This one is kinda tough. I know that RO water will cause a deficiency of all micronutrients not in the potting mix. Once it shows, it is too late. I frequently had that problem when using RO and even though I bolstered with cal/mag the damage was never reversible. IT only seems to get the big leaves and the tips of new growth.

BUT....Rust fungus sounds terrible and I am not positive what it looks like, though similar to what you show. Again, by stopping RO water, just using tap water that set for 24 hours, has eliminated the problem and my temps are lower and humidity higher than most folks this time of year.

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WashougalWonder Reviewed by WashougalWonder on . Mg deficiency or Rust fungi!! Alright lets start by saying i started noticing this on one of my plants about 2 weeks ago. At the time i was give a full dose of FF Grow Big and Big Bloom. I water with R/O water and everything is always buffered to 6.5-6.8 pH. Runoff is 6.2-6.5 pH. Last weekend I up potted everything to 3 gal pots using FFOF with 20% perlite added. Tuesday i noticed that it had moved from just the one plant now to almost all. At first i thought it was a Mg def. so I added Cal-Mag 3 days ago, but now Rating: 5