Transcript Planet Earth and Its Environment A 5000

A Search For Better Health
Topic 13: Diseases of Plants
Biology in Focus, HSC Course
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.
www.conservationcorridor.org
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.
www.apsnet.org
Diseases caused by pathogens
Fungi are the most common
causes of plant disease, with at
least 10 000 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.
www.ipm.iastate.edu
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.
www.ars-grin.gov
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.
www.dpi.vic.gov.au
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.
www.abc.net.au
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