Plant Disease

• All types of pathogen have a simple life cycle.

  They infect a host, reproduce (or replicate themselves if a virus), spread from their host and then infect other organisms.
• Diseases caused by pathogens are called communicable diseases. This means they can be caught.

  The spread of communicable diseases is called transmission.
• Plant pathogens kill or reduce the growth of many plants, which in turn can reduce biodiversity.

  Plant pathogens of food crops are especially serious, such as the Irish potato famine around 1850, which caused deaths of large numbers of people through starvation.

  The pathogen caused 'blight' - an infestation that discolours the leaves but also rotted the tubers making them inedible.
• A pathogen is an organism that causes a disease.

  There are four main types of pathogen:

  • Virus.
  • Bacteria.
  • Fungi.
  • Protist.
• Tobacco mosaic virus - Viral

  • Infects tobacco or species like tomatoes and peppers.
  • Transmitted by contact between plants, either naturally or by the hands of farmers.
  • Infects the chloroplasts of plant leaves and changes their colour from green to yellow or white in a mosaic pattern.
  • It can also make leaves crinkle or curl up.
  • Reduces the plant's ability to photosynthesise and grow properly, which can reduce farmers' crop yields.

  There is no cure, thus farmers' only option is to stop their crops from being infected or reduce the spread if they have.
• Rose black spot - Fungi

  • Caused by a fungus which infects roses.
  • It infects leaves and causes black or purple spots on the leaves.
  • The rest of the leaves often turn yellow and can drop off the plant.
  • This reduces the plant's ability to photosynthesise and reduces growth.
  • Black rose spot can be transmitted in air or water as well as direct contact by gardeners.
  • It is treatable using fungicides and by removing and destroying infected leaves.
• Insects

  • As well as microbial pathogens, insects can infest a plant.
  • Aphids are a common example of these seen in UK gardens and greenhouses.
  • In particular, they infest roses and tomatoes.
  • They are found in large numbers on new shoots, from which they suck sap.
  • This reduces that growth rate of plants and can eventually kill them.
  • Aphids are eaten by ladybirds, so gardeners often try to cultivate these in their gardens as a natural way of reducing aphids.
• Deficiency diseases are diseases caused by a lack of essential vitamins or mineral ions. Plants suffer from deficiency diseases when they do not have enough mineral ions.
• Nitrate Ions

  • Plants use nitrates as a supply of nitrogen, which is needed to make proteins for healthy growth.
  • Plants absorb nitrates in water through their roots.
  • Nitrates are present in high levels in plant fertilisers.
• Nitrate Ions

  • Without nitrate ions, the amount of chlorophyll in leaves decreases.
  • This means leaves turn a pale green or yellow colour.
  • This reduces the plant's ability to photosynthesise and grow properly, which reduces the farmers' crop yield.
  • Farmers or gardeners can add chemical or natural fertilisers, such as manure to increase nitrate levels.
• Magnesium Ions

  • Plants use magnesium ions to make chlorophyll in their leaves.
  • Like in nitrate deficiency, the plant is limited in terms of its photosynthetic ability and the plant growth is compromised.
  • Magnesium is a limiting factor in healthy plant growth.
  • A deficiency in magnesium ions causes chlorosis (discoloured leaves).
• GROWTH OF PLANTS IN DIFFERENT NUTRIENT SOLUTIONS PRACTICAL NOT DONE!!!!!!!!!!!!
• Mealybugs cause stunted growth in plants.
• Rose black spot causes purple or black spots on leaves.
• The rice blast fungus causes decay.
• Identification

  • Farmers and gardeners often use books and the internet to identify plant diseases.
  • They can also take a small cutting of an infected plant (or a photograph of it) to a local garden centre, which have staff that can often help identify and treat the disease.
  • For diseases that are more difficult to identify, cuttings of the plant may be analysed by scientists in laboratories using testing kits containing monoclonal antibodies.
• Plant defences

  • Plants constantly defend themselves from attacks from pathogens.
  • Plants have physical and chemical defences which stop pathogens from infecting them and causing disease.
• Physical defences

  • Many plants are covered with a thick bark. This is an external layer of dead cells which forms a physical barrier against infection. In this way it is very much like our skin.
  • Beyond bark, each plant cell has a cellulose cell wall which acts as another barrier against infection.
  • Leaves are covered with a waxy cuticle which also stops their cells from becoming infected.
• Chemical defences

  • Some plants such as mint and witch hazel produce antibacterial chemicals. These limit the spread of bacteria that were not stopped by physical defences. These chemical defences are now used in antiseptics for humans.
  • Other plants, like stinging nettles, have developed poisons to stop themselves being eaten by herbivores. These do not defend plants from infection by pathogens.
• Other defences

  • Farmers can now genetically engineer crop plants to be resistant to infections.
• Mechanical plant defences

  • Plants have also evolved other defences from attack. These defences are often to avoid being eaten by herbivores.
• Mechanical Plant Defences

  Thorns and hairs

  • Plants like roses have evolved large thorns to avoid being eaten, while plants like cacti have evolved thin spines.
  • These adaptations protect plants from damage and keeps their vital water stores safe.
• Mechanical Plant Defences

  Drooping leaves

  • The Mimosa pudica plant is very special.
  • It has evolved to close its leaves and then point its stems towards the ground when touched by an insect as it lands on it, making it difficult for the insect to feed on the leaves.
• Mechanical Plant Defences

  Mimicry

  • Some plants have evolved the ability to mimic what is around them.
  • The passion flower vine has evolved small yellow spots on its leaves. These look like the eggs left behind by butterflies. Female butterflies are unable to tell the difference between these and real eggs, so they lay eggs on other leaves to avoid competition when they 'might' turn into caterpillars.
  • Therefore, the passion flower vine suffers less damage to its leaves from feeding caterpillars.