Biology in Focus, HSC Course A Search For Better Health Topic 13: Diseases of Plants

DOT Point(s) perform an investigation to examine plant shoots and leaves and gather first-hand information of evidence of pathogens and insect pests

Introduction Pathogens and insect pests cause many different types of plant disease. The pathogens that cause disease in plants are viruses, bacteria, fungi and nematodes (roundworms). Some examples of insect pests that cause disease in plants are mites, aphids, scale insects and caterpillars of moths and butterflies.

Introduction Regardless of the cause of the plant disease, there is always some sort of visible evidence of the effect of the disease-causing organism on the plant. Similar types of diseases can be caused by different pathogens; for example, blights and galls may be caused by bacteria or fungi. strawberryplants.org

Introduction Definitions of some of the terms used to describe diseases of plants are given below: blight—a disease that causes death of plant tissue gall—a growth or swelling caused by invasion of pathogens mosaic—variation in the colour of leaves, usually light and dark green patches.

Diseases caused by pathogens Fungi are the most common causes of plant disease, with at least different types of fungi capable of causing disease in plants. They are responsible for diseases such as rusts, wood rot, powdery mildews, downy mildews, petal blights and black spot. web2.newtown-h.schools.nsw.edu.au

Diseases caused by pathogens Black spot is a fungal disease of roses that causes the leaves to develop black spots, turn yellow and eventually drop off. Powdery mildew is a fungal disease that causes a white coating on the leaf; if not treated, it will cause the leaf to become misshapen.

Diseases caused by pathogens Viral causes of disease in plants are nearly always characterised by colour changes and patterning of the leaf (mottling, mosaics or streaking). Malformations of the leaves, stems, flowers and fruit can also be an indication of infection by viruses.

Diseases caused by pathogens Some examples of viral diseases in plants are rose mosaic virus, which causes yellowing of the leaves in a mosaic pattern and celery mosaic virus in coriander. Another viral disease is tomato leaf curl virus, which causes tomato plants to become stunted and the leaves to be rolled upwards and inwards. forums.gardenweb.com

Diseases caused by pathogens Bacterial diseases of plants are characterised by spots on leaves and stems, soft rots, galls and wilts. Bacterial blight in pea plants is characterised by water-soaked lesions that become dark brown. The infected tissue becomes very thin and causes distortion of the leaf.

Diseases caused by pathogens Nematodes are tiny roundworms that cause small lumps on the leaves, leaf distortion and scorches or blotches on the leaf. Nematodes also cause extensive damage to the roots. When the stem and bulb of garlic become infested by nematodes, the leaves become bloated, twisted and swollen. Potatoes can also be affected by nematodes.

Diseases caused by insect pests Many diseases are caused by insect pests. Some examples of these are: ■ lace bugs, which affect many plants including the azalea. These bugs inhabit the underside of the leaves and suck out the sap, causing the leaves to turn speckled brown. Sugary liquid called honeydew is also secreted and sometimes a sooty mould develops on this, causing the leaves to appear a rusty colour. web2.newtown-h.schools.nsw.edu.au

Diseases caused by insect pests ■ two-spotted mites (formerly known as red spider mites), which cause the leaves to turn a dull green with pale mottling; then the leaves turn yellow and the webbing spun by the mites becomes visible. New growth is curled under and has a slightly brownish tinge. web2.newtown-h.schools.nsw.edu.au

Diseases caused by insect pests ■ aphids, which are sap-sucking insects that infest many different types of plants. They cause the growth of the plants to be stunted, the leaves to wilt and the buds to drop. They also cause distortion of the leaf. antsbeesbutterfliesnature.blogspot.com

Diseases caused by insect pests ■ leaf miners, which are the larvae of an introduced moth. After the female moth lays her eggs and the larvae hatch, the larvae burrow between the upper and lower leaf surface of the young leaves. The leaf becomes twisted and misshapen as the larvae eat and mine their way through it. A light-coloured trail is left as they burrow through the leaf. commons.wikimedia.org

Activity -Examine plant shoots and leaves for evidence of pathogens and insect pests web2.newtown-h.schools.nsw.edu.au