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Diseases : Control of diseases with chemicals

The principle of control of plant diseases using chemicals is to parallel the plant’s own defences. Just as the natural wax layer on a plant leaf prevents the germination of fungal spores, chemicals – mostly based on copper or sulphur – form a protective layer on the plant’s surface. Some chemicals work within the sap of the plant, giving it systemic protection, in the same way as the plant’s own internal defence chemicals.

Control of plant diseases using chemicals depends on the accuracy of placement of the chemical and its efficacy in preventing entry or development. If a barrier chemical is placed on the leaves after the fungus has already entered, it will have no effect.

If the barrier is not complete, some leaves or parts of leaves not having been sprayed, the fungus will enter through the undefended area. If the attacking fungus has mutated a new strain that is not susceptible to the chemical, it will not be hindered in its development.

To achieve correct coverage, even using an effective chemical in good time, is not all that easy to achieve. Results using chemical disease control in amateur gardening can be quite poor.

The question arises in the amateur context of the desirability of depending on chemical disease control, given that applying chemicals routinely is time-consuming, carries some expense, is not likely to be hugely effective, and exposes the user and the environment to some risk, however small.

A question mark must be placed over any plant that routinely needs spraying. If spraying is the only way to grow a plant successfully, it should be applied properly and to a limited number of plants, only those considered essential.

Plant viruses cannot be treated by chemicals at all because they operate within the plant cells.

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