Plants suffer from many diseases caused by minute living organisms called viruses. In most cases the plant merely tolerates the presence of the virus and exhibits certain symptoms indicating its presence—usually these symptoms are typified by a yellow mottling or mosaic effect on the leaves. These symptoms, however, should not be confused with the yellowing caused by magnesium or iron deficiencies (see page 15).
The effect of virus diseases is usually simply to reduce the vigour of the plant. This reduction in vigour sometimes means that a plant produces very little suitable propagating material as its growth is weaker than that of a virus-free plant of the same species or variety. It is, however, unwise to propagate from any plant that you suspect is affected by virus as the virus will be passed on to any vegetatively propagated offspring.
Unfortunately it is not possible for gardeners to reduce or eliminate the disease from a virus-infected plant. All that can be done is to attempt to prevent the spread of the disease by digging up and burning the whole of the infected plant.
Some virus diseases are carried by aphids that infect other plants when feeding; the spread of virus diseases to uninfected plants is therefore best controlled by the routine use of pesticide smoke canisters or sprays. #
Virus diseases can also be spread by eel-worms in unsterilized soil, or merely by contact from, for example, a propagation knife. After cutting virus-infected plants a knife will be a fertile area for infection. Always disinfect propagating tools and your hands if virus diseases are suspected.
The only satisfactory way to avoid trouble when propagating is by using plant material that is virus free. This is a counsel of perfection as it is not easy for the gardener to recognize with certainty virus symptoms in many plants. Although viruses may be passed on through the seed it is possible to raise fresh virus-free stock of certain plants, such as Daphne mezereum, where viruses do not appear to be transmitted by seed. Virus-free rootstocks are labelled emla, and they are available for some fruit trees arad a few ornamental crabs and cherries.
Some virus diseases, such as Daphne mosaic, can be recognized by holding the leaf up to the light and by identifying the mosaic pattern of lighter yellow colour. Others are more difficult to recognize and may frequently be confused with mineral deficiency symptoms or even pest damage.
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