Powdery Mildew and Fungicides: Part 1: Background

Powdery mildew is a common fungal disease of angiosperms (basically, any plant but conifers). The growing fungus looks grayish white and powdery.

The beginning of a powdery mildew infection.

Powdery mildew disease is caused by an infection from one of the many fungal species of the family Erysiphaceae (ee-rhy-suh-facie).

Powdery mildews live, for the most part, external to their host not within it. In other words, they are ectoparasites, not endoparasites. Furthermore, powdery mildews’ parasitic process requires the host cells remaining alive…making them what scientists call obligate biotrophic parasites. Biotrophic pathogens usually cause disease on only a relatively small group of host plants. This is because of the different genetics and molecular processes/structures required for wider ranging host-pathogen interactions.

Powdery mildew is not systemic.  In other words, powdery mildew doesn’t grow into a part of the plant, propagate internally throughout the plant, and then pop-out somewhere else on the plant.

There’s a common misconception about the word “systemic”. Use of Eagle 20 would be an example of a systemic fungicide attacking a non-systemic fungus. Eagle 20 is a systemic fungicide—after you spray a plant with Eagle 20, the fungicide moves throughout the plant. The fungus doesn’t move throughout the plant, only the fungicide. Just because a systemic fungicide attacks a fungus, doesn’t mean that the fungus was systemic.


First symptoms of a powdery mildew infection are dusty gray or white spots on either side of the leaves. If untreated, the growing fungus will produce spores and rapidly spread through leaves and into stems and flowers. Powdery mildew fungi doesn’t usually kill the host plant, but it can cause early senescence and it does stunt the plant’s growth and development by stealing nutrients and increasing plant stress. Some plants will show local necrosis when infected badly.

The fungus can be transmitted by pests such as aphids or by touch, tools, clothing, shoes, and even by airflow through previously contaminated spaces. Those are all sometimes called “transmission vectors”, or simply vectors, i.e. – “Aphids on that clone were the vector that brought PM into my garden.”

Relative humidity plays an essential role in the control or spread of mold.  Molds and fungus particularly like dry leaf surfaces and humid environments. For the best inherent protection during growth phase, relative humidity (RH) should be kept between 40% and 55%. Relative humidity over 50% poses danger during flowering.

Most critical are the night time RH levels. The night time RH spike is often underestimated by growers. Consistent climate control is very important during growth phase, but it’s critically essential during flowering. Flowering requires drier conditions, humidity should be held between 25-45% RH.

Crowding your plants can produce pockets of stagnant, high humidity air—so give your girls some room. Wide RH swings are also conducive to powdery mildew infection. It’s good to keep the lower fan leaves trimmed because these are the most vulnerable to infection. Washing leaf surfaces can help flush fungal spores off the leaves. But be sure to wash early in the day so the leaves will be thoroughly dry before the dark period.

Sanitization = Standard Operating Procedure

Dedicated shoes, or a boot bath, and washing hands (alcohol based hand sanitizers are good) before entering the grow area are your first line of defense. Also, it’s a good idea to not wear your outdoor clothes into your grow.

Clean and sanitize tools and equipment that come into contact with multiple plants. Immersing tools for a couple minutes in a sanitizing solution helps prevent contaminating healthy plants. 1-pint of household bleach made up to 1-gallon with tap water will work well.

Use your sanitizer solution to spray pots, plant trays, water tanks & pipes, and all other hard surfaces of equipment. Wetting porous surfaces, even with bleach, doesn’t do the good things you’d hope it does—don’t spray water solutions onto porous surfaces. Minimize porous surfaces in your grow rooms.

Monitor and inspect for plant infection on a regular basis (at least daily). Upon noticing the start of a new infection, the infected plant parts should be clipped-off when possible. Put them directly into a plastic bag, without waving them around. This is your alarm bell to get moving or to adjust your treatment program. More on this in the Treatment section.

Between grows, thoroughly sanitize rooms and all surfaces with a commercial-duty hard surface disinfectant. Always pre-clean before using any disinfectant. Let the cleaners clean and the disinfectants disinfect.

To clean between grows, there are several effective products. Bleach is one of them. Bleach is easily available and inexpensive, it does have downsides though: irritating odor, it can produce noxious chlorine gas, and is inactivated by relatively small amounts organic matter.

Quaternary ammonium compounds (Quats) have been used for many decades as high performance sanitizers. They work by disrupting the cell walls of bacteria. Quats are known to also apply a heavy selection pressure on fungi. A couple of representatives are: Naccosan, it sanitizes hard surfaces and inhibits the growth of mold and mildew. Physan-20 is similar to Naccosan but also has a bit of ethanol (should you feel ethanol ever helps).

Be sure to change all your air filters between grows. Bag the filters as you pull them out and remove them from the premises into a closed trash container. Don’t go walking around with a used filter full of who-knows-what.

