Pests and diseases can thrive and multiply in the warmth and shelter of a greenhouse, and once established, may be difficult to eradicate.

The key to avoiding pests and diseases is to maintain a clean, tidy greenhouse using the maintenance techniques discussed earlier in this section and to keep a sharp eye out for problems so that you can deal with outbreaks whilst still in their early stages. Greenhouse pests include spider mites, whitefly and vine weevil larvae, whilst common diseases to watch out for are grey mould, damping off of seedlings and mildew on plants.

Damping Off

Damping (or dampening) off is a common fungal disease affecting seedlings, causing them to collapse and die. Prevention may be carried out by using disinfected pots and trays when raising seeds and seedlings, along with fresh compost. Use tap water in a clean watering can for seeds and seedlings rather than water stored in a water butt; you may also want to consider using a copper-based fungicide.

Other Fungal Diseases

Fungal disease like powdery mildew and grey mould may be prevented by ensuring that there is plenty of air movement around the greenhouse. This is one of the reasons that an effective ventilation system is essential - even during the winter months. Fungal diseases may be controlled using a systemic fungicide; try to use one with a different active ingredient each year.

Spider Mites

Spider mite populations can build up during the summer when the greenhouse gets hot and dry. Move as many plants as you can outdoors and protect the ones that remain by misting them with water using a hand held sprayer to keep the humidity high. Paved and gravel floors may also be washed down with water to increase humidity levels.

If plant leaves become dull and yellowed, with a fine, pale mottling, then they have probably become infected. Use a systemic insecticide such as dimethoate or try a biological control such as the predatory mite Phytoseiulus perimilis.

Flying Pests

Non-toxic sticky traps may be hung above plants to catch a wide range of flying pests such as whitefly, aphids, thrips, midges, leaf miners and hoppers, mites and fungus gnats. They are attracted by the bright yellow of the trap, becoming caught on the non-drying, non-drip 'sticky glue' on the trap's surface.

The traps are also useful to gauge how many flying pests are in the greenhouse, to decide whether further controls are necessary. Whitefly may be controlled using a contact insecticides or by introducing a biological control such as Encarsia formosa. However, do bear in mind that these two systems cannot be combined; if you want to use an insecticide before trying out biological control, make sure that you use one based on pyrethrum as it does not persist.

Vine Weevil Grubs

Plants grown in pots can have their roots destroyed by vine weevil grubs. Pick out any you find whilst re-potting and check new purchases carefully before introducing them into the greenhouse. A band of non-drying glue around pots can stop adult weevils crawling up pots to lay their eggs. Soil drenches such as HCH or pirimiphos-methyl and biological controls (Steinernema and Heterorhabditis spp.) are available to control vine weevil larvae.