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Looks like PM, I'm battling a case of it now. Mine built up at the bottom of the leaves as yours is doing.
Test I read online, don't know it's surety - drop of Iso alcohol on the spot, if it's PM it stays and doesn't get washed away.
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I've tried H2O2 with mixed results;
I'm doing a sulfur burn on one of my tents right now, the other tent tonight;
Dr Symes (I think) is recommended if you live in the US, no shipping to Canada;
I bought a UVC light from a company called CleanLight out of the NL. It claims to kill PM with a swipe of the light but I don't see any difference scoping before and after. Maybe PM doesn't fall apart when it's killed by UVC. Suppose to be good for killing spores.
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