Fungi are essential in breaking down dead organic matter to produce the humus which is needed for good soil structure - saprophytes. They do not have any chlorophyll so cannot use light to capture energy, instead they derive their energy by breaking down plant and animal material - alive or dead. The widespread use of chemical control can damage the balance of these beneficial fungi and this forms part of the principals of Organic management.

Viruses dwell inside the cells and cannot be treated with chemicals so affected plants must be destroyed (special microculture techniques may overcome the infection by taking cells from the growing tip, but this is restricted to the laboratory). There are no antibiotics for plants, so bacterial attacks, eg. fireblight, are untreatable as well. Some fungi can be killed with chemicals without damaging the host because their growth habit is different, ie. they tend to grow on the plant and not in it, using root-like structures to extract nourishment.

Since killing the pathogens is difficult or impossible, "prevention is better than cure". By observing good hygiene when propagating and growing your plants, you can prevent a lot of diseases from taking hold.

• Destroy diseased plants, clear up dead leaves and other debris.

• As the spores can have tough outer coatings, do not add diseased material to the domestic compost heap as they do not usually achieve high enough temperatures to destroy them.

• Prune fruit trees and bushes regularly to keep an open structure allowing a good air flow and to remove damaged branches.

• Disinfect secateurs, saws or knives used for cutting out diseased branches with methylated spirits or a flame (a cigarette lighter comes in handy for this). Household bleach or other disinfectants can also be used as a dip for shears and clippers, including electric or petrol machines. This also helps when taking cuttings.

• Only use new or well-washed containers when growing cuttings and sowing seeds.

• Crop rotation in the vegetable plot will prevent a build up of disease.

• Space plants well apart especially crops where similar plants are growing together, to allow good air flow. Fungal diseases in particular, thrive in still, damp air and there is a greater chance for them to be transmitted to surrounding plants if they are in close proximity.

• Catching disease early is important so keep an eye out for it at all times.

• Plants are more susceptible to disease if they are not growing well. This can be due to poor soil, drought or both. So prepare the site well adding plenty of organic matter and give plants an occasional feed. As gardeners we are usually trying to grow plants which most likely are not native to the region or in a place they would not choose for themselves, so there is a greater probability that they could be under stress.

Of course not all plant diseases can be prevented by good hygiene as some are transmitted by insects and others are wind-borne. Aphids and other sap-sucking insects are the main vectors of viruses. Dutch Elm Disease is fungal transmitted by Elm Bark Beetles and the bacterial Zylella festidiosa disease by Froghoppers. Tobacco mosaic virus can be harboured in cigarettes, so smokers are banned from greenhouses where Surfinia Petunias (susceptible to the virus) are raised from cuttings - all stocks had to be destroyed in 1995 when they became infected. The spores of some fungal diseases are carried in the air, and in rain drops and splashes.
Be careful when bringing plant material into the garden. Diseases can be in the soil as well as the plants themselves. Some gardeners will only raise their plants from seed, especially vegetables as some onion sets may have White Rot or brassicas may have Clubroot and this would infest the plot for years to come.

Some plants have a natural resistance to diseases, particularly if they have evolved together. With detailed knowledge of genetics, this resistance can be transferred to susceptible plants. This is new technology and these Genetically Modified Organisms are controversial.

The globalisation of the nursery industry means that plant material is being carried from far-flung parts of the world and just like the movement of people in past centuries nearly wiped out indigenous populations with previously unencountered diseases, there is an increased risk to our native flora. This has been shown most recently with Ash Dieback (Chalara fraxinea) on imported saplings from the European mainland. A few years ago the most of the stock of Impatiens bedding plants was hit by a Downy Mildew infection