Having seen a number of posts and enquiries on forum Q&A’s about mildew I thought I would tell you my way.
Milk!
It’s nothing new, it’s been used effectively for over fifty years now. It works really well on all trees. It will prevent an attack on susceptible trees such as acer campestre or hornbeams if done at the bud break stage. So if you know when mildew is likely to strike use the formula below to prevent it, or generally at bud break (as I do). It doesn’t cure mildew but it knocks it back, prevents it spreading further and lets the leaves photosynthesise again.
Mix about 30% milk with water and spray it on the leaves / needles ensuring you do both sides.
It is best sprayed in the morning of a sunny day so it dries really fast as this helps the proteins go to work and forms a seal over the mildew. If it rains within two days you need to spray again plus a second and maybe a third dose may be needed if the mildew is really bad. I found this to be better than any chemical sprays on the market.
The only downside is on spray days it tends to attract all the neighbourhood cats!!
I use the following mix, it seems to have worked for me time and time again.
In 1 litre of water add :
1 1/2 tablespoons of baking soda
1 tablespoon of Neem oil
2 tablespoons of Olive oil black soap.
This is the mix i used on my Devilwood. There is a post on that.
Hope you are all doing well.
I was thinking of using baking soda but I think you may be using too much in your formula and risk damaging your trees long term or maybe even causing burning to the leaves.
This is the formula most professional gardeners use:
A solution of 1 teaspoon (5 mL.) baking soda to 1 gallon a (4 L.) of water reduces instances of leaf burn. Add 1 teaspoon (5 mL.) dormant oil and ½ teaspoon (2.5 mL.) of dish soap or horticultural soap as a surfactant to help the mixture stick. Keep in mind the solution is water soluble, so apply on a dry cloudy day for best results.
I find milk works quicker and is better. It can also be used on conifers at no detriment to the tree whereas because you have quite a bit of oil in yours I’m not sure it would be good for them. Plus I always have milk available so for me it’s really convenient.
I’m not saying you’re wrong, only that I prefer milk…
Are you going whole fat, skim or something else? 
Hi Evan, it actually doesn’t matter as it’s the protein in the milk that’s the active ingredient. As full fat, semi skimmed and skimmed all have the same protein value it doesn’t matter which you use. I use semi skimmed as that’s what we drink.
I think it’s also important to look at why powdery mildew is prevelant in the first place. Can come from a looot of factors including health of the tree but most likely from high moisture content combined with low quality air-flow or “stagnant air.” If you just treat the symptoms and don’t look at the cause you’ll be setting yourself up for a lot more work in the future. PM can spread like wildfire (in wetter seasons) so it is extremely important to deal with it fully.
Hi Luke, and welcome to the fun. I agree with you to get the best growing conditions for the tree. However, some trees are more prone to PM than others, field maples, English Oak, blackthorn to name a few. The seasons seem to have skewed over the last decade. Whereas it used to be June / July that PM would strike due to high humidity, last couple of years it was September. It normally occurs at roughly the same time as blight on potatoes (in UK).
By using milk at bud break then at partial defoliation time I have managed to prevent PM on my trees this year all bar one which I partially defoliated but then missed the treatment. Now it is showing signs of PM. I will spray with milk again which will eradicate it - hopefully for the rest of the year.