Powdery mildew

Some types of powdery mildew

One common species of powdery mildew has white, thread-like filaments which are invisible to the naked eye and gradually spread across the surface of the plant under attack. Throughout the winter, they lie dormant and begin to produce new spores in early spring when temperatures rise. The filaments are connected to plant cells underneath the surface of leaves by fine runners and feed off the contents of the cells. They will eventually dehydrate and kill the host plant.

powdery-mildewSmall, fruiting bodies called cleistotherciene are produced by a different type of mildew fungus, the bodies are perfectly round and yellow at first, turning brown or black as they mature. Each cleistotherciene contains several sacs, each with between two and eight spores, and they become dormant as temperatures drop during autumn. They may overwinter on healthy plants or ones which are unhealthy and diseased. In spring the spores appear and start to spread to other plants.

Recognising the symptoms

The symptoms of powdery mildew are easy to recognise. Grey or white powdery deposits appear on leaves and stems and can spread rapidly in hot, dry conditions, especially in homes that have central heating.

The powdery deposits are really thousands of small spores of the mildew fungus which reproduce rapidly and, spread through the air to plants of the same type. There is often more than one type of fungus on each plant, but they are all closely related. The spores are usually transferred from one species to another by touch. The damaged parts of a plant -,’ vary in size from tiny spots to large patches which join up to cover the surface Mildew can gradually attack the whole plant and may mildew-depositseventually kill it.

The fungus will gradually attack the whole plant and may eventually kill it. A healthy, strong-growing specimen will suffer less than a weak, sickly plant.

Preventing mildew

The best way to prevent attacks of mildew fungus is to give your plants the best growing conditions possible to keep them healthy. It will also help if you raise the humidity round groups of plants by mist spraying. Begonias and chrysanthemums are particularly affected by this fungus and they should be mist sprayed with

Raising humidity

A simple method or raising humidity is to group pots in deep trays containing a layer of pebbles or other material which is kept constantly moist. This will create a humid micro climate and help prevent mildew, especially in centrally heated atmospheres. Fill the trays to a depth of 5cm (2in) with pebbles, gravel or a horitcultural aggregate for cultivating without soil. Add water, and stand the pots on top, keeping them above the water level.

  • Increase the plants’ humidity by standing pots on pebbles which are kept constantly moist.
  • This fungus disease spreads rapidly in hot, dry conditions indoors. It can quickly reach epidemic proportions and severe cases may prove fatal.

Keeping foliage clean

For plants to be really healthy and in good condition to fight against the onset of a disease like powdery mildew, it is essential to keep the foliage clean. Dust and dirt can accumulate, eventually clogging the pores through which plants breathe. Small plants can be cleaned by spraying the foliage with water at room temperature. On bigger plants and those with large leaves, wipe each leaf with a damp cloth or sponge. Support the leaf with your hand to prevent damage. Never wet succulents or cacti, instead, clean with a soft brush.

Clean large-leafed plants by wiping with a damp cloth or sponge.



  • Keep slightly affected plants in ‘quarantine’, well away from others of the same variety. Remove affected leaves or wipe powdery deposits off large leaves with damp cotton wool.
  • Treat quarantined plants with a systemic fungicide like benomyl or triforine. Always spray plants outside and follow the manufacturer’s instructions carefully, applying at regular intervals.
  • Remember that fungus spores can be transferred from a sick to a healthy plant by a gardening tool or carried on your hands, so always wash them in hot, soapy water when you have been dealing with a diseased plant.
  • Destroy seriously affected plants, preferably by burning.
  • Remember that many plants such as cacti and succulents dislike high humidity and should not be mist sprayed. Display these in a different part of the room, away from more susceptible plants.


  • Let compost dry out when temperatures are high as plants will be more susceptible to attack.
  • Take cuttings from a plant suffering from powdery mildew, or use any offsets as they may be affected.

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