Powdery Mildew - Ask Extension
I had a problem with powdery mildew in my flower beds and flower pots last season.
It is my understanding that powdery mildew overwinters.
1. Do yo...
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Powdery Mildew #925813
Asked March 10, 2026, 8:55 PM EDT
I had a problem with powdery mildew in my flower beds and flower pots last season.
It is my understanding that powdery mildew overwinters.
1. Do you recommend I change wash out my pots and change the pot soil between seasons?
2. Should other than removing debris, is there any product I can use in my flower bed to reduce the potential of the development of powdery mildew this season.
Thank you!
Frederick County Maryland
Expert Response
Most fungal spores overwinter, though some blow back into our region on weather fronts from areas to our south. There are many species of powdery mildew based on the host plant it infects; even though the symptoms all tend to look the same, they are caused by different fungal species, and we don't have life cycle details for all of them.
You do not need to sterilize pots, though giving empty pots a cursory clean-out between plants is helpful. A rinse to remove as much clinging soil as possible, and perhaps a wash in soapy water, is sufficient in most cases. Only in instances where a plant died from a wilt infection or root rot might it be useful to also sterilize (such as with a ready-to-use bleach spray) the container, which will only work well if it's free of any soil residue first. Even so, not all infectious fungi can survive for long without a host plant to infect.
Keep in mind that, even if you sterilize the containers, a disease could always appear later from spores that blew around or splashed onto the plant or soil in rain or irrigation water, so nothing stays sterile for long. They key is to give plants the growing conditions they need so they can resist most infections, and in some cases, certain cultivars of some plants have resistance to common diseases for that species. Powdery mildew thrives in situations of high humidity and poor air circulation, so avoiding crowding plants helps with reducing outbreaks.
Replacing the potting mix or garden soil is not needed, although potting mix that is several years old in the same container benefits from freshening-up by adding new mix to "fluff" it up again to restore good drainage for roots. The mix can be completely replaced if you prefer, or just mixed 50-50 with an equal amount of new material. Adding compost to garden beds can help introduce beneficial microbes that help plants stay healthy, and just layering it on top (for soil life to mix in gradually) or mixing it into each planting hole is fine.
There is no fungicide product we'd recommend as a preventative, and you definitely don't need to pre-treat the bed or containers. Fungicides can risk harm to other organisms, including pollinators, and if used, they should be a last resort. They can only protect healthy growth, and usually cannot cure existing infections (with powdery mildew, sometime they can suppress an existing outbreak, though still cannot reverse any symptoms or leaf damage from the infection).
Miri
You do not need to sterilize pots, though giving empty pots a cursory clean-out between plants is helpful. A rinse to remove as much clinging soil as possible, and perhaps a wash in soapy water, is sufficient in most cases. Only in instances where a plant died from a wilt infection or root rot might it be useful to also sterilize (such as with a ready-to-use bleach spray) the container, which will only work well if it's free of any soil residue first. Even so, not all infectious fungi can survive for long without a host plant to infect.
Keep in mind that, even if you sterilize the containers, a disease could always appear later from spores that blew around or splashed onto the plant or soil in rain or irrigation water, so nothing stays sterile for long. They key is to give plants the growing conditions they need so they can resist most infections, and in some cases, certain cultivars of some plants have resistance to common diseases for that species. Powdery mildew thrives in situations of high humidity and poor air circulation, so avoiding crowding plants helps with reducing outbreaks.
Replacing the potting mix or garden soil is not needed, although potting mix that is several years old in the same container benefits from freshening-up by adding new mix to "fluff" it up again to restore good drainage for roots. The mix can be completely replaced if you prefer, or just mixed 50-50 with an equal amount of new material. Adding compost to garden beds can help introduce beneficial microbes that help plants stay healthy, and just layering it on top (for soil life to mix in gradually) or mixing it into each planting hole is fine.
There is no fungicide product we'd recommend as a preventative, and you definitely don't need to pre-treat the bed or containers. Fungicides can risk harm to other organisms, including pollinators, and if used, they should be a last resort. They can only protect healthy growth, and usually cannot cure existing infections (with powdery mildew, sometime they can suppress an existing outbreak, though still cannot reverse any symptoms or leaf damage from the infection).
Miri
