Powdery Mildew

powdery mildew white mould

Powdery Mildew

I.D., Prevention and Treatment

A Basic Powdery Mildew Guide for Growers


If you have been gardening for any length of time, indoors, outdoors or most certainly in a greenhouse, there is a good chance you have seen powdery mildew the white mould on plant leaves.  This is  a plant disease that causes lots of losses around the world in a huge range of crops, whether being grown for food, fibre or medicine. Sometimes growers get Powdery Mildew confused with Botrytis (AKA “The Grey Ghost”).  While both can ruin your crop if left unchecked, they are very different.  For one, powdery mildew is a fungus-and Botrytis is a bacteria.  In Tsung-Tsu’s “Art of War” one of the commandments is to Know Your Enemy.  Once you understand what’s infesting and bothering your plants, ie the nature of it, what it likes and doesn’t like, you have halfway won the battle already.  Let’s have a quick and effective lesson in identifying, preventing and treating powdery mildew, as it pertains to hydroponics, horticultural or greenhouse crops.




Usually you’ll see a very light white “spot” first, typically on dark, well fertilized foliage first.  It will almost look like somebody spilled some milk or other white organic liquid onto a plant leaf and left it there to dry.  As the disease progresses, entire leaves or even flowers get covered, looking like they have been dusted in a fuzzy looking all white icing sugar–but don’t be fooled, this stuff is anything but sweet to you and your crop!  This is how powdery mildew or Erysiphe graminis is able to take ahold-it looks rather benign, or not that big of a trouble.  That is, until your crop is ruined because it is a covered in this white mould.

identify white mould on plant leaves


If left unchecked, young plants that get infected will perform poorly, and in some plant varieties may not even flower, ie plant growth points succumb to being overwhelmed with the disease. Typically, while yields are affected negatively, the biggest damage or even loss comes from what the disease does to Crop Quality.  In some plant varieties it makes the harvest completely unmarketable-ie, upon inspection, nobody will want it, nor should they.

powdery mildew leaf

Conditions that Favor P.M.

Humid and warm weather favors the development and spread of the this crop disease (15-24 Deg C).  If it’s cooler, the disease spread is slower. Development and spread is suppressed by conditions over 30 Deg C.

Why Did My Crop Get P.M?

    • An abundance of Nitrogen from fertilizer is typically a vector-soft tissue is the result of lush fast growth.  the fungus has to penetrate into the leaf (like a mosquito) to gain a foot hold.
    • Climate conditions were favorable
    • Significant spore count, ie there is enough disease causing spores in the air and eventually the plant can’t fight it off; overwhelmed.

How to Prevent Powdery Mildew:

  1. Grow plants or strains less susceptible to the disease.  Some types of indoor strains are super predisposed to contracting P.M., while others resist it very well.
  2. Don’t buy problems.  Commercial clones can sometimes already have the problem–even if you don’t see it on the outside.
  3. Sanitation, sanitation, sanitation. Remove any damaged foliage from plants, and discard plant debris away from the growing area right away.
  4. Apply preventative measures-foliar sprays with potassium carbonate or aerobic compost teas or beneficial microbes or even baking soda can be utilized when there may be a risk of getting P.M.
  5. Don’t let plant tissue get dehydrated or “tired” plants right next to fans or dehumidifiers are often the first to show signs.
  6. Keep spore count low indoors-use an activated carbon filter to scrub indoor grow room air 24/7


 Treating Powdery Mildew:

First, determine how bad the problem is and how far along the crop is.  Is it better to simply terminate a young crop and start a new one after some scrubbing and precautions? I

Types of Treatment

  • MicrobialSince the problem is microbial, ie a fungus, so may the solution.  foliar sprays of specialty beneficial bacteria or fungal products specially selected for the kinds of crops you grow can be effective.  If you plants are covered in “friendly” living fungus or bacteria (that you can’t even see or otherwise detect) there will be “no room” for powdery mildew spores to make a home.  Kind of like a hotel: fill it with desirables, and there is no vacancy for any undesirables to come along and stay–they will need to look elsewhere, as many types of beneficial fungi and bacteria my be antagonistic, preventing and eliminating P.M; a highly unwanted guest in the garden.


  • Biological-Plants can produce there own anti-bodies and are very effective in fighting off diseases and other problems.  Remember plants have evolved without the ability to turn and run–they must stand to fight, and they can be pretty good at it too.  If you supply your crop with the right kinds of organic building blocks, they can often create their own defense.  Chitin and Salicylic Acid are a couple of naturally occuring substances that contribute to  your plants defenses (while having other benefits).  These can be found in refined bottled products from your hydro shop or garden cenmtre, or may “brewed” naturally via compost teas and applied to crops.


  • pH-The relative acidity/alkalinity on the surface of your plant’s leaves can have a big effect on how hospitable a place it is for P.M. to live.  Remember it’s a living growing thing, like your plant–who favor certain pH ranges in order to function properly. By creating a pH that is unfriendly to P.M. on your leaves, you can drive it away or prevent it in the first place.  Potassium Silicate, Potassium Carbonate and even Baking Soda are prime examples of common sprays you can use to treat your plants–all the ones listed here will raise the pH on the surface of the leaf, while not causing much or any harm to the plant.  However, sprays can leave residues, so choose carefully as not to ruin the taste of your harvest, if you are just treating for a couple of spots.


powdery mildew rose hydroponics
BEFORE spray treatment with beneficial Trichoderma fungi.

spray to treat powdery mildew

powdery mildew-trewated hydroponic rose after
AFTER treatment with spray. NOTE-the “corpse” doesn’t go away–but is no longer active ie still discolored but not “fuzzy”


A Note of Caution:

Moulds of all kinds can be hazardous to your health if inhaled-some causing serious or even potentially fatal respiratory issues via the lungs. Make sure you aren’t breathing in spores, indoors or out and dispose of diseased plant materials right away-incinerating is the best, and can feel the most satisfying too.

About Erik Biksa 247 Articles
Erik Biksa has been writing about and discussing hydroponics growing, related technologies and cropping methods since 1999 in a variety of professional publications and platforms globally Erik has travelled the world learning and teaching modern growing techniques and technologies and is appreciated by many growers for his informative yet hands on approaches. Presently, he is the Editor at Grozine Hydroponics Mag.