I've been trying to find the solution for sev years :). and I look at undersides of leaves in bot gardens in Miami: I see the same spots too. they are outside all year.

it can be cultural: some mineral missing ...but I have not found out yet which one..

however I had another 2 philos next to pthe plant with spots and they were ok. so it's not something that crawls. I had it on philo Xanadu the worst, and it seemed to spread to Philo subincisum and then to Philo Gloriosum, both of which are fast growers with large leaves.

after 3 years of controlling it on Xanadu I got frustrated and chucked it.

I noticed that the oldest leaves are effected the worst, the youngest are clean for some time. I rooted the tips by wrapping them in moist sphag in hopes that they will not get it.

when I keep them tented and water minimally while they are very small they seem to be OK, but then it starts again slowly. I read that it can be bacterial and can only be controlled chemically. which is what nurseries do, but I don't want to use commercial products indoors for obvious reasons. I tried Azamax, which is a super concentrated Neem extract: It definitely reduces the spots to bearable, but you need to apply it again and again.

they recommend keeping the effected plants on a drier and cooler side when it's bacterial.

I read thru most common bacterial symptoms and none match. the plants grow vigorously and well despite the spotting.

When I tent the humidity rises and I can keep them drier and water less. and then it helps, there is less 'dew spots' on underleaf. but there is still some.

I tried spraying occasionally with soap and a bit of peroxide added: then they stop 'dewing' for like a 1-2 weeks..

I do get mites occasionally on subincisum, but it's different, though they can cause spots obviously and some dew too.

My Philo 'Painted Lady' and 'Jose Buono' are clean and they never are bothered by mites. but they grow much slower and have tougher leaves.

perhaps, some are not as susceptible to it? but if it's bacterial I would think that all of them

would be? My P. gloriousum was clean for years, but started getting this spots 6mo ago.

it's unsightly to say the least. I just hate it! That makes me think some mineral deficiency is causing it. I do feed them with fertilizer with trace minerals and with cal-mag supplement too.

Aroids like that. and I flush too, though I grow most plants on self-watering wicks in the well draining light 'tropical mix' that I mix myself (you can get more info from my other posts: too lengthy to go into here).

P. Xanadu outside in landscaping in Miami seem to all have it badly and P. subincisum too (the worst). But outside plants here are constantly under attack from various pests, even in botanical gardens.

If you manage to get more info, please , post :).