As long as it isn't brought to a water-boiling temperature (~100C), potassium bicarbonate is completely stable. Not sure about Neem, but I think you need to keep it out of direct sunlight.

I have it stored indoors in a cool place. I'm unsure how long I can keep it that way and it still work. Some store bought fungicides that are mixed say that the mixture needs to be thrown away after 24 hours but these aren't organic. Thanks for the feedback

Store bought fungicides start to break down when they are combined with water. They are liquid when you buy them because they are mixed with ethylene glycol, which is a cheap liquid that is not water and doesn't hurt the fungicide. That's why when you mix up some fungicide from concentrate, with water, it expires quickly. That, of course, has nothing to do with potassium bicarb.

Have you considered removing the affected plants?

I wonder if the soil is infected?

That's good to know. Thanks again for the guidance. I'll make sure it is properly stored.

@Jm Mat I'm using this more as a preventive. I have a myriad of squash and cucumbers this season and I find I have better results when I'm proactive when it comes to powdery mildew. I had a container of strawberries that showed some early signs so I disposed of the seemingly infected leaves and sprayed the plant and the rest of my veggies . Everything seems to be under control. I just wanted a little information to make sure my efforts were on the right track.

As I said, potassium bicarbonate is completely stable, unless it's at about 200F. Read below, please. That's what the OP was asking about. Potassium carbonate is stable at temperatures up to more than 1000F. So yes, one could say it is a bit more stable than Neem. I think potassium carbonate has been used as a fungicide, but research has shown that the bicarbonate actually works better.

I'm aware it's stable, but likely by itself in dry form.. but the OP is mixing it with several other things..will it still work? sure it might, but a solution stored with neem is going to stink..

i've been using potassium carbonate for years as a fungicide since Greencure started selling it with a wetting agent, so i have great experience with it. it is more effective than neem as a fungicide on already diseased plants.

Powdery mildew is one of the few fungi that reproduces when dry. Hose off your plants in the morning and you should notice an improvement. Powdery mildew IPM

Mixed in water, it's very stable. Now, of course, if you mix it with some acid, it will break down rapidly. Neem oil is somewhat acidic, so that isn't good. Soap, on the other hand, is quite alkaline. So ... it depends, also on the particular dilution. Now if you mix a bicarbonate in an acid solution, it isn't going to hang around long at all. We're talking minutes, not days.

The bottom line is that it's probably very unwise to combine bicarbonates with Neem.

"I wonder if the soil is infected?"

Not really a concern with powdery mildew. This pathogen requires live plant material to survive.

Such great info.

@stevie I was using the neem oil in the mixture to not only assist with the powdery mildew but as deterrent for some spider mites that had made their presence known. I was using 100% neem oil and since it is hydrophobic, a couple drops of soap as a wetting agent is necessary. The soap is also an assist for the potassium bicarbonate. I again use a general use bicarbonate so a wetting agent is required.

@daninthedirt (USDA 9a, HZ9, CentTX, Sunset z30, Cfa) the acidity of the neem oil makes sense so I will definitely do it as separate applications from now on.

Again great info from everyone. Thanks for all your help.