Hi all - northern california here, growing cymbidiums outdoors, I have about 30 of them. three of them in particular had multiple highly yellow leaves - though lots of new growth - and I just looked into the media and there is while mildewy moldy growth on the roots and on some of the media. Interestingly it's been hot here and these weren't watered much this summer, so I don't think this is overwatering.

Also, I think these three plants have had this problem since i got them a few years ago - call me stubborn or dense, I never saw what was going on below the surface of the medium before.

I'm wonder if it is almost certainly a mold, or if it could be an insect/powderly mildew. The fact that it's on the bark and not only the media makes me think, mold. AND - most importantly, now that I've separated these plants from the rest - if anyone has experience successfully and conclusively curing something like this. Because I may decide it's simpler to dump these particular plants than spend weeks trying to treat this with small chance of success - or putting them back with others, thinking the problem is over when it's not. Though to be fair I think this has had ample opportunity to spread and hasn't, that I know of.

I pot in a custom mix of perlite, redwood shredding, fir bark, and red volcanic rock. Fertilize with osmocote and irregular applications of GrowMore Blue and Pink, water 1x/week in dry season.

Thanks!