Powdery and downy mildew: causes and cures
I love Calendula (pot marigolds) and Centaurea (perennial cornflower), but they always fall victim to powdery mildew by August at the latest.
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, curcurbits, 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 of powdery mildew:
- White, powdery spreading patches on upper or lower leaf surfaces, stems, flowers and fruit.
- Tissues sometimes become stunted or distorted.
- Most infected tissues show little reaction in the early stages, but in a few cases, such as buckthorns, tissue turns dark brown.
- Powdery mildew is not the same as downy mildew. Both produce light-coloured spores on foliage, but downy mildew develops spores only on the undersides of leaves – powdery mildew appears everywhere.
- 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 turn yellow, die and fall off.
Powdery mildew organic control
CLEANLINESS: Destroy fallen infected leaves. Mulching and watering reduce water stress. Pruning out infected shoots will reduce subsequent infection.
VARIETIES: Try to buy resistant cultivars.
MILK AND WATER SOLUTION: I’ve seen a 50/50 milk/water solution recommended; one part milk to two parts water and a 10% milk solution. No-one’s sure why it works – it may boost a plant’s immune system or act on the mildew directly. Spray weekly, starting with the weakest strength.
WATER: Washes off spores before they have time to develop, but that creates an environment for downy mildew. 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 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.
BORDEAUX MIXTURE: A mix of copper sulphate and hydrated lime, which prevents spores from developing. The lime enables the solution to penetrate leaves. However, it can burn plant tissue, is damaging to micro-organisms in the soil and is harmful to beneficial insects, despite being approved as organic. It is also moderately toxic to mammals and humans and can build up copper concentrations in the soil. Avoid it, in my opinion.
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. The active compound allicin prevents germination of spores.
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. Commercial horticultural oils have an emulsifier added. Spray every seven to 14 days.
Downy mildew is different from the powdery variety. Although both produce light-coloured masses of spores on foliage, downy mildew develops spores only on the undersides of leaves, whereas powdery mildew will appear on both sides, plus shoots, buds and sometimes flowers.
Its ideal conditions are the opposite of the powdery stuff – it appears when it’s cool and wet and is halted by warm, dry, windy weather.
It’s caused by a fungus-like organism, spread by airborne spores.
Affected plants include brassicas, carrots, foxgloves, Geum, grape vines, Hebe, busy Lizzie (Impatiens), lettuce, onions, pansies, parsnips, peas, poppies, rhubarb, roses, spinach and Nicotiana.
Symptoms of downy mildew:
- Discoloured blotches on the upper leaf surface – pale green, yellow, purple or brown, sometimes with straight edges if bordered by leaf veins.
- A mould-like growth, white, grey or purple, on the underside of the leaf, corresponding to the blotch.
- Severely-affected leaves may shrivel and turn brown or yellow and fall prematurely. Occasionally, other plant parts can be affected.
- Plants are often stunted and lack vigour, leading to death, especially in busy Lizzies and Nicotiana.
- Pick off and burn affected leaves and remove and destroy badly-affected plants.
- Avoid dense planting for good air circulation.
- In greenhouses, avoid prolonged leaf wetness or high humidity. Open the doors and vents.
- Don’t water plants in the evening.
- To avoid infection from soil-borne spores, employ crop rotation for veg, and don’t use the same ornamental plant in that piece of ground for a year.
- Buy resistant varieties – such as lettuce and onions.