Greenhouse Hygiene: Pests and Disease


A high standard of hygiene is essential to successful greenhouse horticulture. The congenial environment you provide for your plants is also one in which pests and diseases thrive and quickly multiply, spreading from plant to plant.

The greenhouse structure and all equipment must, therefore, be kept clean, and sickly plants must be kept apart from healthy ones.

Crude animal manures, unsterilized leaf mould and garden soil must never be brought inside. Dirty rainwater collected from roofs is another source of infection. The dangers of using the ground soil have been explained. Pots, seed trays, and other similar equipment must be kept clean, and preferably sterilized before use each year. Seed and potting composts should be stored in clean plastic bags or lidded plastic bins to keep them free from contamination.


The term ‘sterilization’ does not have the same implications as it does when used to describe an operating theatre, for example. The aim in the greenhouse is to destroy as many as possible of the pests and diseases which might overwinter there. These are frequently found in the form of eggs or fungal spores. For the home gardener only a limited number of disinfectants are available, for safety reasons. The most widely used are those based on phenol. These are relatively safe, provided they are used strictly according to the instructions on the label.

The best time to disinfect and clean your greenhouse is when all plants can be removed, preferably before the main growing season begins. Autumn to late winter is often most convenient. You should start by washing down the entire interior structure, glass and frame, with a soft brush soaked with the manufacturer’s recommended dilution of the disinfectant. Apply the solution generously and make sure it gets into cracks and joints, and into the (T’ slots of alloy structures. The floor and staging should be soaked too.

After this treatment, leave the house closed for a couple of days, then open vents and allow a few days for vapours to clear. Although there is usually a persistent odour, as long as all plants are container grown they can be moved back in without fear of damage. If the ground soil is treated, a longer period of time must elapse before it can be used for growing. A test can be made by sowing some cress seed in samples of the treated soil. If this germinates and grows normally, the soil is safe to use.

The phenolic disinfectants will clear algae, slimes and mosses. There is also a disinfectant called Algofen that can be used for this purpose during the growing period since it’s safe with plants. It’s excellent for keeping automatic watering systems and capillary matting, for example, algae free and hygienic. It can also be used on the surface of potting composts.

Don’t use your greenhouse to store junk. Clutter will provide hiding places for pests and diseases and make cleaning difficult. Keep the exterior of the greenhouse crystal clear in winter to admit maximum light. Also, try to keep the surroundings weed free at all times, particularly because many weeds can be hosts for invasive pests, whitefly, for example.


The most important point before buying or using pesticides is to read the label. Then always follow the instructions exactly. Some pesticides damage certain plants and should not be used near them. With regard to food crops, there are usually restrictions on how soon they can be picked after spraying. Always avoid personal contact in any form. Never store pesticides in unlabelled containers or domestic bottles, and keep them out of reach of children.

Sprays are generally more effective than pesticidal dusts but the most efficient contact method is fumigation, either with ‘smokes’ or aerosols. Systemic pesticides are also extremely efficient since they are absorbed by the plant, making all parts poisonous to the pest. They remain active for a long time but they are not initially as fast acting as contact methods. Always make sure that the under-surfaces of leaves get thorough treatment, since this is usually where pests and diseases first congregate. It is wise to make routine inspections of your plants every few days. A small magnifying glass is helpful to spot minute pests such as red spider mite and mould or mildew spots in the early stages. Act immediately if trouble is suspected. Treatment is usually easier and more effective in the early stages.



When large numbers of ants invade the compost in pots they cause considerable root disturbance and severe plant wilting. They usually get in via the drainage holes. Several specific antkiller dusts are now available which are very effective.


Greenfly are the most common aphids in the greenhouse but others frequently find their way in too. Like a number of similar insects, they deposit a sticky secretion called ‘honeydew’ on which an unsightly blackish mould grows. There are many aphid killers on the market and you should choose according to the plants you are growing.


This is a problem affecting seedlings after pricking out, and is caused by several fungi. It is common when unsterilized composts, garden soil or crude manures are used, and where hygiene is poor. It causes the seedlings to topple over and shrivel. The trouble can spread very quickly unless immediately checked. You should water with Cheshunt compound, available from garden shops. It is advisable to do this as a routine preventive measure after all pricking out.


