Everything You Need To Know About Powdery Mildew, Including How To Control It.

The dreaded, unsightly silver-grey spots on your leaves will eventually reduce plant health to a noticeable point. While most powdery mildews will not kill your plant, they are often associated with plant decline from other processes. So it’s no surprise you’ve come here searching-out how to control it.

Listed below are some methods to control and prevent it, and how each method it works. Hopefully your greater understanding of powdery mildew and these solutions will make you a more confident grower.

Increase the Humidity:

This can seem counter-intuitive because “word on the street” is that hight humidity causes powdery mildew. But, it’s actually the opposite. Increasing the humidity of your environment (indoors or in a greenhouse) will make the development of spores less likely. Don’t go crazy with humidity. 60% – 80% relative humidity is standard and fine. Maintain air-flow as well… but this does not mean crank-up the fans. Air movement spreads spores. But a plant with adequate spacing allows humidity from transpiration to be whisked away. 

How it works: Like most fungi, there are distinct life stages at play. What you are typically seeing with a powdery mildew infection is the reproductive stage of the fungal infection. These are the spores being created on the leaf surface so that wind may blow them elsewhere. And, like some fungi, the dramatic changes in humidity trigger sporulation. 

It is wrong to assume that keeping your room dry will help, because you are forgetting that as the leaves transpire they trap humidity amongst their tiny hairs. This is especially true on the undersides of leaves. Despite how dry your house is you can find pockets of extremely high humidity right along the leaf surface. So, the drier the air, the more extreme the change in humidity and the more likely to develop spores. 

Outdoors, ground covers and lots of densely placed plants maintain ground level humidity which helps. But too much crowding develops other problems. Pick your poison.

Example: A cannabis grower maintains ultra-low humidity and use high speed fans to firm the stems and contribute to drying. Their humidity sensor is central in the room. There is only a small spike in humidity during watering, or spraying. However, measuring the humidity at plant level found that during watering, spraying and at certain times of day caused spikes over 90% humidity, caused immediately by a reduction back to 20% or lower. These conditions resulted in catastrophic mildew problems. To make things worse, the spray used to control the mildew also caused a dramatic spike it the humidity.

Overhead Water:

Wash your plant leaves or occasionally water them overhead as if it is raining. This will prevent mildews from taking hold. (You can also wash off the spores for cosmetic reasons…although they remain viable once rinsed off, and the leaves can quickly return to their messy look if conditions allow).

How it works: Mildew spores are floating around – right now. They are everywhere and settling on leaves of all plants. Rain, is one of the natural factors that inhibits mildew spread and infections. So, occasionally wash those spores off before they infect. But remember, keeping your plants wet, or using a soap or acid/base, detergent or oil, will result in other issues. Just use water. And don’t get carried away.

Add sulphur to your soil.

This is likely a new one for most people. It’s a practice that is catching on amongst North American professional greenhouse growers. Simply mix elemental sulphur into your soil mix to give your plants a boost in protection against powdery mildew infections. 

How it works: Burning sulphur (more on that below) has been used for decades to control powdery mildew. But recently it’s been found that a plant with adequate sulphur in the soil naturally develops better resistance to the infection. Think of it as boosting your plant’s immune system. 

However, you’ve heard of adding sulphur to your soil before – for decreasing your ph. Keep in mind that minerals don’t really go anywhere, so you only need a tiny bit. The label on your sulphur will indicate how much to use over a designated space to drop the ph by one point. Don’t get carried away and greatly disrupt the ph for the sake of mildew control, otherwise you’ll have other problems. Just add a little bit and mix into into the soil (not directly on the surface).

Burning Sulphur

Burning sulphur causes it to fill an air space (inside). This is best left to professionals as it requires specialized equipment and safety precautions. But look it up if you need it. Some growers also use it to suppress some pests.

