Please note: under UK and EC law is is illegal to use any preparation as a pesticide that is not approved for such use. The information here is for historical reference only and does not imply a recommendation for use. If you disregard this warning and make any of the preparations you do so entirely at your own risk.
Update 09 July 2010
Evidently cider vinegar works (5% acidity) as a control for apple scab, leaf spot and mildew. Dilute 3 tablespoons (45ml) with one (US) gallon (3.8l) of water and spray in the morning.
Update 22 June 2008
A few years back there was news that a German research group had found that milk diluted with water made a good fungicide. There were various suggestions about the ratio some said 1 pint of milk to 8 pints of water (1:8) while others advised 1 pint milk to 4 pints water (1:4). I recently had some mildew on the main onion crop and used the 1:4 dilution (plus a very small amount of approved liquid detergent wetting agent) to spray the soil as a nutrient additive. Unfortunately, because the onions were quite closely spaced, a lot of the spray went on the leaves of the onions and cured the mildew problem!
The information below is from “Organic Gardening” by Roy Lacey, published by David & Charles, London, 1988, now out of print.
The insecticidal properties of ordinary soapy water should be more widely known because it would undoubtedly save inorganic gardeners a small fortune in poisonous sprays and save many helpful insects into the bargain.
I’ve used nothing but soapy water as a spray on my roses for many years and have never had more than minor trouble with aphids. Soft soap, a pharmaceutical product used as a liquid in enemas, is a very safe and very effective insecticide to control brassica whitefly and cabbage white caterpillars. Dissolve 56g (20z) in 4.5 litres (1 gal) hot water and use diluted when cool enough. Soft soap can be bought from Garden Organic
Savona insecticidal soap is a relatively new product for the control of aphids, whitefly, red spider mite and scale insects. It is perfectly safe to human beings, mammals, bees, ladybirds and other predators and is recommended for use in greenhouses before the introduction of biological controls if there are high pest levels. It is diluted with fifty parts of rain-water and is now widely available from garden shops.
Do NOT use washing up liquid as it contains many other chemicals which can damage plants.
There are any number of recipes for home-made pesticides, many of them handed down over the centuries (and some of doubtful efficacy).
For example, an old remedy to deter snails and slugs is to collect as many as possible, morning and evening. Tip them into a bucket of boiling water and let it stand for a few days until the smell becomes fearsome, then strain off the liquid and use it to sprinkle round vulnerable plants, such as the young growth of delphiniums, lettuce and so on – but not on them. The remains of the slugs and snails can also be scattered.
Effective against blackfly on, for example, broad beans and runner beans but not against cherry blackfly.
The bracken must be gathered when brown and brittle dry. Pulp the leaves and store in paper bags until wanted. Using a graduated jar, measure out 120cc (4fl oz) of the bracken and pour on 420cc (14fl oz) of hot water, stir and allow to soak for twenty-four hours, strain, then bottle into airtight jars and keep out of reach of children, of course.
For use as a spray, dilute 25cc (1 fl oz) to 4.5 litres (1 gal) of rainwater and spray each day for three days.
This kills aphids, small caterpillars and is useful as a fungicide for mildew and blackspot on roses. The toxic agent is hydro-cyanic acid, so in preparing the spray use an old saucepan.
Gather 450g (1 lb) leaves and young stems of elder prefer-ably in spring when the sap is rising. Place in the saucepan and add 3.3 litres (6pt) water. Boil for half an hour, topping up as necessary. Strain through old tights and use the liquid cold and undiluted. It will keep for three months if bottled tightly while still hot.
Twigs of elder, cut in the spring and placed at intervals, inverted V-wise, over early turnip rows, are said to ward off attack by flea beetles.
Horsetail (Equisetum arvense) is a pernicious weed which spreads by underground stems which may go down as deep as ten feet, forming horizontal rhizome systems at intervals. This makes it particularly difficult to control. If you have a horsetail problem, there’s a bright side to it because an infusion of the weed makes a good fungicide for control of mildew on strawberries and other crops, and checks rust on celery and celeriac.
Collect the horsetail, foliage, stems, rhizomes and all, and for each 28g (1 oz) pour on 1.1 Litres (2pt) hot, not boiling, water, and allow to stand for twenty-four hours. Strain off the ‘tea’ and use undiluted.
Bio-dynamic gardeners and growers have a very high regard for the common stinging nettle, using the leaves in sprays of several kinds. As well as using nettles as an activator on the compost heap (page 27) the organic gardener can use them as a liquid manure and as an aphicide.
Gather 224g (l/2 lb) young nettles and soak in a bucket of water for a week. Strain and use undiluted as a control of aphids on roses and celery leaf miner. Add the mushy nettles to the compost heap.
The oxalic acid in rhubarb leaves is a safe control agent for aphids, particularly those on roses. Cut 450g (1 lb) rhubarb leaves, place in an old saucepan with 1.1 litres (2pt) water and boil for half an hour, topping up as necessary. When cool, add 1 dsp soap flakes dissolved in 275ml (l/2 pt) warm water. This acts as the wetting agent when added to the strained rhubarb liquid. Stir the mixture thoroughly and use undiluted as a spray.
Use the recipe above as an effective weapon against aphids on many crops.
The value of seaweed as a liquid manure is well known. Used as a foliar feed on a wide range of vegetable crops, it also has an insecticidal and fungicidal effect possibly because the alginates make the surface of the foliage less attractive to pests and the spores of fungi.
The advice from the book quoted above is dated. As far as I know none of the fungicides shown below are now approved for organic use. Never use anything containing sulphur on cucumbers, marrows, courgettes or any other squash as it will kill the plant. A few years back I heard that horsetail tea was once used as a fungicide.
This was discovered in 1845, but not used as a fungicide until 1885 when it was found to control downy mildew . It is the most important preventative of potato blight and is made from copper sulphate and lime. Applied before the fungus spores of blight settle on the leaves, the copper sulphate gradually releases small amounts of soluble copper which kills the germinating spores. It is equally effective in preventing blight attack on tomatoes outdoors.
Substantially the same as Bordeaux mixture, but the lime is replaced by washing soda. It can be used as a prevention against mildew on roses and gooseberries by spraying the bushes in January.
A combined pesticide and fungicide that offers some control over aphids and powdery mildew on roses, delphiniums, chrysanthemums and other plants.
Usually sold as flowers of sulphur or yellow sulphur, it is also available as a brand-named spray for control of black spot on roses, powdery mildew and scab on fruit vegetables and ornamentals. It can harm parasitic wasps and predatory mites.
Also available as a combined insecticide and fungicide when mixed with lime. It is used as a winter wash on fruit trees and is useful in controlling big bud and other mites.
Used neat it is an effective winter wash for soft and top fruit. For gooseberry mildew use 0.5 litre (1 pt) urine to 3.9 litres (7 pts) hot water into which 84g (3oz) washing soda and 28g (1 oz) soap flakes have been dissolved. Spray when cool.