The presence of this pest should be suspected if flower petals or leaves are being eaten away and left with tattered and ragged edges. Seedlings can also be attacked. Earwigs hide during daylight and become active at night so die damage may seem something of a mystery. Plants with large flowers, such as chrysanthemums, are very susceptible. Pots stuffed with straw and inverted on canes can be used to trap the earwigs. An effective chemical that is safe for most plants, including food crops, is permethrin.

GREY MOULD (Botrytis cinerea).

This mould always appears sooner or later, even in the best kept greenhouses. It attacks both living and dead plant tissues, forming a greyish-to-brownish furry covering. It is frequently seen on damaged tissue. If the mould is disturbed, a smoke-like cloud of spores is released which spreads the infection. Pelargoniums, lettuces, and chrysanthemum flowers are often affected. On tomatoes die fungus causes small whitish rings with a tiny central black speck on the fruits (ghost spotting). The fruit is, however, safe to eat. The fungus is encouraged by generally bad hygiene, poor ventilation and excessive humidity. Fortunately, TCNB fumigation and spraying with a mixture of benomyl and a foliar feed (a proprietary formulation is available) can both prevent and control the fungus. It’s wise to use them as a preventive measure.


This pest particularly affects chrysanthemums, and sometimes polyanthus. It manifests itself as wandering brownish lines on the foliage, and a tiny burrowing grub is responsible. It can be prevented and controlled by spraying with permethrin, pirimphos-methyl, or gamma-HCH.


This appears as tiny scales with a mealy, whitish waxy covering. It is common on succulents and thick-leaved plants. Wipe off by hand when possible, using a tuft of cotton wool saturated in methylated spirit. Where infestation is considerable, spray with pirimphos-methyl.


There is a wide variety of these fungi. Most of them form whitish-to-greyish velvety coatings on foliage, usually appearing at first as isolated patches. They are encouraged by poor ventilation, overcrowding and cold damp conditions. There are now a number of general-purpose fungicides available, including systemic types, which effectively control a wide spectrum of these fungi. The black sooty mould which grows on insect secretions is not in itself harmful, merely unsightly, but it could interfere with plant metabolism. It is best to wipe it off using cotton wool soaked in a weak solution of detergent in water.


This can be a very serious pest if allowed to gain a hold. It usually appears during the summer months. Foliage may turn yellowish and plants become sickly for no apparent reason. A look at the under-surface of the leaves through a powerful lens will reveal tiny spider-like greyish-to-red mites and spherical whitish eggs. In severe attacks they may swarm and form an obvious web, by which time the plants are usually ruined and are best burnt. Sterilization helps to prevent the pest and kills overwintering eggs. Probably the best treatment and control is fumigation at seven-day intervals with a pirimphos-methyl smoke cone.


These are similar to mealy bugs in habit and they attack similar plants, but they appear as cream-to-brownish scales without the mealy coating. They often cause a sooty mould on foliage just below where they congregate. Treatment is the same as for mealy bug.


These are tiny whitish-coloured maggots which attack roots and the base of plants and seedlings. They can do a surprisingly large amount of damage; wilting may be the first sign of their presence. They are the larvae of tiny flies, which may be noticed hovering nearby. Peat composts and” damp conditions seem to encourage them. If possible, let pots become dryish and then water with pirimphos-methyl. The flies should be controlled by general fumigation with a general-purpose smoke cone.


These common garden pests find their way into the greenhouse and cause severe damage. Just one snail or slug can eat a box of seedlings overnight. They hide during the day, under staging, for example. There are a number of baits available. The miniature pellets are particularly useful in the greenhouse. Woodlice cat roots and seedlings but are easily controlled with antkiller dusts.


These minute insects produce whitish patches surrounded by black specks on leaves. To confirm their presence, place white paper under the plant and shake the foliage. Wriggling insects will fall and show up clearly. Use a general systemic insecticide.


Symptoms include yellowing and mottling of foliage, distortion, stunting, striping of flower colours, and general unsatisfactory growth. They can be spread by insects, pruning and cutting tools, and by handling. Plants should be burnt and not used for propagation. There’s no cure.


This is a tiny fly with whitish triangular wings. It usually occurs in considerable numbers and, like aphids, causes sooty mould. Weeds around the greenhouse can harbour whitefly. Fumigate with special whitefly smoke cones.

03. September 2013 by admin
Categories: Featured, Garden Management, Top Tips | Tags: , | Comments Off on Greenhouse Hygiene: Pests and Disease


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