Use the links on the right to view details of some common plant diseases. The link at the top is for pictures of some of the diseases.
Plants can suffer from bacterial,
viral and fungal attack just as we can ourselves.
The organisms themselves (pathogens)
are different, but at the microbial level the
infection is much the same since one cell is as
good a host as another. No matter which part of
the plant is attacked the effect is usually to
weaken or kill it. By infecting the leaves the
plant's ability to produce its food is reduced.
Some pathogens block the vessels in the stems
which supply the leaves and by attacking the
roots, the uptake of water and nutrients is
reduced or stopped completely.
When a plant is attacked by one of these microorganisms the damage caused provides an opportunity for the others to get in and often it is the combined onslaught with secondary and tertiary infections which deals the final blow. Also if it is under stress, such as through drought or poor nutrition it is more susceptible.
Sometimes the 'infection' is symbiotic where both
organisms derive a benefit. A good example of
this is the nitrogen fixing bacteria ( Rhizobium ) which reside in
nodules on the roots of leguminous plants (pea family), the plant provides food and
protection, the bacteria takes nitrogen from the
air and converts it to a form usable by the host.
The Mycorrhizal fungi have a similar symbiotic relationship with many plants, but in this case the plant provides the photosynthetic process in exchange for an enhanced uptake of nutrients. Lichens are an algae and a fungus growing together as a symbiotic conjunction, ie. the fungus provides physical support for the algae and the algae produces food.(*Lichens are not harmful, but a monograph is included as they are sometimes thought to be.)
There are some less welcome fungi which attack living plants and weaken or kill them - these are the ones which are mentioned in more detail here.
In other cases of interest to gardeners, the plant is not
benefited but the changes caused produce more
attractive features. This is what happens with
Aucuba japonica where a viral infection produces
the mottled leaves in the 'Variegata' variety.
The Tulipomania craze in the 17th century was
caused when 'Broken' varieties of tulips began to
appear with streaked and mottled petals. This occurred at random and increased the desire and fascination for the bizarre effects. Prized
examples were valued at more than "a mansion with
servants", although it was not discovered until
the early 20th century that viruses were to blame
for the unusual colours and effects. The virus inhibits the production of the pigment in the petals and the underlying white tissue is exposed giving random streaks. In Holland,
where the craze caused great hardship when it
crashed in 1637, they now grow the pure forms
with no streaks or frills.
Fungi are essential in
breaking down dead organic matter to produce the
humus which is needed for good soil structure - saprophytes. They do not have any chlorophyll so cannot use light to capture energy, instead they derive their energy by breaking down plant and animal material - alive or dead.
The widespread use of chemical
control can damage the balance of these
beneficial fungi and this forms part of the
principals of Organic management.
dwell inside the cells and cannot be treated with
chemicals so affected plants must be destroyed (special
techniques may overcome the infection by taking
cells from the growing tip, but this is
restricted to the laboratory). There are no
antibiotics for plants, so bacterial attacks, eg. fireblight, are
untreatable as well. Some fungi can be killed with
chemicals without damaging the host because their
growth habit is different, ie. they tend to grow
on the plant and not in it, using root-like
structures to extract nourishment.
Since killing the pathogens is
difficult or impossible, "prevention is
better than cure". By observing good hygiene
when propagating and growing your plants, you can
prevent a lot of diseases from taking hold.
Destroy diseased plants,
clear up dead leaves and other debris.
- As the spores can have tough outer coatings, do not add diseased material to the domestic compost heap as they do not usually achieve high enough temperatures to destroy them.
Prune fruit trees and
bushes regularly to keep an open
structure allowing a good air flow and
to remove damaged branches.
Disinfect secateurs, saws or
knives used for cutting out diseased
branches with methylated spirits or a
flame (a cigarette lighter comes in handy
for this). Household bleach or other disinfectants can also be used as a dip for shears and clippers, including electric or petrol machines. This also helps when taking
Only use new or well-washed
containers when growing cuttings and
Crop rotation in the
vegetable plot will prevent a build up of
Space plants well apart
especially crops where similar plants are
growing together, to allow good air flow. Fungal diseases in particular, thrive in still, damp air and there is a greater chance for them to be transmitted to surrounding plants if they are in close proximity.
Catching disease early is
important so keep an eye out for it at
- Plants are more susceptible to disease if they are not growing well. This can be due to poor soil, drought or both. So prepare the site well adding plenty of organic matter and give plants an occasional feed. As gardeners we are usually trying to grow plants which most likely are not native to the region or in a place they would not choose for themselves, so there is a greater probability that they could be under stress.
Of course not all plant diseases
can be prevented by good hygiene as some are
transmitted by insects and others are wind-borne.
Aphids and other sap-sucking insects are the main
vectors of viruses. Dutch Elm Disease is fungal transmitted by Elm Bark Beetles and the bacterial Zylella festidiosa disease by Froghoppers. Tobacco mosaic virus can be harboured in cigarettes, so smokers are
banned from greenhouses where Surfinia Petunias (susceptible
to the virus) are raised from cuttings - all stocks had to be destroyed in 1995 when they became infected. The spores of some fungal diseases are carried in the air, and in rain drops and splashes.
Be careful when bringing plant material into the garden. Diseases can be in the soil as well as the plants themselves. Some gardeners will only raise their plants from seed, especially vegetables as some onion sets may have White Rot or brassicas may have Clubroot and this would infest the plot for years to come.
Some plants have a natural resistance to diseases, particularly if they have evolved together. With detailed knowledge of genetics, this resistance can be transferred to susceptible plants. This is new technology and these Genetically Modified Organisms are controversial.
The globalisation of the nursery industry means that plant material is being carried from far-flung parts of the world and just like the movement of people in past centuries nearly wiped out indigenous populations with previously unencountered diseases, there is an increased risk to our native flora. This has been shown most recently with Ash Dieback (Chalara fraxinea) on imported saplings from the European mainland. A few years ago the most of the stock of Impatiens bedding plants was hit by a Downy Mildew infection
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Try this link for pictures of plant diseases.
Apple or Fungal Canker
Blackleg on Cuttings
Blackleg on Potato
Blackspot of Roses
Entomosporium Leaf Spot
Onion White Rot
Phytophthora Root Rot
Potato Brown Rot
Potato Ring Rot
Potato black Scurf
Rhododendron Bud Blast
Sudden Oak Death