Any idea whats the cause?

what's the PPM and PH

 

If it was me. I would look under the leaves with a magnifying glass to see if you got bugs.

 

Looks like it could be a cry for calcium. Possibly.

 

Looks like "Leaf Septoria" which would be a fungus.

EDIT:

After looking up Calcium Deficiency in Cannabis Leaf that appears to be closer resemblance to your issue.

 

Buy a cheap PPM and PH meter from amazon or ebay. They run 10 bucks each.

Do you spray your leaves with something?

 

Ok flush 3 times with water (no nutes) as amount of dirt that is PH'ed to about 6.7
If you got a gallon pot, flush with 3 gallons of water, 5 gallon pot 15 gallons of water.
And after the last water goes in, catch the last part of the run off.
Use this to test PH and PPM with those meters you need to order.

 

6.0 in dirt is Cal-Mag lockout, study the chart above

 

Is this chart Universal for all/majority of plants?

 

It's for weed.
Other plants like acid or alkaline

 

Mine are showing similar symptoms, so I did the flush that HardDrive suggested (my ph was way low 5.8), after the flush it is 6.54. I did this 7/17, so waiting for things to dry out, but new growth looks real good. Mine are in week 2 of flower

 

P.S. My ppms were only 340 after flush