I don't think Bt insecticide actually has active bacteria in it. Bt strains produce proteinaceous inclusions that do the insecticidal work. That's not to say that fungicides don't interfere with it, just that they probably aren't killing anything useful. FWIW, many Bt insecticides actually do show antifungal action.

thanks dan. Yeah, I thought I read that somewhere before.

Interesting regarding the bt being antifungal. Do you know which ones?

FWIW, I see here

http://pmep.cce.cornell.edu/profiles/extoxnet/24d-captan/bt-ext.html

that, about Bt, "Formulated products are compatible with most insecticides,
acaricides, fungicides and plant growth regulators; they are not compatible
with captafol, dinocap, alkaline sprays or, under some conditions, leaf or
foliar nutrient."

Now, potassium bicarbonate is mildly alkaline.

Re fungicide activity, this is the only thing I could find about Bt

https://www.ncbi.nlm.nih.gov/pubmed/18167458

But that doesn't help much because there are different Bt strains.

"Now, potassium bicarbonate is mildly alkaline."

Right. It's little things like that that make me have to play spraying roulette between spraying bt, then waiting a few days later to spray the potassium bicarbonate. But then the temps skyrocket, so I have to hold off on spraying. Next thing I know, powdery mildew starts, so I have to spray. Meanwhile, the rotation of the bt gets thrown off. So....between having to spray for PM when the temps are elevated and infrequent bt applications, I end up with chewed and burnt plants.

Ahh...the plights of summer gardening. I have no idea how people deal with it in much hotter parts of the country.

It's a little odd, because it turns out the pH of an insect gut is HIGHLY alkaline (as in, pH~10 or so). Very different than us, for which are own guts are HIGHLY acidic (pH~2 or so). Turns out that the Bt toxin needs that high pH in order to do its stuff. The Bt toxin has a rather narrow range of pH in which it'll work, and that range is on the high side. It may be that high pH activates the Bt proteins, and you don't want that to happen outside the insect gut. That being said, the pH you can make with potassium bicarbonate isn't that high. So I'm guessing it won't interfere with Bt action.

I should add that that fact that vertebrate guts are highly acidic is one of the main reasons that Bt is nontoxic to people. In fact, it is rapidly deactivated in acidic conditions. So if you have alkaline soil, it may sit around there in an active state for a while.

That "sounds" all well and good, but there must be something else at work and chemistry wasn't my favorite. The way you put it, I should just be able to spray my plants with just plain water(municipal of 8.1) and I would never get PM. But yeah, I'm not going to worry too much about it. I've never had any problems with Bt NOT working.

I don't think anyone said that it was high pH that kills or suppresses PM. While the mode of action is not well understood, research shows that bicarbonates seem to damage the cell wall membrane in PM spores. They are contact fungicides. Potassium works somewhat better than sodium, for some reason. pH may have a role, but it's not the dominant one.

The issue was whether the pH caused by bicarbonates in solution was high enough to damage Bt. I'm suggesting that it isn't. BTW, my soil and water supply are alkaline because of limestone (calcium carbonate). No bicarbonates in my juice. That being said, there is some work that has been done suggesting that carbonates (as opposed to bicarbonates) have some antifungal activity. If true, my soil and irrigation water should be antifungal. Nice! But I'll bet standard sprays have a lot more. Again, pH is not a measure of carbonate/bicarbonate concentration.

Actually, that's why I thought we were talking about pH. From my readings, I was under the impression that some of the fungicides out there relied on changing the pH of the surface of the plant to make it an inhospitable environment for the PM spores.

Much why I asked about the use of bt and potassium bicarbonate.

Kevin

Whether or not pH has the dominant role in suppressing PM, I think we've concluded that the pH of potassium bicarbonate is unlikely to be high enough to disable the Bt. Potassium bicarb buffers to pH of around 8, and that pH sure doesn't inactivate the Bt.