Common Pests and Diseases in the Greenhouse
Although pests and diseases are generally easier to control under glass than in the open, they will multiply and spread more freely unless you make routine checks and take prompt action. In addition, whereas winter in the outdoor garden is a fairly trouble-free time, many common pests and diseases can be troublesome all year round in the greenhouse.
A clean and tidy greenhouse will often deter many pests and diseases. By notrubbish and garden oddments there, you deprive pests of excellent hiding places, and by throwing away dead foliage and flowers you can discourage fungus diseases such as moulds, mildews and rusts. Make a daily inspection of the underside of the foliage; many pests and diseases make their first appearance here. Discard plants that are sickly for unknown reasons; they can endanger others.
Plants with pale or distorted flowers or foliage (sometimes stunted or mottled, or with striped blooms that fail to open properly) may be suffering from one or other of a number of virus diseases. These diseases can be spread by sap-sucking insects (like greenfly), by knives or scissors, or by your fingers. There is no cure for virus diseases and affected plants are best burned. Tomatoes are especially vulnerable, and ideally grown alone.
Many familiar garden pests invade the greenhouse. Those that fly can enter through the vents. As a good preventive measure, fit screens of muslin to greenhouse vents in summer. This will stop butterflies and moths getting in; the eggs they lay may turn overnight into caterpillars that can do enormous damage before you spot them. Birds can also enter through the vents and may do much damage to plants, besides injuring themselves while trying to escape.
`Creepy crawlies’ may be brought indoors on the bottom of pots that have been standing in the garden. Slugs and snails are easily controlled by the usual baits used outdoors. These pests will often climb up the glass sides and gain access to pots and trays on the staging. Just one slug or snail can eat a whole tray of small seedlings during the night.
Never stand pots directly on the ground soil of the greenhouse (or outdoors). Stand them on sharp shingle, tile, stone or plastic. This prevents worms entering through the drainage holes. If they do get in, the constant disturbance to the roots will make plants wilt.
Ants have a similar effect, but can be eradicated with a proprietary ant bait; there are several kinds (all very effective) on the market. Woodlice are not, as many believe, harmless to plants and they are especially damaging to seedlings. You will find these pests hiding among greenhouse rubbish. Dusts containing BHC will control them.
Earwigs can hide in the smallest cracks and crevices of the greenhouse structure. There they stay unseen during the day. At night they fly out (it is often not known that they can fly) and eat plants — flowers particularly — leaving the petals holed and ragged. Because they operate at night you may not realize that earwigs are the culprits, and the cause of the damage will remain a mystery. Chrysanthemums need special protection so use BHC dusts liberally. Ant bait is also often effective against earwigs, and can be put in possible hiding places.
Aphides and whitefly
Aphides (or plant lice) are a group of insects that include greenfly and blackfly as well as numerous other closely-related pests that suck plant sap. Few plants are safe from attack, so take routine preventive measures. Systemic pesticides give long-term protection under glass, and there are numerous quick-acting products now sold. Those containing malathion act very quickly, but the old-fashioned liquid derris pesticides are still efficient and very safe.
Whitefly has become a serious greenhouse pest. It often infests outdoor weeds, like nettles, and will then be a source of trouble to any nearby greenhouse in summer, and overwinter in greenhouses that are free from frost. In appearance, the fly is minute, with pale grey wings. Like aphides, it sucks sap and also causes leaves and surfaces below them to become sticky with honeydew secretion. This often encourages a growth of black mould which, though not harmful, makes the plants look most unsightly.
Fortunately, a recently-introduced pesticide called resmethrin is particularly effective for whitefly control, and generally useful for other similar pests. Apply it by spraying. Alternatively, malathion and BHC smokes can be used.
Thrips and red spider mite
The tiny thrips cause foliage to become marked with whitish patches that are often encircled by dark specks. To check for their presence, put white paper below the foliage and then shake the leaves. The thrips fall off and you will be able to see them squirming against the white background. Most general pesticides will control thrips; systemics will be absorbed into the plant tissues and remain effective for a long time.
If, during summer, foliage becomes yellow and mottled and tends to fall, suspect a pest called red spider mite. If you look at the undersides of the leaves with a powerful magnifying glass, what you will see (if the pest is present) is a very tiny, mite-like creature and minute round, whitish eggs. In large and severe infestations the mites will show up as many thousands of pale-reddish ‘spiders’fine webbing. Since they enjoy hot, dry conditions, you can discourage them by keeping up humidity. Fumigation with azobenzene is very effective, but numerous greenhouse plants may be slightly damaged by this treatment, so be sure to check with the maker’s label. Liquid extract of derris (not dusts) is a good general control if applied thoroughly.
With the now widespread use of peat composts, sciarid fly maggots are becoming increasingly common. These are tiny, whitish, wriggling worms infesting the composts, or the peat spread over staging. There will also be tiny flies about — the `worms’ being their maggots. These maggots can do enormous harm to plant roots, especially seedlings, and may also eat lower parts of stems. Moist conditions encourage them. Water with a malathion insecticide, but try to keep moisture lower subsequently. The flies must be killed since they are the source of the trouble. Most general insecticides will do this.
Two very common diseases caused by fungi are damping-off of seedlings and grey mould. If pricked-out seedlings topple over, it is a sure sign of damping-off. Always water in with Cheshunt compound as a precaution. Damping-off is less prevalent now that modern sterilized composts are used. Grey mould (or botrytis) will attack both living and dead plant tissue. It is usually seen as a
brownish-grey, furry mould which, if disturbed, distributes a cloud of fine `dust’ — the spores that spread the disease. On tomatoes the mould will cause flowers and fruit to drop, and fruit may be marked with small whitish rings with a black speck at the centre. Lettuce and chrysanthemums are especially prone to attack. To check the disease, increase ventilation and, at night, lower the humidity. Benlate sprays and TCNB fumigation are excellent controls and preventives of grey mould.
Whenever possible, buy pesticides especially designed for greenhouse use. Read the labels carefully because some plants can be damaged by certain pesticides. In certain cases you can simply remove these plants from the greenhouse, or cover them with plastic bags, while other plants are being treated. Follow all safety precautions exactly (especially where edible crops are concerned) and, in the enclosed atmosphere of your greenhouse, take care that you do not inhale any pesticides yourself.