If you had a sick child, would you give him/her the
first medicine on the shelf? Or would you first want to identify
the illness, and use the appropriate treatment?
In the field and garden, the same principle holds true; effective treatment
begins with accurate diagnosis. Miss diagnoses can result in a
significant loss of crop yield and/or reduced quality and may lead to
excessive use of pesticides, including organic pesticides. It is
worth remembering that organic pesticides may be natural compounds but
they are not universally harmless.
Correct diagnosis can lead to a more complete understanding of the life
cycle of the disease at hand, and such knowledge provides the most effective
means of control. As an example, bacterial leaf spot of pepper
can be controlled by applications of copper, but may require frequent
applications if the disease is abundant.
However, the best control strategy is prevention! In
this case, the bacterial pathogen does not survive the winter in the
soil, but can survive on debris from last years crop and on seeds from
infected plants. In this case, using disease free seed and crop rotation
are the best controls. Plant
debris should be deeply buried, or composted at high temperatures.
For those pathogens that do survive through the winter, the introduction
back into the crop the following year can be greatly reduced if you identify
the source. For example, several foliar diseases of tomatoes overwinter
in plant debris, seeds, stakes, fencing, twine, pots and flats used to
start seedlings. Here, using new flats, pots, fencing, disease
free seed, crop rotation and heavy mulching and removal of crop residue
all help to reduce disease.
Recognizing that the spores of some pathogens are
spread by tiny droplets of water during rains suggests that increased
plant spacing, perhaps interplanting with another crop, eliminating
overhead irrigation would reduce the spread of such diseases. Some diseases, such as black
rot of grapes, are prevalent in the wild populations of these plants. One
control option is to eliminate these reservoirs. Other diseases,
such as cucumber wilt, are spread by insects, and are best controlled
by targeting the insect vectors.
For more information you can contact your extension agent.
The American Phytopathological Society has published a series of compendia
on many common diseases. On the internet information is available
on the Ohio State Agricultural Studies