Plant disease - Lecture notes 7-10 PDF

Preview

Summary

Lecture notes for Biology these notes are a key asset to revision they are very helpful when it comes to revising they also help to assist in learning there are key subheading that can be used to create your own notes so you can understand them better! Diagrams are included to help understand the co...

Description

Plant disease – Biology Pathogens as causes of plant diseases. A pathogen is an organism that causes a disease. There are four main types of pathogen: -

Virus Bacteria Fungi protists

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.

VirusesA tobacco leaf infected with tobacco mosaic virus -

Tobacco mosaic virus (TMV) chlorosis seen in a backlit tobacco leaf (Nicotiana tabacum)

The tobacco mosaic virus infects tobacco and lots of other closely related species like tomatoes and peppers. It is transmitted by contact between plants, either naturally or on the hands of farmers. It 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 crinkled or curled up. This reduces the plant's ability to photosynthesise and grow properly, which can reduce farmers' crop yields. There is no cure, the farmers' only option is to stop their crops from being infected or reduce the spread if they have.

FungiA leaf infected with rose black fungus -

Rose black spot fungus causes black and purple spots on leaves

Rose black spot is 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-

Green Aphid sucking sap from stem of nettle

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.

Mineral ion deficienciesDeficiency diseases are diseases caused by a lack of essential vitamins or mineral ions. Scurvy is a deficiency disease historically found in sailors who did not have enough fresh fruit or vegetables, so were deficient in Vitamin C. Plants suffer from deficiency diseases when they do not have enough mineral ions.

Nitrate deficiency -

Chlorosis in orange leaves caused by magnesium deficiency

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. Without nitrates, the amount of chlorophyll in leaves reduces. 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 ion deficiencyPlants 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.

Symptoms of plant diseases and identification of pathogens SymptomsPlant pathogens cause diseases with a range of different symptoms. These symptoms can be used to identify the pathogen and then treat the disease, or limit its effects. 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 Just like animals, plants are constantly defending themselves from attack from pathogens. Again like us, 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.

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.

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.

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.

Communities These are important key terms: -

An ecosystem is the interaction between a community of living organisms and their environment. A community is two or more populations of organisms. An ecosystem is two or more populations of organisms (usually many more) in their environment. A population is all the organisms of the same or closely-related species in an area.

Interdependence All organisms in an ecosystem depend upon each other. If the population of one organism rises or falls, then this can affect the rest of the ecosystem. A simple food chain is: grass → rabbit → fox If the foxes in the food chain above were killed, the population of rabbits would increase because they are no longer prey to the foxes. As a result the amount of grass would decrease because the increased population of rabbits would be eating it.

Often very small changes to ecosystems have large consequences, which can be difficult to predict. This means that all the organisms in an ecosystem are dependent upon each other. We call this interdependence.

Competition All photosynthesising plants and algae in an ecosystem compete for light, space, water and minerals from the soil. Animals in an ecosystem compete for food, mates and their territory. Organisms which have more of these resources tend to grow more healthily and are more likely to have offspring. -

Competition can be interspecific or intraspecific depending on whether organisms from different species or the same species are competing for resources.

Stable communities A stable community is one in which the size of the populations of all species remain relatively constant over time. Example- above the amount of grass, and the numbers of rabbits and foxes all remain relatively constant. -

The different populations are living in a healthy balance with their environment.

Abiotic factors affecting the abundance & distribution of organisms The abundance is the number of organisms in an ecosystem and their distribution is affected by abiotic factors. These are factors that are non-living. They include: -

Light intensity

Some plants have evolved for optimum growth in bright sunlight. An example of this is a cactus houseplant. Cacti originally come from deserts where they grow in bright sunlight. Other plants have evolved to grow in shade. Many orchids, which are also kept as houseplants, grow on trees in the rainforest and have evolved for optimum growth in darker conditions. If you were to put an orchid on a bright windowsill and a cactus in a dark corner of your room neither plant would grow well. -

Temperature

Both animals and plants have evolved to grow healthily at their optimum temperatures. If you planted either your cactus or orchid houseplants outside in cold temperatures, they would die. Similarly, animals that have evolved to live at the North Pole, such as the polar bear, could not survive in warmer conditions.

-

Moisture levels

More people kill houseplants by overwatering than by under-watering them. Many plants cannot survive in waterlogged soils. Their roots are unable to respire, they rot and the plant dies. Other plants, such as pitcher plants, grow best in bogs where the moisture levels are high. Soil moisture meters can accurately determine how wet an area is. -

Soil pH content

The pH of soils can have a huge effect on the plants that are able to grow in them. Some plants, like azaleas, grow best in acidic soils and will quickly die if planted in alkaline soils. Others, like clematis, prefer alkaline soils. Some, like the hydrangea, can grow in both. These plants are unusual in that their flower colour changes in different soils. Just like universal indicator paper, hydrangea flowers are pink in acidic soils and blue in alkaline soils....