What is Powdery Mildew?
Powdery Mildew is a widely-spread fungal disease, which can affect commercial crops such as grapes and it is all too common in the home garden. What causes it? There is not just one fungus but a number of fungi, which are species-specific e.g. the one that affects grapes is different from the one that affects lilac. In the home food garden, powdery mildew can attack a range of crops including cucurbits (cucumber, zucchini etc), peas and apple. In the flower garden roses, dahlias, sweet peas are amongst the species that will succumb to it. To get rid of powdery mildew you are best advised to take care to avoid it and, if it appears, use organic sprays to treat it.
The Powdery Mildew fungus appears as tiny heads on plants, looking like a fine powder. If you look closely you can probably see the very small, white or grey, spherical fruiting bodies that will later turn brown and then black. The fungus is more common on the upper sides of leaves though it may also attack the undersides as well as buds, twigs and flowers. Leaves may become distorted and fall off prematurely.
Hot, dry weather is conducive to the arrival and spread of Powdery Mildew in the garden. High humidity is favourable but not rain – the fungus won’t grow on wet leaves. Humidity needs to be high to enable spore germination and crowded conditions with poor air circulation enhance the development and growth. Older plants, new growth, weak or stressed plants and those growing in shaded places are most susceptible. Stress from drought is also a common cause.
Get Rid of Powdery Mildew
Your best plan is to look for plant or seed varieties that are resistant to the disease but there may not be any available or you may like a particular variety in spite of this disadvantage. There are various ways to attack the problem. Since we garden organically, I’ll mention only practices that fit this category.
1. Avoid planting susceptible species in low, shady places. If you know that a certain crop is likely to develop the disease, try to plant it in a sunny spot with plenty of airflow.
2. Remove any diseased plant material. This material should be binned, not composted since home composting is usually not hot enough to destroy diseases.
3. When watering, aim the flow low down and avoid getting the leaves wet as wetness higher up will increase humidity.
4. Prune overcrowded plants and bushes to open them up and decrease humidity.
5. If you want to use chemicals then choose Neem, sulphur, or potassium carbonate. These are suitable for use in an organic garden. Use them according to instructions and don’t breathe in the spray.
6. An easy treatment is to use baking soda (sodium bicarbonate) and water, with or without light horticultural oil (cooking oil will do if you don’t have the horticultural oil).
1 tablespoon baking soda
2.5 tablespoons oil
1 gallon (4 litres) water
Mix together and pour into spray bottle. Shake well before use.
If you don’t use oil in the mixture then add 1 dessertspoon washing up liquid. Do not use in sunlight as it may result in leaf scorch.
This baking soda mixture has not had full scientific testing but it has been used by organic gardeners for a very long time. It is safe to use but avoid using it in sunshine.