Powdery mildew is instantly recognisable as a white bloom on the leaves, fruit or stems of an infected plant. It looks like the plant as been dusted with talcum powder on the top surface of the leaves but brown patches usually occur on the lower side of the leaf.
Powdery mildew proliferates in high temperatures but it is also associated with dry weather and high humidity. Powdery mildew peaks during the summer and is very difficult to cure once a plant is heavily infected.
Of the crops we grow we find the following are the most susceptible.
- Crataegus monogyna (Hawthorn)
- Apples and Pears
- Gosseberry and Raspberry
- Acer platanoides and cultivars (Norway Maple)
- Quercus spp. (Oak)
- Amelanchier
- Rhododendron and Azaleas
- Lonicera spp. (Honeysuckles)
Control
From April monitor susceptible plants during periods of hot weather and ensure adequate watering and a balanced supply of soil nutrients. Newly planted native hedging, Oak trees, roses, soft fruit and fruit trees should always be monitored.
When the disease is spotted, if practicable remove and destroy infected leaves and apply a suitable fungicide. Through the summer it is good practice to alternate fungicide products with different active ingredients to reduce the chance of fungicide resistance.
In glasshouse and conservatories long periods of high humidity should be avoided. After irrigating any plants, vent well and open doors as is practical.