Pests & Diseases: Mildew

Oidium and Mildew - Pests & Diseases

Mildew is known as ‘downy mildew’. As the disease spreads, the leaves curl up, necrotise and eventually fall off. The parts of the mycelium containing the spores of this fungus emerge through the stomata of the plant. In good light it can readily be identified as a gray to purple felt on the back of the leaves.

About Mildew

What are Oidium and Mildew?
The terms Oidium and mildew refer to a group of phytopathogenic fungi that cause diseases in plants and have similar symptoms.
What can you see?
In general mildew is found on the upper side of the leaf, but there are exceptions. The leaf looks as if it’s been dusted with powder.
What can you do?
Keep a low humidity and keep your growing area clean.

About Oidium

Oidium is known as ‘powdery mildew’ Before any symptoms become clear the leaf starts to develop blister-like patches, and this is followed by the characteristic white powder where the blister was. The leaf looks as if it’s been dusted with powder. In general mildew is found on the upper side of the leaf, but there are exceptions. One type of mildew only grows on the underside of the leaf, so it’s no surprise it often gets overlooked. However, as the disease advances, the leaves can end up being completely covered in this white layer and it can even colonize the buds, with subsequent losses in crop size and quality.

Oidium and Mildew

How to prevent the pest?

The best treatment against these types of fungi is prevention; once they have set in and developed they are very difficult to eradicate, sometimes even with chemical fungicides. Try to prevent spores coming in from elsewhere and contaminating your plants by keep your growing area clean. You do this by using only clean equipment and wash your hands thoroughly before entering.

Solutions to control the pest

Check older leaves regularly for light yellow discoloration and fungal growth.

  • You can remove suspect leaves and keep these in a resealable freezer bag with some moist paper in a warm place. After two days you can check the leaves for mildew, maybe using a magnifying glass.
  • Remove any contaminated leaves, but also make sure that you don’t spread the disease yourself. Make sure you wash your hands regularly, preferably with an alcohol solution.
  • Burn infected material.
  • Use a fungicide.
  • Don’t forget that you’ll need to repeat the spraying several times.

Oidium and Mildew

If you want to use biological products to stop the fungus you will need to bear in mind that the effect is not very longlasting; so unless you get the timing right, all you’ll be doing is wasting time and money. A product that is effective against one type of Oidium may not work against another similar-looking mildewy fungus.

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