Greenhouse Cleansing against Pests and Diseases
Greenhouse Cleansing against Pests and Disorders
Whilst routine inspection of the growing plants is helpful in reducing the chances of disease and pest spread, the periodic cleansing of the glasshouse structure is also essential. To do this it is, of course, necessary to move out all plants to a suitable environment when a further examination of them can be made. The house should then be thoroughly washed down either with a proprietary cresylic acid solution or with dilute formalin (2%) plus a wetting agent, making sure that cracks and crannies in the structure and on the bench are not neglected, and that old bits of string used for plant supports and remnants of plant material are removed and burnt.
Both formalin and cresylic acid can be applied by brush or by sprayer, but whatever method is used it is advisable to wear an eye-shield or goggles. When formaldehyde is used the structure should be completely closed for 24 hours after treatment, and if possible the temperature should be raised to about 21°C (70°F) during this period. Following this treatment the house should be opened up and thoroughly ventilated until all traces of formaldehyde smell have gone.
In larger glasshouses it may be more convenient to wash down with a proprietary detergent solution and then to fumigate with formaldehyde or with sulphur (provided the main structure of the house is not made of metal). The formaldehyde can be vaporized either by placing the solution in a container over a heater, or by placing crystals of potassium permanganate in a twist of soft paper and dropping it into a metal bucket containing the formaldehyde solution. Approximately 5 litre (1 pint) of formalin and 43g (4oz) potassium permanganate are required for each 29m3 (1,000cu ft) of house. Care should be taken to use a large enough metal container for the formalin since the addition of the potassium per manganate causes violent bubbling and evolution of heat during which the formalin is vaporized. Allowing a 4.5 litre (1 gallon) container for 1 pint of formalin should be satisfactory. When sulphur is used, it can be placed in small bags 190g per 29m3 (approx 6-1/2oz per 1,000cu ft) with wood wool and ignited. Close tightly all the doors and vents in the house; these are subsequently opened after 24 hours (for formaldehyde) or 2-3 days (for sulphur) to allow the fumes to escape.
Many pesticides are relatively harmless, even when exposure to them is considerable, but some are quite toxic, and a few are extremely dangerous both in the concentrated and diluted state. It is, therefore, essential to treat all pesticides with respect, and to err on the side of extra care when storing and handling them.
The following points should be constantly borne in mind:
1. Store all pesticides (if possible under lock and key) out of reach of children, in clearly labelled containers. DO NOT TRANSFER CHEMICALS TO SOFT DRINK OR OTHER TYPES OF BOTTLE.
2. Amateur growers should obtain their fungicides and insecticides from the appropriate retail sources. Where possible, use chemicals that carry the seal of approval of the official Agricultural Chemicals Approval Scheme in preference to materials not carrying such approval.
3. Carefully read the instructions on the pesticide container and follow them. Wear rubber gloves when handling pesticide concentrates and also when spraying. Also use a face mask and other protection if so instructed. A face mask should be worn when handling and applying certain dusts.
4. Thoroughly cleanse the spraying equipment after use.
5. Wash hands and face, and all protective clothing after spraying or dusting.
6. Do not eat, drink or smoke when spraying or dusting, nor until protective clothing has been removed and hands and face have been thoroughly washed.
Just as the more stable environmental conditions of the greenhouse are favourable for pests they are also extremely suitable for some of the natural enemies of the pests (the parasites and predators). Concern over the hazards and expense of pesticides, together with increasing problems of pest resistance to chemicals, has refocused attention on biological agents. The most notable of the techniques developed at Horticultural Research International, Littlehampton is the use of the predatory mite Phytoseiulus persimilis for the control of glasshouse red spider mite (Tetranychus urticae) onand . There are also techniques for the control of aphids and whiteflies on glasshouse crops and other pests and diseases.