How it works: It can kill and disrupt powdery mildew spores. The plants benefit from it directly on their leaves, displacing other microbials, and inhibiting pests. A hot plate burns the sulphur until it mixes with the air. Typically growers turn these on for several hours over night, and then exhaust the house before re-entry of personnel the next day. While this can suppress some pests, it is unlikely to outright control them. And, it is guaranteed to kill beneficials. So always be prepared to reapply any beneficials after sulphur burning. 

Leave it alone.

Mildew is a natural process. Sometimes leaving it alone allows it to take care of itself. Not all powdery mildews are the same, either So powdery mildew on your maple is not necessarily the same species that will infect your squash, which means you don’t always have to worry about it spreading to other kinds of plants. But be concerned when it’s on a rose in a garden filled with roses. That mildew will spread (actually it probably already has).

How it works: Your squash will get powdery mildew, especially late in the season. The truth is your plants have had this infection all along, but were fighting it off. Come fall, the plants’ energy and nutrients are all sent to the fruit and the weakened leaves become susceptible to the infection – this is why you suddenly see it. But just let it be. There are many tiny invertebrates (and some larger ones, like beetles) that feed on the mildew spores. Leaving the spores might increase the populations of these special creatures making subsequent infections less noticeable. 

Think about it this way: A good year for rats (if left alone) will result in the following year being good for owls. Therefore, a good year for powdery mildew will be followed by a good year for whatever feeds on it. And, you might not have to wait a year. With invertebrates and pathogens there is often more generations per year, so you may see the results more quickly.

You might also find that the infection simply comes and goes. It is opportunistic and really needs relatively warm and dry conditions to spread. So during its ideal time, it may flourish, only to be fought-off by the plant when conditions favour vigorous plant growth. 

Apply Milk or baking soda.

Yes, some people apply milk mixed with water (1:10). This has positive results. While baking soda (also in a water solution) is relatively simple, milk has sugars, fats and proteins which may develop other problems. Before you spray anything (whether homemade or not) consider the consequences. In most cases it’s simpler to carefully remove infected leaves, or just leave it alone. 

Or course, saying “you can leave it alone,” isn’t always true. Sometimes you need an intervention. If, in mid-season, or indoors, you suddenly have a spike in powdery mildew, one of the broader interventions my be required. But, while you treat that symptom, be sure you’re not forgetting about the cause (likely environment) and correcting it as well. Otherwise, it will be an on-going problem. 

There is one last thought I’ll leave you with. There are now products available for (and continuing research into) displacing fungi and other infections by…..get this….introducing a different infection. Hear me out! One of the reasons the milk application might work is because it can stick to the leaf. Once there, spores of wild yeasts and others will land and feed on the milk residue. While they are there and flourishing (you might not be able to see them) it leaves less room (and in some cases a barrier) for other spores to infect the leaves. I personally know two commercial cut-rose growers who use a spray of primarily molasses to do something similar. And some of the emerging products are sprays of harmless bacteria that can displace bacteria that would otherwise infect a plant. 

I don’t bring up this idea so that you start emptying your fridge by spraying condiments all over you yard. The idea I want you to consider goes hand-in-hand with the idea of leaving it alone: There is so much going on on the surface of a leaf. You can’t see even half of it. A mildew infection may have an infection of its own, or the infection on one leaf may cause the plant to harden off other leaves to prevent the spread. My point is, plants don’t normally need our help unless we’re growing them outside of their natural environment. Outside, you rarely have to worry. So don’t get too hung up on leaf blemishes, fungi or mildews, if you’re not a commercial grower or tending to a mono-culture. 

So, remember these simple facts:

Powdery Mildew, when visible on the leaves, is a late stage infection. It is already inside the plant. So some plants without noticeable infections likely already have it.

Powdery Mildew needs relatively warm conditions. Above 30C, and below 15C it slows.

It needs dry conditions and spikes in humidity.

Powdery Mildew usually starts as spots that look like a dusting of something white/grey, but can quickly develop into random shapes or cover the entire leaf.

It usually is most prominent on the upper surface of leaves.

Powdery Mildew is natural, and all around us all the time. Environment and plant stress/condition is what allows it to get a foot-hold.

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