It looks to me like some kind of pest or virus. I've never seen damage like that, but if it's only affecting new growth in multiple plants, it's contagious, so I would move them away from all of your healthy plants. You'll have to google to see if and how you can treat this ailment.

The only other thing I can think that might cause it is perhaps if you're getting water in the crown when you water it? That can lead to leaf damage or rot.

Thanks for your comment and advise, dannie 317. That is also what I thought. I spent quite time searching in google for a solution on treatment, but no success. If anyone knows of a good website showing the whole range of orchid diseases (and treatments) it would be much appreciated. Thanks in advance!

It would help us help you if you gave more information. Where are you growing these plants? What zone, Country, State, etc.

How much light and what type are they getting? Indoors/outdoors. How often do you water, fertilize and how are you doing it? Are you spraying these plants?

From the photos you provided, it appears the plant is potted too deep. It should be raised so the crown is above the rim of the pot.

I can't guess what is happening without more info.

Jane

Hello Jane. Lest see if with a bit more information, anyone can find out what it is. The plants are in a flat in Budapest, Hungary.

How do you water? I submerge the pots in water up to just below the rim of the pot

How often do you water? Do you water on a schedule? Once per week (Sundays)

What's your water quality like? Do you use water softeners? What's your water source? The water is from the tap. I believe water in Budapest is quite good, but no idea about its quality/composition/pH. I don't use any water softener.

How do you feed? What kind of fertilizer do you use? What's your feeding rate? Guano fertilizer (liquid) once every two weeks

Tell us about your growing temperatures. Do they fluctuate through the year? Do they fluctuate through the day? Yes, they fluctuate all year, but not greatly, as they are in the living room, so we try to keep a pleasant temperature for ourselves. The won't go under 15 degrees in winter and not over 28 in Summer.

Do you use a fan or other means to increase air flow? No

What's the humidity level? Does it fluctuate through the day/month/year? No idea about the humidity level. According to google, the air humidity is much higher in summer than in winter. The annual average water vapour content is 7.4 g/m3 (0.0074 oz/cu ft) at a hydrostatic pressure of 7.3 millimetres (0.29 in). This is obviously outside, so I guess it won't translate precisely to the same values inside.

What's your potting mix? The one that came with the plants when we bought them about 1-1.5 years ago

How often do you repot? When did you last repot? I haven't reppoted yet.

Is the potting mix in good shape? Looks like

Does this issue with the leaves happen over a long time period? Did it just start happening? How long has it been going on? I started noticing about 3 months ago, maybe longer

Do you spray for pests? Do you spray for disease? If so, what pests / diseases? What chemicals do you use? How often do you treat? No treatment yet. I'm trying to find out first what the problem may be

What are the light levels like? What light sources? Can you provide specific light readings? Do these change through the year? Does that correlate with the issues you're having? Natural light (plenty and not direct), which changes throughout the year due to the season.

Are there kids or pets near your plants? No

Do the plants go outside, ever? If so when? No

How long have you had these plants? 1-1.5 years

Where did you get them from? A big shop for gardening (OBI)

Are you doing anything special to the crown (center of the plant) where the leaves emerge? No

Wow! Great answers. I have no idea what the climate is like in Hungary but it sounds similar to NY where I lived most of my life.

This is what I would do. Repot the plants into new mix. Pot them higher than they are now so they get good circulation around the leaves. If you can put them outside in a shady spot that would be great.

Do not spritz them with water. It gets into the crown and can cause problems. Only water when the mix is almost totally dry. Stick your finger into the mix and feel how damp it is. If is is damp do not water. The roots need to get air.

These plants grow naturally attached to trees with lots of air movement around them. The roots are not planted in dirt but attached to the tree.

If you can't put them out, maybe an open window with some air. But most important, repot them and raise them in the pot. Do not overpot. Judge the size of the root ball when you pull them out of the pot. Only put them in a pot about 1 inch larger.

In the Fall, they will need a temperature drop to initiate spiking. If you have a cooler spot in your house, put them there at the end of summer. They need at least a 10-15 degree drop in temp during the night. If you don't do that you won't get them to bloom. They need that drop in temperature.

I'm guessing you have some sort of fungal issue with those leaves. You could try a little alcohol on the edges and down into the crown, but my guess is they will improve with a repot into fresh bark. Use bark mixed with some sphag. That will work best for your environment.

Jane