Online Gardener's Handbook 2010
Chapter 1: Causes of Plant Injury
Causing Plant Injury
Table of Contents
and Bacterial Damage
are caused by fungi, bacteria, viruses and nematodes. These organisms attack plants,
living at the expense of their hosts.
Fungal and bacterial diseases can be broadly categorised
by the type of damage they cause.
Foliage diseases attack leaves. Leaves
may turn yellow, drop off or become deformed due to vascular wilt or secondary
factors such as air pollution or drought. Common diseases include Botrytis, powdery
mildew, downy mildew, leaf spots, black spot, rusts, anthracnose and needlecast.
and root rots often attack seedlings. The lower stem of the plant appears water
soaked. Eventually, the plant rots off at the soil line. Soil fungi such as Pythium
and Phytopthora attack herbaceous and woody plants. Rhizoctonia and Fusarium cause
brown to black lesions on lower stems and roots.
Canker diseases are fungal,
and commonly attack plants weakened by drought, root disturbance or poor root
environment. Cankers kill bark and underlying tissue in patches on branches and
stems. The bark splits as it dries and is easily removed, exposing the wood below.
Fruiting bodies from the fungus are often seen on the dead bark areas.
wilts occur when fungi or bacteria enter a plant's vascular system and reduce
water movement to the leaves. Leaves often recover in the early stages during
cool, moist periods. Eventually, however, leaves and branches wilt and die; the
entire plant dies soon after. Serious wilt diseases include verticillium wilt
and Dutch elm disease.
Many fungal and bacterial diseases have similar names
on different crops, even though they are caused by different pathogen species.
For example the fungus causing powdery mildew on grapes is be different from the
fungus causing powdery mildew on cucumbers. This can be important in some cases,
because the course of the disease and the management practices taken can vary
with the pathogen causing the disease. For example, downy mildew on cucumber is
often much more serious than downy mildew on broccoli.
are a group of living organisms that have some similarities to plants, but unlike
plants they do not have chlorophyll and so must take their nourishment from living
or dead organisms to survive and reproduce. On infected plants, they appear as
delicate strands (filamentous mycelium) that grow on or in plant tissue and may
not be visible to the naked eye. Most reproduce by spores produced by the millions
and spread by air currents, water, soil and insects. Under suitable environmental
conditions, and if a suitable host is near, the spores will germinate and infect
the plant. They enter through wound sites and natural plant openings or by forcing
their way through the outer layer (epidermis) of plant tissue. Many fungi also
produce resting spores that can withstand long periods of unsuitable conditions.
that attack foliage, flowers and stems are usually spread by air currents or splashing
water. Fungi that attack plant roots, crowns or the sap stream of plants (wilts)
survive indefinitely in the soil, until suitable germination conditions exist.
The damage that they cause includes a breakdown of tissues (soft rot), cell death
(necrosis or leaf spots) and the blockage of water conducting tissues.
Bacteria are microscopic single-celled organisms that reproduce rapidly inside
the plant under favourable conditions. Bacteria enter plants through wounds, pruning
cuts and the pores of leaves. They can also be spread mechanically, though splashing
water and rain is the most common delivery system. Once inside, they affect most
parts of the plant.
Bacteria are often classified as wilts
(as in geranium wilt), blights (as in lilac blight), or soft
rots. Wilt-causing bacteria tend to clog up the vascular tissue, inhibiting
the flow of water and nutrients. Blights attack the young shoots and the outer
layers of tissue first. Soft rots develop on fleshy tissues in wounded areas of
the plant that remain wet for long periods.
Symptoms of wilt disease appear
primarily on the leaves, and include large V shaped areas of dead tissue, yellowing,
and a wilted appearance. Grey or black streaking may be apparent on leaves and
stems. Under warm, humid conditions, plants collapse very quickly.
may first appear as black spotting on leaves and stems. With many woody ornamentals,
flower buds, young leaves, and shoots may turn completely black and die off. Infection
occurs in early spring during extended periods of wet weather.
often develop on fruits or vegetables when water sits next to them for long periods.
This usually occurs where a wound already exists on the plant. Bacterial soft
rots usually have a foul odor and appear wet and gooey.
are only visible with electron microscopy. They reproduce only in the body of
a living host. The most common viral diseases are classified by their symptoms:
mosaics, yellows, stunts, mottles and streaks.
Viral diseases exhibit a variety
of symptoms. The most common is the uneven yellowing of the leaf, which gives
rise to a mosaic pattern of yellow and green. With some systemic viruses, the
areas adjacent to the veins turn yellow. Still other viruses can cause ring spots,
stunting, distortions of leaves and flowers, and premature death.
are carried from infected to healthy plants by insects. Infection can also be
spread by humans when handling plants, taking cuttings or pruning. Many viruses
are carried over from season to season in tubers, bulbs, corms and occasionally
Nematodes are microscopic worms
that attack plant roots to obtain food. Leaves may also be attacked, though this
is rare. In some cases, the roots become stubby, with swollen knots or growths.
(Do not confuse the nitrogen fixing nodules on leguminous plants with damage from
nematode infestation.) Small lesions are often also visible. As the disease progresses,
root rot due to invasion by other organisms usually occurs. The plant gradually
loses vigour, showing symptoms of drought, stress, nutrient deficiencies, and
stunted growth. Nematodes thrive best in sandy soils.