Control of Powdery Mildew Organically

At Glyn Bach, we hold a National Collection of Monarda; these plants are notoriously prone to powdery mildew, so we have had to learn how to control it, in order to keep our collection healthy and happy.

Mildew is around us, every day.  It is in the atmosphere and there are many different types.  Monarda only succumb to mildew when they are stressed.  To alleviate stress we have learnt the following:-

  1.  Plants need to have the right nutrients.  Monarda are heavy feeders and will not thrive if their feeding requirements are not met. Plants in pots soon exhaust the nutrients in the potting compost which will weaken them and they will succumb more easily to mildew.  We replace potting compost in pots annually and add Osmocote as a slow release fertiliser.  We also feed pots, from May onwards to the end of September, with a mixture of extract of seaweed and our home-made nettle tea.  Nettle tea is particularly advantageous as it encourages the leaves to produce a thicker coating. 
  • Whitefly and greenfly can weaken plants and spread disease so they must be controlled.
  • Plants in pots and in flower beds must not be allowed to dry out.  All Monarda require a certain amount of moisture and dry conditions will cause them stress which will exacerbate mildew.
  • Mildew descends on the plants in the mornings so overhead spraying during dry weather is advantageous as it washes off any mildew that is about to settle.  Be sure to spray before midday.
  • Plants should be routinely sprayed with an anti-fungal, from March onwards; as soon as growth begins to show, we spray weekly with our own organic mix (see mixture details below).  In April, nettle tea is added to this spray.
  • Humidity definitely increases the risk of mildew occurring but it is impossible to control outside so ensuring plants are healthy, making them more resistant to mildew, is important.
  • Many report that allowing air to circulate around plants is important.  We have not really found this factor to be too much of a problem.

Nettle Tea

We find nettle tea to be a brilliant free resource.  As soon as the nettles begin to appear in April, we pick the young shoots and place them in a 2 gallon glass container.  This is covered with water and left to mature for about 3 weeks.  As soon as the mixture gives off a pale green froth it is ready to use.  Every time we use some of the mix, we top it up with fresh nettles and water so we have a continuous supply until late Autumn.

Nettles contain formic acid which kills whitefly and greenfly. It is also rich in the following vitamins and minerals:-

vitamin A, various B vitamins (including B-1, B-2, B-3, and B-5), vitamin C, amino acids, calcium, fatty acids, folic acid, iron, magnesium, manganese, phosphorus, and potassium.

Nettle Tea makes a good all round food source for plants and is a great organic insecticide.

Mildew Spray

We make our own spray as follows:-

1 litre of water

½ tsp Bicarbonate of Soda

½ tsp Wood oil

½tsp Washing up liquid

Bicarbonate of Soda (Sodium hydrogen carbonate –  NaHCO3) is one of the best organic controls to use in the garden – it causes minimal damage to all wildlife and no detrimental impact has been noted on honey bees.   It creates an alkaline environment so powdery mildew cannot colonize the surface of the leaf since they need a neutral pH (around 7.0) to mildly acid to survive and thrive.

Washing up liquid is pH8 so also helps to neutralise mildew, but also acts as a surfactant for whitefly and greenfly. We use Ecover, but Fairy App

Wood oil, such as Murphy’s wood oil (classed as organic) is also a surfactant and it acts as a wetting agent on the surface of the leaf, allowing the Bicarbonate of Soda to neutralise the acidic mildew. 

The above products have limited impact on the environment and minimal to no impact on the important pollinators that we encourage into the garden.  We find this simple, cost effective regime is effective in controlling powdery mildew on our Monarda. 

Whilst we are primarily concerned with keeping our Monarda collection in pristine condition, we do also use our spray on our Roses and courgettes. We have also heard that milk neutralises mildew and black spot too, although we have never tried this.

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