GARDENING: How to solve widespread fungal disease

The white fungus shows up clearly on this purple Berberis.

The white fungus shows up clearly on this purple Berberis.

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I love Calendula (pot marigolds) and Centaurea (perennial cornflower), but they always fall victim to powdery mildew by now.

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

A breeding ground for powdery mildew on these Calendula.

A breeding ground for powdery mildew on these Calendula.

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 have a narrow range of host plants.

The symptoms are:

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

Getting rid of dead flower heads improves air circulation.

Getting rid of dead flower heads improves air circulation.

l Tissues sometimes become stunted or distorted.

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

Spores overwinter on perennial crops, or in plant debris. When conditions are right, it spreads quickly – cool, humid nights and hot, dry days. Unchecked, leaves to turn yellow, die and fall off.

Organic controls are:

CLEANLINESS: Destroy fallen infected leaves. Mulching and watering reduces water stress. Pruning out infected shoots will reduce subsequent infection.

VARIETIES: Try to buy resistant cultivars.

MILK AND WATER SOLUTION: 50/50 milk/water solution; one part milk to two parts water and a 10% milk solution recommended. Spray weekly, starting with the weakest strength.

WATER: Washes off spores before they have time to develop. Spray early in the day so foliage has time to dry quickly.

MOUTHWASH SPRAY: One part ethanol-based mouthwash to three parts water has been cited but can damage new foliage.

VINEGAR: A mixture 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: 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.

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