White spots on my licorice mint.

[Ferry]

Hello,

Can someone please tell me what the white spots on the leaves of my licorice plant are?

I have read some topics on powdery mildew but i don't think that this is the case. Where powdery mildew can easily be wiped of the plant leaves, these spots can't be wiped off. Is it possible that this is a deficiency? I had a major magnesium shortage 2 weeks ago. I have put in magnesium sulfate in my fishtanks and now two weeks later most of the tip burn and interveinal chlorosis is gone but got this in return. I also have peppermint and the larger shoots are all white due to these white spots while the shorter shoots are green and no spots are to be found. I also have Maroccan mint and they don't have any spots on them. They all grow in the same media bed. I know powdery mildew is contagious. I also have a wickerbed sytem and the photo's are from mint in this wickerbed. The leaves in my wickerbed are a littlebit lighter then the mint in my aquaponics system. I also have micro cucumber and honeytomatos in the same wickerbed and they doesn't have these spots on them.

I have a very high humidity in my greenhouse. 80%-100% all day round. For me it is hard to ventilate because the outside temperature is below 14 degrees celsius by day and 6-7 degrees celsius by night. Because my heating sytem isn't ready yet i temporarily heat my greenhouse with an electrical heater. if i ventilate, the temperature won't maintain and i have trouble to raise it again to 20 degrees celsius. during the day the temperature is between 20-30 degrees celsius and by night it is 17-20 degrees celsius.

If i don't find an answer to this soon, i have to stop growing during winter and have to wait till next year april. Licorice mint can't be sold with these white spots on them.

Does anyone have a suggestion?

Thanks in advance

Ferry