Pests And Diseases Of Plants Under Glass

Unlike so many of the regular plants of the greenhouse, the cinerarias and primulas for example, most of the bulbs and tuberous plants suffer little from pest attacks. Two pests which may occasionally prove troublesome are Red Spider and White Fly.

Red Spider

Red Spider

This mite may occasionally be found on the under sides of the leaves of begonias and achimenes, causing the leaves to turn brown and shrivel. As it is not easily controlled by spraying which does not reach the undersides of the leaves, fumigation is carried out. This may be done during June when the plants have made some leaf. The modern method is to use either azobenzene or napthalene, placed at intervals and a light set to the small ‘piles’. Care must be taken when carrying out the work and before the preparations are placed in the greenhouse, the plants and floor of the house should be well sprayed with water to increase humidity which will give the most effective results.

White Fly

This pest causes considerable damage to greenhouse plants particularly in summer. It sucks the sap from the leaves of plants causing serious loss of stamina. It is also, like all aphides, thought to be the carrier of virus diseases. The commercial grower will use the extremely poisonous hydrocyanic acid gas which gives complete control, but the amateur grower will be advised to fumigate with an insecticde in the same way as described for red spider mite. Most of the research stations supply a parasite which, if introduced to the greenhouse in early June, will give complete control of white fly.

Millipedes

These pests sometimes attack the succulent stems of tuberous plants causing them to die back. The use of napthalene for fumigation for the control of red spider will also prove effective against millipedes.

Botrytis

This trouble may be the cause of leaf-spotting of begonias and cyclamen. It is generally noticed in a greenhouse which is overcrowded and badly ventilated, a greyish mould first appearing on the leaves. Until recently there was no cure, but now if the plants are dusted as soon as the fungus is noticed, a preparation called `Folosan’ will give control.

Mildew

Similar but not so destructive as Botrytis. Mildew also appears where excessive humidity and poor ventilation are experienced. As a preventative, weekly dusting of the plants with green flowers of sulphur will prove reliable.

Virus Diseases

Virus in its many forms may be transmitted to greenhouse plants through aphides. The plants are often stunted and the leaves spotted and striped, later turning completely yellow. There is no known cure and the plants should be removed and burnt. Correct ventilation and the control of aphides, red spider and other pests should keep virus troubles at a minimum.

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