Hi

I live in Bangalore and trying to grow some potatoes and tomatoes in peat soil and compost in containers

i have come across patches on leaves as in the picture

the plants are getting enough sun 2 hrs a day so not sure whats causing this. i am adding compost every week once on top .This is a common observation on potato and tomato leaves

some videos suggest it could be magnesium deficit or ph problem but i am a bit hesitant before applying a cure

can one of you understand what could be the cause and how to fix

Thanks

Sujeet