9.4 The Search for Better Health
You have been looking at the different types of diseases that affect the human body and the natural defence mechanisms that maintain health. In this last part of the module you will look at how increased understanding has led to the development of a wide range of strategies to prevent and control disease. In this part you will have the opportunity to: • discuss the role of quarantine in preventing the spread of disease and plants and animals into Australia and across regions of Australia. • explain how one of the following strategies has controlled and/or prevented disease: - public health programs - pesticides - genetic engineering to produce disease resistant plants and animals • perform an investigation to examine plant shoots and leaves and gather first-hand information of evidence of pathogens and insect pests • process and analyse information from secondary sources to evaluate the effectiveness of quarantine in preventing the spread of plant and animal disease into Australia and across regions of Australia. • gather and process information and use available evidence to discuss the changing methods of dealing with plant and animal diseases, including the shift in emphasis from treatment and control to management or prevention of disease.
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Investigating plant diseases As you learned in an earlier Study Guide, many different organisms can cause disease in plants, including bacteria, viruses, fungi, nematodes and insects. All parts of a plant may be attacked, including roots, stems, leaves and flowers. It is sometimes difficult to identify the causes of plant diseases but black spots caused by fungi are usually easier to identify and viruses may cause leaf deformities or patchy discolouration on leaves or flowers. It is usually easy to identify damage to plants caused by insect attack, too. The edges of leaves may be chewed by caterpillars, there may be white tracks caused by leaf miner insects, or there may be large round swellings of galls caused by wasp larvae. On gum leaves you may see white lerps (the sugary houses of psyllid bugs) or on wattle stems, you may see resin plugs indicating that a borer (witchetty grub) is inside. Fungi are the most common cause of plant disease. Rusts, rot, mildews, petal blight and black spot are examples of fungal diseases. Viral diseases include rose mosaic virus which results in yellowing of the leaf, or tomato leaf curl virus, which causes the plant to be stunted and the leaves to be rolled inwards. Bacterial diseases of plants include blight, galls and bacterial wilts. Nematodes are roundworms that infect leaves, roots or underground bulbs and tubers. Insect pests include lace bugs that live on the underside of leaves such as azaleas where they suck sap, causing a speckled brown appearance. Aphids are sap-sucking insects that cause leaves to become distorted. Leaf miners are the larvae of a moth. They burrow through the leaf, leaving behind a silvery trail.
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In this activity, you will examine some plants shoots and leaves and record evidence of pathogens and insect attack. Using a hand lens, examine the selection of plant material available. Look for evidence of plant disease (in other words, look for the symptoms of disease) and look for evidence of insect attack. Separate the plants into two groups - those suffering a disease, and those with evidence of insect attack. Evidence of disease could include discolouration, browning, blotching, swelling, leaf curling, local death of tissue, or even visible mould or powder on the surface of a leaf. Use the photographs or the Internet to help you identify the pathogen. Record your observations in the table below and, if necessary, use the Internet to find the scientific, binomial name of the pathogen. Examine more closely the plants showing evidence of insect attack. Describe the damage that you observe and suggest...