The old adage that the best fertilizer is the gardener’s shadow is never more true than in summer, when your presence can mean spotting a plant that’s running short of water rather than coming home to find it parched. And the more time you spend with your plants, the more likely you are to spot pests and diseases. Swift action in the early stages of an attack can often save your crop.

Beat blight

The scourge of summer is late blight, attacking tomatoes and potatoes and turning plants to mush in days. Blight is caused by fungus-like organisms, and is most likely to attack during hot and humid weather.

Blight is rampant during Smith periods, when temperatures stay above 10°C for 11 hours per day over two days, with a relative humidity of over 90%. Rather than having to remember this formula, you can sign up for alerts from Blightwatch. If, following a Smith period, you notice black blotches on your potato plants, cut down the foliage and bin it. Leave the soil undisturbed for two weeks, during which time the skin on the potato tubers will set and help prevent attack by the blight. Then dig the potatoes up and enjoy them fresh - they’re unlikely to store. If your potato harvest is regularly blighted, consider buying resistant Sárpo varieties next year.

Catch out the carrot fly

Carrot fly larvae can ruin your carrot (parsnip, parsley, celery and celeriac) harvests during the summer. As they don’t fly high, the easiest way to stop them in their tracks is to erect a barrier around your carrot patch. You can also try and confuse them by planting your carrots among aromatic plants - carrot fly hunt by smell. Avoid touching carrot foliage more than you need to, as it releases the scent and draws flies in. There are two hatching periods each year, from May-June and August-September, but these vary according to the weather. For the latest information, download the current carrot fly forecast.

Avoid aphids

Aphids tuck into fresh growth, weakening and distorting plants and leave behind a sticky secretion that allows the growth of mould. They can also spread viral diseases as they move from plant to plant.

A quick blast from the hosepipe is enough to dislodge small colonies -total eradication is counter-productive. Aphids attract ladybirds, whose larvae eat aphids. If you tolerate small populations of aphids then you should find that the ladybirds will control these for you. You can keep an eye on the aphid numbers via the aphid bulletin, which may help you decide when you might need to take further action - the most effective is to wipe off aphids with your fingers.

Manage moths

Codling moth caterpillars bore into your apples and pears during the summer, and the plum moth into plums, damsons and greengages. Installing a pheromone trap that attracts male moths and prevents them from mating should be enough to prevent problems.

Remove any infected fruit (you can see the entry holes) and throw these away. Over the winter, remove any mulch from around the base of the trees and encourage insect-eating birds into the garden, as they will eat any overwintering caterpillars that they find.

Vanquish vine weevil

Plants in containers fall prey to vine weevil. The first signs are usually notches eaten out of the leaves by the adults, but it’s their larvae that do the most damage, eating their way through the roots. There are no chemical controls safe for use on edible crops, but there is a very effective biological control. You can also keep numbers down by going out at dusk and shaking plants to dislodge the adults; squashing them underfoot is the order of the day.

Rout rosemary beetle

The rosemary beetle is a pretty pest, metallic green with purple stripes. It feeds on the leaves of rosemary, lavender, thyme and sage as well as other related plants. A new arrival to British gardens, it is now widespread in England. Hand-picking off adults and larvae is the advised treatment-putting newspaper under the plant and giving it a good shake speeds up that process.

Knock out gnats

One of the biggest indoor pests when the weather is warm is the fungus gnat, whose larvae munch on plant roots and can build up to problematic levels. Conventional wisdom says that allowing the soil to dry out between waterings puts paid to gnats - but it’s a fine line between that and killing the plants. Watering from the bottom, so that the top layer of compost remains dry, does help - as does a layer of gravel or vermiculite

The best defence is yellow sticky traps, which deal with the more adventurous males while leaving the females to wander the compost in search of a mate. You can also buy a biological control if you have a real problem.

Stymie spider mite

The Spider mite is also problematic indoors. The first sign you see is the webbing the mite uses to move around. The mites (usually, but not always, red) are visible to the naked eye, and once they become established they cause golden patches on leaves where they have sucked out the sap.

Spider mites love the drier conditions indoors, and prevention and early control involve misting plants to increase the humidity. If your infected plant can stand a trip outdoors then that helps control the pest and prevents the infestation spreading. A biological control is available, but fast action is needed. It may be best to sacrifice one plant to avoid the problem spreading to others nearby.

And finally

The best gardeners know that varied planting and a wildlife-friendly garden can take care of most pest problems before they get out of control. And keeping your plants fighting fit helps them to take care of themselves.

By Emma Cooper, author of ‘The Alternative Kitchen Garden: An A to Z‘ (Permanent Publications).

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