onto the target area in order to be effective.
Resulting issues with the use of these protective plant chemicals include leaching into the wider environment and potentially spreading through food chains, toxicity to certain animal species, and providing a strong selection force that results in resistance against the further use of pesticides, similar to the development of antibiotic resistance in pathogenic bacteria.
In order to mitigate these issues and provide the most efficient protection to crops, biological control strategies as well as integrated pest management strategies are employed.
Biological control involves the use of a natural predator of the pest being used to keep its spread in check, while the integrated management (IPM) involves the combination of both chemical control and biological control.
Introducing often exotic species to a new area can successfully result in a drop in the number of their prey. However, it is not a predictable setup, and it can cause unforeseen ecological problems as well as fail to accomplish its goal. By placing a new predator species into a new environment it may target the intended species, but also feed on non-intended species. This can cause an imbalance in the preexisting food chains, and disrupt the ecology of the area.
IPM aims to combine a number of principles and strategies in order to achieve crop protection. It includes responsible pesticide use, biological control methods, preventative measures, monitoring and maintaining a threshold of acceptable levels of pests as opposed to their complete annihilation.
Additionally, in the event of a breakout, mechanical methods of removal such as hand-picking and traps are prioritised over the use of chemicals.