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Knowledge is the most effective
weapon against pathogens

By Dr. Art Trese

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 Website.