Yellow and see-through spots on oncidium and bratonia leaves

[CJ Green]

I would love some help diagnosing some spots! I have three Oncidiums and a Bratonia together on a windowsill, and just recently noticed spots on their leaves. I initially thought the spots on the Oncidiums were light yellow, but on looking more closely I realized they're translucent, so they appear light when the light is behind them, but otherwise look dark. On the Bratonia, the spots really are yellow and extend down into the bulb area, mostly on new growth.

I tried searching around through different threads on this forum but didn't see anything that exactly matches, although a recent thread started by orchidgirl610 looks like a similar problem. I saw suggestions that it could be a nutrient deficiency or something to do with the water. I have a lot of orchids and use the same water for all of them, though, and this is the only group with the markings. I've also wiped the backs of the leaves and don't see any signs of mites. I can't find a good match when looking at pictures of various fungi or viruses on orchid sites, either, so I'm stumped. Also, their environment this past winter has been a bit on the cool, dry side, with no direct sunlight--which I know is not ideal.

Any suggestions?

[Ray]

Two possibilities come to mind (and either or both could be wrong...)

►Droplets of cold water hitting the leaves (small regions of mesophyll collapse) - damage is done; don't sweat it further.

►Guignardia/phyllosticta - two stages of the same fungal disease - treat with a systemic fungicide or a probiotic like Inocucor Garden Solution.

[CJ Green]

Thank you, Ray. I think both of those are good possibilities. I do sometimes mist these plants and the water in the spray bottle might be pretty cold, although don't know how cold it would have to be to cause damage. And I did see some photos of phyllosticta-affected plants that look similar.

[Ray]

Yeah, it's hard to judge what water temperature will cause issues. I believe it is the delta-T that's the factor - the warmer the leaves, the warmer the watershoudl be.

Stop misting the plants. It is an inefficient way to water, and if the intention is raising the humidity, you need to mist the air, not the plants.

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[CJ Green]

I mist to raise the humidity but yes, it probably does more for me than the plants.