Post category: Diseases


Plant diseases are generally caused by fungi, bacteria and viruses. A disease is a plant malfunction, not the causal organism.


Grey mould disease on tomato


Fungi are like very tiny plants, often microscopic, with no chlorophyll. To stay alive, they must feed off dead, or living, organic material and they cause most plant diseases. Bacteria consist of single cells, so small they are visible only with a microscope. There are not many major bacterial plant diseases. Viruses are even smaller than cells; they consist mainly of the genetic code for their own production. They cause many plant diseases.


The outer surface of a plant – bark, skin or wax layer – is the main line of defence. If this is damaged, and the plant is too weak to heal the injury, fungi, bacteria and viruses can get in. When the outer defences are breached, plants react by producing natural chemicals that inhibit fungal and bacterial growth.



The plant’s ability to make these defensive chemicals depends on its state of health. Weak plants, growing in unsuitable conditions, are less able to defend themselves, and therefore more likely to suffer disease, and this can be very commonly observed in gardens. Strong vigorous plants with balanced growth are very good at warding off disease attacks.

Many diseases are specific to the plants they attack, other plants being resistant. Some varieties of a particular plant have resistance to certain diseases. In some cases, plants have been bred specifically for disease resistance. For instance, most modern tomato varieties are resistant to tomato mosaic virus and leaf mould, formerly both serious diseases of greenhouse tomatoes.


Just as plants have conditions in which they thrive, the disease-causing fungi and bacteria have conditions that favour them. Moist conditions generally favour fungal diseases. Some kinds, such as apple scab and potato blight, need moisture on the plant leaves for a period of hours in order that their spores can successfully germinate and grow.


Powdery mildew on maple


Some soil-borne fungal diseases, such as root rots, prefer wet soil and actually travel through the soil water to reach new hosts. Bacteria generally need warm weather and this explains the small number of bacterial diseases in this country.

Do not splash water about indoors, or stand pots in trays of water. Outdoors, plant disease susceptible plants so that the foliage dries out more quickly. Take care not to buy diseased plants and do not leave old plants as sources of infection. Removing diseased plant parts gives good control in some cases.


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.