Could you advise if these spots are normal or they're signs of a serious disease?
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I suspect that if the rest of the plant is looking healthy, this might be a case of just cutting this leaf off and monitoring the plant. I notice that you have quite a large quantity of Dynamic lifter spread over the surface of the pot. How often are you fertilising like this, and could you please tell us about your watering regime?
Thanks Mitchell and Noelle for your helpful advice.
I spread two handful of Dynamic lifter every 6 weeks and water it once a week. Was it the receipe for killing the plant?!
It might not be a recipe for killing the plant @Shima, but you could reduce the amount of fertilizer you apply. I'd recommend one handful every two months. You'll want to up that watering as well to at least two times a week and make sure they are good five-minute waterings, so the soil is totally saturated.
Agree with everyone's comments but would add... look like the leaves have also been eaten by some sort of leaf skeletoniser - a small caterpillar of some sort.