After many weeks of testing, I'm here to offer my endorsement of 0.3% canola oil spray for powdery mildew (PM). And to break it all down for you, here is the Thursday update (after you've congratulated
@Grand Daddy Black for his triple OTM win last night...I'll wait!).
Text, then pictorial:
As I mentioned back in March, I discovered a South Korean study from 2008 and seems well-researched and documented, and on the plus side, it's written in mostly non-technical language, dealing in percentages and ml rather than molecular weights and such:
https://circadiancropsciences.com/wp-content/uploads/2020/04/PM-and-DM-Control-Cooking-Oil.pdf
My post on it
is here in case you missed it. Might be worth your time if you get PM.
[BACKGROUND: Normally I use neem oil in veg for PM, but that leaves me searching for another solution in flower. I had tried milk and H2O2 (neither of which were effective for me), citric acid (which worked but did some leaf and pistil damage), and potassium bicarbonate (which I found created more damage and seemed less effective than citric acid). Also, neem isn't even available for plants in Canada or the UK, which makes canola oil a viable alternative during all stages of growth for those members of the Commonwealth.
]
The testing process I used started by letting PM grow on a couple of my Candida plants to use as test subjects. Once I had enough of the plants covered in PM I sprayed the mother with the canola oil mix and let the other one go. I kept them mostly in the shade during this time to prevent the heat or sun from affecting the PM growth, and I sprayed every 5 days as listed in the study.
Because I had let the leaves get so covered with PM I ended up having to spray every 3 days (to runoff) until it was gone, but it definitely worked. And after it was gone, every 5 days seemed to keep it away.
Once I was satisfied with that I needed to wait for some pistils to try it out on to see what it would do there.
Here is the flower I chose to spray with the 0.3% canola oil mix:
I sprayed it and made sure it was shaded until it dried.
Here is the same flower 72 hours later
Now, you are correct if you see some browning of the ends of the pistils, but here is a nearby flower that
didn't get sprayed:
And an unsprayed top flower:
So you can see that the pistils are starting to brown across the plant rather than specifically from the canola oil. I will spray it again this weekend to see if every 5 days accelerates the browning effect. But either way, a little browning of the pistils seems like a reasonable trade-off for the treatment of PM on your flowering plants.
Also, spraying almost anything on the flowers will tend to brown the pistils somewhat, so if you have to spray, spray something that works!
All that said, here is how I went about making my 0.3% canola oil spray. Keep in mind that the beauty of the metric system is that it's easily scaleable.
500ml of distilled water:
I don't trust any single set of measuring lines so I made sure that the 500ml mark on the spray bottle was the same as the .5L line on the blender:
After that I needed the canola oil:
But really hardly any, so a small syringe is needed to get 1.5ml:
Here is the canola oil in water:
Oil and water don't stay mixed for long, even with a blender, so I used Dr Bronner's unscented castile soap as an emulsifier:
It took only 2 drops using this pipette I had on hand, though you could also use the same syringe from the oil:
After running the blender for a minute, here is the result:
I poured it into the spray bottle and sprayed down the plant in the shade.
Even though it's blended and emulsified, I still do a lot of shaking of the bottle to make sure before and in between sprays.
I'm now making 1L batches since that's as much as my blender will hold (3ml oil, 4 drops Dr B's), and if I end up with a lot of PM this fall I'll be doing four 1L batches to fill my gallon sprayer.
That's it from me, and I hope this helps someone out with their PM problem!
. I was wondering about the copper thing. The lady at the garden store said it should be ok because she read the flower thing on the instructions.