Powdery Mildew Treatment

Fungicides are most often grouped by “mode of action” (MOA), they’ve been split into approximately 42 groups (some groups aren’t numbered…and some don’t apply to powdery mildew). Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. The mode of action simply means the way in which they attack the fungus. The two mode-of-action groups most commonly mentioned are Group 3, the demethylation inhibitors (DMI), and Group 11, the quinone outside inhibitors (QoI).

Fungicides kill by damaging fungal cell membranes, inactivating critical enzymes or proteins, and/or interfering with key metabolic processes like fungal respiration. The best method to prevent fungal infection is a multi-pronged approach where you rotate through a few various systemic and contact (surface) fungicides. The idea is to use methods having different modes of action. Your combinations should aim to provide contact kill, spore suppression, and to limit fungal spread.

Fungi tend to develop resistance from overuse of some treatment methods, those products are labeled on their application notes to not be used more than (usually) twice on the same crop. Rotate through different fungicides having different modes of action. The goal here is to reduce the opportunity for those few individuals with strongly-resistant genomes to take over your fungal population. You do not want to develop your own superfungus. Rotation gives you much more effective control.

Remember to always test for phytotoxicity (toxic to plant growth) on a small area of the plant before use. Some amount of slight damage is usually tolerable when weighed against the alternative of powdery mildew.

Demethylation inhibitors (AKA-, sterol biosynthesis inhibitors) must be inside plant tissue to be absorbed by the fungus. This group inhibits sterol biosynthesis in the fungal cell membrane, DMIs do not prevent spore germination or germ tube growth. Triazoles are in this group, other examples are Banner MAXX, Eagle, Hoist, Rubigan, and Terraguard.

Quinone outside inhibitors act on a broad spectrum of fungi. They inhibit respiration and impede spore germination. They are excellent preventative treatments. They should be applied prior to infection or in the very early stages of disease development. One type of QoI is kresoxim-methyl products (such as Compass O, Cygnus, and Heritage). Another is the strobilurins such as Quadris® (azoxystrobin). Due to the narrow targeting action of strobilurins, fungal resistance is a major concern.

Fungicides designed to attack one single protein or enzyme molecule are said to be “single site acting”. Some examples of this are the triazoles (Group 3), which focus on the demethylase enzyme; the strobilurins (Group 11), which target the succinate dehydrogenase enzyme; and the carboxamides (Group 7) that go after the ubiquinol oxidase enzyme.

Fungicides that attack via a wide spectrum of pathways are termed “multi-site acting”. Multi-site fungicides are often used with single site products to reduce the risk of developing treatment resistant fungal strains. Examples are copper (sulfates, oxides, hydroxides, and copper linked to organic molecules) and chlorothalonil (Bravo, Echo, and Daconil). Chlorothalonil targets one fungal molecule, but that molecule is widely used. Copper ions do general damage to the enzymes critical for fungal cell function.

Another way to describe a fungicide’s mode of action is to say it is used as a protectant, as a curative, or as an eradicant.

Protectants are applied to healthy plants, they employ contact action to prevent fungal spores from germinating or penetrating host tissue. It follows from that that they must be applied before fungal spores infect the plant. Protectants generally are not effective once the fungus grows into plant tissues, so plant tissue that develops after application may not be protected.

Curatives are systemic fungicides that generally act within the plant and are effective against fungi shortly after fungal germination. These products must be applied within a certain time after infection starts.

Eradicants kill fungi on contact. Recent formulations such as triadimefon and myclobutanil not only kill growing mold but also prevent spore production.

Resistance Management

Resistance management is a critical practice and must be considered during decisions setting the treatment course. There are two types of resistance: qualitative and quantitative resistance. Qualitative describes an either/or situation: Either the fungus is resistant, or it is not. Either it has, or does not have, the quality known as resistance to the treatment. Quantitative, on the other hand, brings percentage into the picture: Some percentage of the fungi may be resistant to treatment. Some quantity of the fungi is resistant.

For instance, DMI fungicides are different than some other types such as the strobilurins. DMI fungicide resistance is almost never complete. Because DMI resistance is due to more than one mutation, the various fungal individuals will have different levels of resistance. The knee-jerk response to use a higher application rate after an application has failed will usually work in the case of DMI fungicides. Other fungicide groups, like the strobilurins, show qualitative resistance where any application rate is ineffective once resistance has developed.

You may have heard the term “cross resistance”. Cross resistance exists when a fungus shows resistance to a fungicide because it has resistance to another fungicide with the same or similar-enough mode of action.

If you’ve had recent PM outbreaks, it’s a good idea to use fungicides protectively (before powdery mildew becomes a problem again). Always apply fungicides at their labelled rates and use as many fungicide modes-of-action (Groups) as possible in your management program.


Note: All pictures and diagrams here were originally produced by others. They are the clearest and best examples available with which to educate and inform. Some were slightly adapted for this use.

This educational document was written, compiled, and adapted by Mike Steffes of Quest Dehumidifiers.