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Pests and diseases

Pests and diseases can affect all growers, with devastating effects but there are lots of ways to prevent and treat them organically, including setting up a healthy growing area.
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Pests and diseases this month

If dealing with aphids, remember washing up liquid is not soap, it is a powerful detergent designed to de-grease a frying pan. Plants' tender new growth can be severely damaged if sprayed with a solution. It is better to squish aphids by hand (wear gloves if squeamish).

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Your organic garden in July

July is the month when all the hard work pays off. Herbs and flowers are fragrant, and the soil rich with your homemade compost. After the rush of May and June growth, you can relax a little and enjoy the fruits of your labour.

Fact sheets - Pests and diseases

Our member factsheets contain in-depth information and guidance on a range of topics. Log in or join us to access them.

  • Pests and diseases

    Pollen beetle

    Take a look at our member fact sheet for detailed information about pollen beetle.

  • Pests and diseases

    Onion white rot

    Tackle the destructive effects of onion white rot with our handy member fact sheet.

  • Pests and diseases

    Woolly Aphid

    Find out how to prevent Woolly Aphid in your organic garden with our handy member fact sheet.

  • Pests and diseases

    New Zealand flatworm

    Find out more about New Zealand flatworm, and how to prevent and control it with our member factsheet.

Ladybird on a Nasturtium leaf

The Principles of Organic Gardening

Organic gardening works within natural systems and cycles. Find out how to practice the five principles.

Frequently asked questions

  • Scale is a common pest on any citrus plant. Small infestations can sometimes be wiped off, but the size of your plant would make this time-consuming and difficult. Try spraying the leaves and stems with a suitable organically acceptable product - available from the Organic Gardening Catalogue.

    The biological control to use against scale is a parasitic wasp, but it is very hard to find a supplier. The wasp, Metaphycus helvolus, requires lots of sun in which to bask, and our summers have not been very sunny in recent years. Optimum temperature 20-30°C. This makes breeding less easy.

    Sprays approved for use in the organic garden can be harmful to useful insects, so only use them as a last resort. Spray flowering crops at dusk when bees are not active. Read the label before you buy. Use pesticides carefully

  • Carrot fly larvae tunnel through the storage root while underground. They can often reduce the carrot to a sorry mess, quite unsuitable for the kitchen. However, you can often salvage something to eat by cutting away the damaged areas.

    The best protection from this pest is to cover where the seed has been sown with horticultural fleece or a fine mesh netting immediately after sowing. Try a carrot variety that has some resistance to this pest, like Resistafly and Flyaway.

  • No, they don't, but it sounds like aphid damage. Aphids suck the sap from young leaves, and the results are twisted, distorted foliage, looking very much like peach leaf curl. The fruit should not be harmed, and all you can do is to make sure that your garden has plenty of predators present to consume the greenfly.

  • Potato tubers that are blight infected may contain higher levels of alkaloids than healthy tubers. The standard advice is that pregnant women should avoid eating them. However, tubers from plants where just the leaves have been blighted are not necessarily themselves infected.

  • From your description of the problem with your courgettes, it does sound as though Cucumber Mosaic Virus (CMV) is the culprit. The source of infection may not be as easy to track down as CMV, despite its name, is able to infect many different species of plant, not just vegetables and not limited to the Cucurbit family. Many ornamental and herbaceous plants can have the disease without showing the severe symptoms that you find on your courgette plants. CMV has been detected in more than 700 plant species in 86 families.

    Some examples below:

    Vegetables: Cucumbers and courgettes, tomatoes, peppers, spinach, celery and carrots

    Flowers: Gladioli, lilies, alstroemeria, begonia, carnation, crocus, cyclamen, dahlia, freesia, hyacinth, hydrangea, impatiens, narcissus, nerine, orchids, pelargonium, phlox, tulip and zantadeschia

    Weeds: Stellaria media (chickweed), Senecio vulgaris (groundsel)

    CMV is spread by sap feeding insects, aphids being the most common vector. Unfortunately, resistant varieties are not always foolproof and it may be necessary to take other precautions. Controlling aphids in the garden is one way that you can reduce the likelihood of infection getting to your plants. Grow flowers amongst your vegetables that will attract aphid predators such as ladybirds and hover flies.

    Another way to prevent virus infection may actually be to isolate the courgette plants in a netted area to prevent aphids from feeding. Use a fine meshed netting, such as the enviromesh available from The Organic Gardening Catalogue. Use this when you plant out the young plants in early summer.