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Pest and Disease Control in Your Greenhouse

By: Dr Gareth Evans - Updated: 2 Jul 2013 | comments*Discuss
 
Spider Mites Aphids Eelworms Weevils

In the close confines of the greenhouse even the smallest outbreak of disease or pest infestation can rapidly get out of hand and spread through your precious plants like wildfire unless it is dealt with very promptly. As a result, the most effective form of control is your own vigilance coupled with a readiness to treat affected plants at the first sign of infection – or even remove and burn them, if needs be – before anything has the chance to get a real hold.

Cleanliness is also essential in the greenhouse, not least because clean and tidy surroundings remove many of the potential hiding places and breeding grounds where all manner of trouble can otherwise be developing unseen. Typical pests to be on the look-out for include spider mites, aphids, eelworms and vine weevils, while grey mould (botrytis), mildew and “damping off” – another kind of fungus, despite the name – are common diseases of greenhouse plants.

Keeping It Clean

Having scrupulously cleaned and disinfected your greenhouse – in the spring before sowing seeds or in the autumn ahead of bringing in your tender plants – and perhaps fumigated it too with one of the many brands of smoke cones available, it is vital not to re-introduce problems. Any plants now being brought into the pristine greenhouse should be carefully checked for signs of pests or disease and appropriate action taken – and this applies doubly to any which have recently come from the garden centre or fellow growers.

Once you have established good, clean growing conditions for your plants, it is obviously important to keep them that way to avoid giving the slightest toe-hold to any unwanted guests. Reduce the likelihood of fungal diseases and deny pests shelter by routinely removing fallen leaves and other debris, discard obviously sick or dying plants and add sweeping to the list of regular jobs to be done. Dirty pots, troughs and other containers too can harbour all manner of trouble, so they need to be carefully washed, paying particular attention to their rims and disinfected before being stored tidily until they are next required.

Dealing with Common Problems

If fungal diseases seem to be a persistent nuisance, it may be worth using a systemic fungicide as a preventative measure – rather like giving your plants a vaccination. However, if you do go down this route, it is a good idea to vary the type used each year to avoid the fungi becoming immune to its effects. Sometimes just slight changes to the way the greenhouse is run can make a big difference in the struggle to control disease. “Damping off”, for instance, is a scourge of seedlings raised under-glass, attacking the young stems at soil level and causing them to collapse and ultimately die. Using only seed trays which have been thoroughly disinfected and fresh compost or sterilised soil, sowing thinly, watering carefully and then pricking out as soon as possible to avoid over-crowding can be a very effective approach to reducing the risk of this major nuisance.

Botrytis (grey mould) affects a wide range of plants and thrives in cool, damp surroundings – so maintaining a slightly dry atmosphere and adopting a careful watering regime can help, especially in spring and autumn when it is most likely to flare up. Ensuring good ventilation and air movement too can help control many problems caused by mildews, moulds and other fungi.

Unfortunately, to a startlingly large array of insects and other creepy-crawlies the strong, lush foliage you have lovingly grown in the greenhouse represents a massive potential food-source. However, the good news is that when it comes to combating these greenhouse pests, there is a wide range of products available – ranging from natural remedies and pesticides through to ultrasonic devices and hormone-baited insect traps. The key is, again, to spot the warning signs and work out the culprit without delay so that the appropriate product can be quickly bought and used – in accordance with the instruction label, of course – to deal with the problem promptly.

For those who prefer a more “green” approach to their gardening, one of the nice things about greenhouses is that natural controls work particularly well in their enclosed environment. More and more forms of biological pest control are appearing on the market, from nematode worms to deal with slugs and parasitic wasps to treat whitefly, to predatory mites which eat spider mites – the ultimate in setting a thief to catch a thief!

Few gardens entirely escape the effects of pests and disease – and the greenhouse is no exception. Although these unwelcome free-loaders can do an awful lot of damage in a very short time, left to their own devices, if they are caught early enough, serious harm can often be avoided. With vigilance and good greenhouse management, you may not avoid them altogether, but you can go a long way towards stopping them making too much of a nuisance of themselves.

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