Is this a disease?
 

Hi, everyone. My first post here. I have a 30 year old Dendrobium (I think) and it has developed speckled leaves. Could you tell me if it is a disease or just from the sun please?

https://i.imgur.com/6A4qDVk.jpg
https://i.imgur.com/R6jYYha.jpg

Welcome to the Orchid Board!

I don't think that's sun. It could be spider mite damage. They are active when it's warm and dry. Take a dry tissue and run it under the leaves from stem to tip. If there are brown or red streaks there are spider mites.

Thank you for your welcome and reply. No, there are no red streaks and no webbing.

To add to ES'es comment, it might also be "false spider mites", a related pest which looks the same but does not produce webs. Adult true spider mites are a bit bigger and can be just seen with the naked eye in good light, false spider mites are small enough you cant see them without a magnifying glass or pocket microscope. Both are a serious issue and can do a lot of damage to a plant in a very short amount of time if the conditions are right. True spider mites are more mobile.

If it is mites use a cloth or wet wipe (test it first) to wipe the leaves down one by one vigorously. You want to use mechanical action of wiping to kill and dislodge as many of the critters and their eggs as possible. Then follow up with a vigorous spray down with as high pressure as you can get. Get the plant wet and clean. Mites like dry dusty environments. Then when it dries treat it with a pesticide formulated for mites, check the label, not all pesticide is appropriate as mites are arachnids, not insects. Some folks use a mineral-oil/soap spray instead. I didn't find it worked well as branded pesticides. You will need to repeat the treatment at least 3 times over 3 weeks.

It goes without saying that you should isolate your plant, and check all the plants that were near it for infection too.

---------- Post added at 08:33 PM ---------- Previous post was at 08:21 PM ----------

False spider mites don't produce webbing, and if the density of mites isn't high right now then it wont show a red streak. Get yourself a magnifying glass or pocket microscope. Currency checking microscopes are super cheap online and perfect for plants. Some mobile phones are capable of very high power zoom, you might see something if you look with your camera very closely. Look for red dots that move. Maybe red dots with a black dot on them.

Thank you I will give it a go. I haven't had any red streaks on the tissue so far.

---------- Post added at 03:19 PM ---------- Previous post was at 03:16 PM ----------

Where would the mites have come from? My orchid has never been outside and I haven't had any new plants for 2 years. And none of my other orchids on the same window have any signs of it. They have all been there for 2 years.

Gosh i wish i knew. :-(

I guess that occasionally they fall off a plant and are carried on the wind. Since they often reproduce parenthetically a single mite can turn into a horde in a short period.

Most leaf spots that look like that come from a fungus such as Guignardia/Phyllosticta, Septoria. and/or Cercosporoids. First picture, looks like Guignardia/Phyllosticta to me. Pretty common in Dens that have gotten too wet plus lack of air movement.

Here's some into on treating it from St. Augustine OS, along with a bunch of others. Good pictures and good info. Scroll down to the section with above-mentioned group of fungus and treatment recommendations. Orchid Diseases

Thank you. I do think this may be the cause. I don't ever spray the leaves with water and only water directly on to the planting medium. It must be due to lack of airflow. Do you know what the names of any fungicide that is available in the UK? Failing that any names that I can look for?

I personally don't, specific to UK. Or just google the ones from St Augustine and see the active ingredients. Or google fungicides for orchids. There are many that work.

Everything I have tried is currently unavailable, we seem to be having supply issues here at the moment. :(