Hi, I’m growing this plant. But now she have this problem on her leaves.
the soil is common and I have never used anything to grow them.
porrebbe essere un parassita o una muffa o una mancanza di potassio?
Can you help me?
Welcome blondi.
When watering the plants, did you by chance splash any water on those leaves?
mudge
Looks like leaf hopper trails to me, but hell, I’m on my first cup of coffee and bong hits and not functioning correctly yet or if at all 
Lets wait for others to chime in besides what @mudge and myself had to say
You can however rule out mold or lack of potassium
@blondie welcome to the network
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Looks like Thrips to me. Mix 1.5 tsp baking soda with a gallon of water and spray the leaves top and bottom three times a week for two weeks. Hope this helps!
Cheers
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Most thrip damage I have ever seen usually leave tiny black dots of excrement on leaves, this don’t look like thrip damage to me, but I could be wrong
It does look like a trail. No idea, though. Would a look on the bottom of the leaf yield a culprit? Maybe with a microscope?