Tiny spots on Cattleya leaves

[ArronOB]

I found some tiny black spots on the leaves of several of my cattleya alliance hybrids today. Typically there is a group of a dozen or so spots on each infected leaf. Usually just one affected leaf per plant - which is the new growth. Mostly they are concentrated more towards the leaf junction, or leaf tip.

The infected plants were growing near to each other, although there were other apparently unaffected plants growing between them.

As you can see in the images below the spots are tiny, and maybe nothing to worry about, but if not id like to deal with them immediately. I've isolated the 6 infected plants by bringing them inside.

I've not seen this before.

A few cultural notes. We've had nights down to about 12 Celsius (53.6 f) lately, although the coldest period is fairly short around 6:00 am. I've also partially enclosed the shadehouse in clear plastic in preparation for winter. This has reduced airflow although its still fairly good as I've left the east-west open for now. There was an period of persistent rain a week or so ago, although these spots have only just appeared so I suspect theyre not related. Otherwise they seem to be very healthy plants, a little Biosduval scale now and then which I deal with.

Any advice appreciated.

cheers
Arron

ps. I'm not 100% sure the spots in second image are related. They may be preexisting. Nonetheless, they worry me too.