Gardening: Got a problem with powdery mildew in your garden?

Calendula ruined by powdery mildew.
Calendula ruined by powdery mildew.
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It seems this year’s number one pest is powdery mildew, caused by our very dry spring – here’s how to identify it and treat outbreaks organically without buying expensive products.

Powdery mildew is a white film that grows on leaves, stems and sometimes flowers and fruit when there isn’t enough air circulation between plants.

Purple Berberis nearly white with the pest.

Purple Berberis nearly white with the pest.

It’s a fungal disease affecting apples, blackcurrants, gooseberries, grapes, brassicas, curcubits, peas, grasses, Acanthus, delphiniums, phlox, the daisy family, honeysuckle, Rhododendrons, Azaleas, roses and oaks.

Each mildew has a narrow range of host plants – the fungus affecting peas is different from the one attacking apples.

Symptoms are:

White, powdery spreading patches on upper or lower leaf surfaces, stems, flowers and fruit.

Euphorbia polychroma purpurea is particularly susceptible in dry weather.

Euphorbia polychroma purpurea is particularly susceptible in dry weather.

Tissues sometimes become stunted or distorted.

Most infected tissues show little reaction in the early stages, but in a few cases, tissue turns dark brown.

Spores produced on both sides of leaves.

It thrives in warm weather when foliage is dry – wind spreads the spores, which can’t germinate or grow when foliage is wet.

Fill out gaps with quick-growing annuals like Godetia Lady in Blue.

Fill out gaps with quick-growing annuals like Godetia Lady in Blue.

Organic control:

Destroy fallen infected leaves. Mulching and watering reduces water stress. Prune out infected shoots.

Try to buy resistant cultivars.

Spray weekly with a 10 per cent milk/water solution, upping the concentration to 50 per cent.

Wash off spores early in the day so foliage has time to dry quickly.

One part ethanol-based mouthwash to three parts water but this can damage new foliage.

A mix of 2-3 tablespoons of apple cider vinegar, containing 5 per cent acetic acid mixed with a gallon of water is the dose, but too much vinegar can burn plants.

Garlic spray: Blend two bulbs in a quart of water with a few drops of liquid soap, strain and refrigerate. This makes a concentrated solution that should be diluted 1:10 with water before spraying.

Oils: Vegetable seed oils such as canola can be used, at a rate of 2.5 to 3 tablespoons per gallon of water, plus a quarter-teaspoon of liquid soap to emulsify the oil. Spray every seven to 14 days.

Powdery mildew is often confused with downy mildew but occurs in totally different conditions – but more of that next time!

JOBS TO DO THIS WEEK

Gaps in herbaceous borders are best filled with annual bedding, such as Calendula, Godetia and Clarkia. Thin out earlier direct sowings in two or three stages at fortnightly intervals. Final spacings should be between 10-20cm (4-8in).

Sow salad crops, such as beetroot, Chinese cabbage, pak choi and radish. Leafy salads do better when sown in light shade, as hot, dry weather leads to bitter leaves.

Sow French, runner and broad beans, peas, squash, sweetcorn, and outdoor cucumbers directly into prepared beds. French beans are best sown in traditional rows, 45cm (18in) apart, at 15-22cm (6-9in) spacing.

Open doors and vents on greenhouses to increase ventilation. Damp down the floor to increase humidity. Give plants a liquid feed to encourage flowering and fruiting.

Plant out summer bedding and seed-raised plants. Make sure they are well watered in and keep moist during dry weather.

Divide Primula (primroses) after flowering, planting them in a nursery bed.

Hoe borders to prevent annual and perennial weeds from spreading and seeding themselves.

Lift clumps of forget-me-not once the display wanes, and before too many seeds are released.

Sweet peas need training and tying in to their supports to encourage them to climb and make a good display.

Liquid feed containerised plants every two to four weeks.

Continue to protect lily, delphiniums, hostas and other susceptible plants from slugs and snails.

Prune overcrowded, dead or diseased stems of Clematis montana once it has finished flowering. Untangling the stems can be fiddly, but it will take cutting back hard.

Sprinkle fertiliser around perennials, shrubs and roses.

Regularly mow lawns and cut edges with a half-moon edger.

Although most winter brassicas need to be sown earlier in the season, calabrese, turnips and kohl rabi can be sown now for an autumn crop.

Start treating potatoes and tomatoes against blight.

Celeriac and celery can be planted out. A well-prepared site with lots of organic matter dug in is essential.

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