Powdery mildew produces a white powdery coating on the surface of plant leaves and fruits and causes plants to become distorted, look unsightly and growth to die back. Affected plants also often display dark brown or bright yellow spots. If it isn’t treated, the problem can cause plants to die and crops to become inedible.
- Powdery mildews are a group of closely related species of fungi that are very similar in appearance, producing a ‘powdery’ effect on the surfaces of plants.
- Each species of powdery mildew has a limited host range, and powdery mildew on one species of plant will generally not spread to other unrelated plants.
- All powdery mildews prefer dry conditions on closely spaced plants with poor air circulation and low light.
- Low levels of fungal infection may show no symptoms but develop when conditions are favourable.
- Powdery mildew spores can be spread great distances by the wind.
Prevention and control
- Monitor crops from flowering onwards.
- Soil moisture: ensure plants have adequate water and keep roots moist by incorporating organic matter and applying mulches.
- Avoid overcrowding plants. Thin as required throughout the season to provide good air circulation and minimise humidity.
- Limit nitrogen-rich fertiliser because this encourages sappy growth which is more susceptible to fungi.
- Prune out infected plant material as soon as you see symptoms. Do this carefully so as not to dislodge spores, seal in a bag and remove from property or burn. Do not compost or reuse.
- Sterilise gardening tools and equipment with disinfectant (including cutting tools between cuts). Do not use the same tools elsewhere in the garden before doing so.
- Some spores of powdery mildew can overwinter on infected material, so remove any sick-looking perennials in the autumn.
- Avoid planting related crops in the same location for another four years.
Chemical / fungicides
Products containing the following chemical ingredients are considered effective for Powdery mildew control: Myclobutanil, Penconazole, Flutriafol, Sulphur (including Mancozeb) as a dust, or incorporated in a spray with fatty-acids eg pest or vegetable oil.
- Milk: 1 part organic milk to 1 parts water
- Bicarbonate Soda or Potassium Bicarbonate
My experiences of Powdery Mildew
A .5% solution of baking soda works for: rosemary and Zucchini (weekly applications needed).
I am currently trialing a Potassium Bicarbonate solution for rockmelon and zucchini on which the powdery mildew has been more persistent.
Whenever is is warm and humid, powdery mildew raises it’s ugly head in my garden.
In late winter/early spring, shelling peas always become affected and I find this definitely reduces yields. Cucumber and (I think the watermelon) got it for the first time in 2013. On the up side it has dramatically increased my ladybug population 🙂