Powdery Mildew- treating soil?

sunnyvalley(Phoenix)February 17, 2012

I just lost all my zucchini to powdery mildew, probably let it go too far before treating since I've never dealt with that before. I sprayed with anything I read about- baking soda, milk, tea tree oil, oregano oil, but it only got worse, and I pulled the plants last week. I need to learn from this before my next crop!

We're having a warm and dry winter in Phoenix, weatherwise I think it'd be fine to start some replacements now, but I'm worried about the PM. I see spots of it on random weeds in my yard, which I don't understand since it's not humid, and those have nothing BUT sun and airflow.

Would I be dooming my new plants if I started now, or is there no point in waiting? Should I bother spraying the soil with anything?

I always end up getting my best garden tips from this forum so I thought I'd ask you folks :)

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Powdery Mildews are plant specific, and no part of the life cycle involves soil - they are always associated with plant tissue. The link below will give you some good info, but the treatment recommendations are:


Several practices will reduce or prevent powdery mildews. Many plants, such as roses, vegetables and Kentucky bluegrass, are developed to be resistant or tolerant to powdery mildew. Use resistant varieties whenever possible.

Once the disease becomes a problem:
� Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue (which is more susceptible to infection).
� Avoid overhead watering to help reduce the relative humidity or water in the early morning to let the tissue dry as soon as possible.
� Remove and destroy all infected plant parts (leaves, etc.). For infected vegetables and other annuals, remove as much of the plant and its debris in the fall. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus.
� Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.
� An alternative nontoxic control for mildew is baking soda combined with a lightweight horticultural oil. Researchers at the University of Rhode Island have confirmed that a combination of 1 tablespoon baking soda plus 2.5 tablespoons oil in 1 gallon of water is effective against powdery mildew on roses. Use of this combination on other crops is still experimental.

Here is a link that might be useful: Powdery problems

    Bookmark   February 18, 2012 at 4:49AM
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Just to reinforce what Bil has already written, while Powdery Mildew does have its optomal growth when the weather is warm and humid that does not mean it will not grow at other times and under other circumstances.
I have had good success using a 50/50 mix of fat free milk and water to control PM

Here is a link that might be useful: Powdery Mildew

    Bookmark   February 18, 2012 at 6:22AM
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How well does the 2.5 tblspoons of oil, mix in the water?

    Bookmark   February 18, 2012 at 11:28AM
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Not well at all, but that makes it stick to the plant. You need a nozzle on your sprayer that won't clog up with the baking soda, and shake it like Helga.

    Bookmark   February 18, 2012 at 1:35PM
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feijoas(New Zealand)

I assume you're talking about last season? Even with the best conditions, cucurbits generally succumb at the end of the season. By then, I'm so tired of zucchini it comes as a great relief!
I find keeping cucurbits very well watered and mulched in very well-composted soil holds off pm.
I'm pretty sure, despite what you'd think about fungi thriving in high humidity, pm tends to take hold when plants are weakend by dry conditions.
I'm spraying my cucurbits with foliar fertilisers and they still look really healthy, with no pm. Usually by now it would be showing up.

    Bookmark   February 18, 2012 at 7:20PM
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feijoas, I have talked with a number of people, over the years, about Powdery Mildew and it is one difficult to control plant disease. Some of these people, myself included, seem to have some success controling PM by periodically hosing susceptible plants down. That seems to wash off any of the PM spores. I realize this is contrary to what many advise on the thought that hosing tghe leaves down would increase the humidity around them and therefore make conditions for PM better. But if washing your hands helps prevent disease transmission for you why would not the same thing, kind of, not do the same for your plants?

    Bookmark   February 19, 2012 at 6:45AM
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gardenlen(s/e qld aust)

g'day SV,

for me i get powdery mildew mostly only when the plant is coming to the end of its productive life, at that stage i see it as the plant is running short of resources, like old age in human's.

we have a milk recipe on our remedies page:

also i plant PM susceptible plants in well drained soil and in a breezy position lots of air movement to keep foliage dry, never water over the foliage just around the roots.

when we get PM it starts with the oldest leaves but eventually brings about the finish of the plant.


Here is a link that might be useful: lens remedies page

    Bookmark   February 19, 2012 at 2:45PM
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I agree. Even with plants that are relatively close, the older ones seem vulnerable while the younger ones are still impervious.

    Bookmark   February 19, 2012 at 3:42PM
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I have read that PM survives in the soil. I live in southern Oregon on the coast, lots of fog. I have used the baking soda and water and am having great luck with that on my zucchini plants. I have a small garden but I do rotate the "crops" each year. How do I make sure the PM is no longer in the soil?

    Bookmark   August 2, 2014 at 7:54PM
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The spores that cause Powdery Mildew need plant debris to exist, so they may live on any plant debris in the soil but they will not survive in just soil.

Here is a link that might be useful: About Powdery Mildew

    Bookmark   August 3, 2014 at 6:03AM
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I have a Banana plant I did not want to lose to powdery mildew... I saw that it had started to grow on the soil (visible area). At first sight of infestation, I took a 32oz. spray bottle and filled it with reverse osmosis water, added 5-10 drops each of pure purification oil, pure lemon oil, and pure frankincense oil. Shake vigorously before each application, inverting the bottle, etc. All oils were from Young Living essential oils, only because my wife had them around and they smell wonderful. I misted the entire plant (foliage and all), I also soaked (via mist) the entire visible sphagnum peat surface area of the pot. Problem solved, 2-3 days later, no visible trace of the powdery mildew. I have continued the tincture for two to three weeks without any visible return of powdery mildew. The plant is looking vigorous and healthy. In the future I will also add 5-10 drops of pure oil of oregano and pure cinnamon oil to the mix, as they are known to be anti-fungal and anti-microbial agents. Their use dates back to the time of Hippocrates.

The purification oil consists of the following...Citronella (Cymbopogon nardus), lemongrass (Cymbopogon flexuosus), rosemary (Rosmarinus officinalis), Melaleuca (Melaleuca alternifolia) lavandin (Lavandula x hybrida), and myrtle (Myrtus communis).

Anyway, I hope my little experiment helps you all. I keep my banana plant indoors, so there was no rain to wash off the water/oil mixture. I would imagine this would make a huge difference into the efficacy of the treatment. All the best!


    Bookmark   October 6, 2014 at 10:30PM
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jean001a(Portland OR 7b)


Hope you check back to learn that powdery mildew does *not* grow on soil.

    Bookmark   October 7, 2014 at 1:34AM
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