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.
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.
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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