Plant disease is a huge and complex subject. This section deals
with the principles of disease control, the many different kinds
of plant and soil diseases, the characteristics and symptoms of
disease, the various ways of preventing and/or treating plant
disease and soil disease, the characteristics of the various
fungicides and bactericides, the problems with the use of these
materials plus an assortment of related issues. A lot of
material! It may be necessary to divide and restructure this
section at some future date.
There are huge numbers of fungi, bacteria, and virus organisms
in the garden. The vast majority of these microorganisms are
beneficial and essential for the health of the garden. A very
small minority of these microorganisms however are also the
cause of the various diseases.
All of the garden fungicides and bactericides used to prevent
or to control plant disease or soil disease are also toxic to
at least some of the beneficial microorganisms. Plant disease
should be prevented and controlled primarily by cultural
practices. There are a number of organically acceptable
fungicides and bactericides that can be used as back-ups in
the event that the cultural practices are not able to give an
acceptable level of control. The gardener needs to articulate
the meaning of "an acceptable level of control".
It is not realistic to expect the garden to be totally
disease-free. Good cultural practices will handle the bulk of
the disease problems on an every day basis. Dependent on a lot
of things there may be an outbreak of disease in an isolated
spot or perhaps in several spots. Good cultural practices can
correct many problems.
The gardener has to deal with the amount of disease that can
be tolerated before going to the use of the organically
acceptable fungicides or bactericides. For the most part, it
involves trust in the cultural practices to do the job. This
sub-section is about the various ways of telling when it is
appropriate to use different levels of control.
Some of the symptoms of a plant or soil disease are very
characteristic which makes the identification of the
particular disease relatively straight forward and simple.
Some of the other symptoms, however, are very general and can
be misleading. Specifically, some of the symptoms of one
disease look like the symptoms of another disease or even
symptoms of other kinds of plant distress. Sometimes it takes
the resources of a plant pathologist to make a positive
This sub-section is about the various symptoms of plant and
soil disease and the different ways of telling if the problems
are one of disease or other factors.
The more one knows about the things that make life comfortable
or miserable for the various pathogens, the better able one is
to devise an effective strategy for their control. The fungus
organism that causes Powdery Mildew on roses, for example,
likes dry foliage and low humidity. Simply using overhead
watering and occasionally washing the foliage with a blast
from the hose helps to prevent Powdery Mildew on roses. A
daily washing, especially with some added garden friendly
soap, makes an effective treatment. This sub-section is about
the various pathogens and the things that make them flourish
The Organic Crops Improvement Association is an organization
of independent farmers who produce "organic" crops.
The various regulatory agencies of government and the various
trade associations could not come up with a standard
acceptable to all so the OCIA organized and drafted their own.
The OCIA's International Certification Standard was well
thought out, they got good input from many reliable sources,
and the Standard became recognized by essentially every
regulatory agency and trade association in the world.
The International Certification Standard lists, among other
things, all of the commercial and generally used pesticides
and classifies these materials as being organically
"acceptable", "restricted" or
"prohibited" as a result of their effect on the
environment and their safety for the consumer. Those materials
that are "prohibited" probably don't belong in the
The fungicides and bactericides listed in the International
Standard as "acceptable" are elemental sulfur,
lime-sulfur, or Bordeaux mixes. Fungicidal soaps and certain
botanicals are considered "acceptable" as well. Some
of these materials, however, will burn the foliage if they are
not used as directed and they should be used primarily as
back-ups to the cultural practices. They are not trouble free.
Farmers growing organic crops and some commercial
growers use these OCIA "acceptable" materials as
their primary control measures because they are cost effective
and easy to use. The cultural practices would give them better
control but they would be too labor intensive in a large scale
operation. This sub-section is about the ways to use these
materials and their limitations.
Some of the simple home remedies touted as being effective for
the prevention or control of plant disease simply don't work.
There are, however, a number that do. Some of the home
remedies that do work may or may not cause other problems. For
example, one can only use so much baking soda for treating
powdery mildew before it starts to create alkalinity problems
in the soil. But the same can be said about some of the high
Both the home remedies and the high tech alternative materials
and methods should be used with caution. The best basic
approach is to first use cultural practices to prevent or
control plant disease and to use OCIA approved materials or
methods as a back up. In the event that the disease problems
are not resolved, to then consider the use of alternative
materials and methods.
There are clearly some alternative methods or materials that
do a good job and are appropriate for the home garden. This
sub-section is about the use of these alternative methods and
This sub-section is about the physical and chemical
characteristics of the various garden fungicides and
bactericides. It is also about their relative toxicities.
Specifically, their effects on the beneficial organisms and the
ecosystems as a whole, the relative dangers of using these
materials or having them around the home, and their suitability
for use on edible crops or on house plants inside the home.
The toxicity of the various pesticides is often measured
in terms of MLD50 which stands for the Minimum Lethal Dose that
will kill 50% of the test animals. It is a figure of mg. of the
test material per kg. of body weight for the test animals. The
lower the MLD50, the higher the toxicity. See
regarding the MLD50 ratings of the various synthetic fungicides.