Ficus tree

Question

My Ficus tree has started to have lots of little black straight lines on some of the leaves.

Answer 1

Leaf Spot fungus can leave black spots on the leaves of the plant.
The black lines that you describe could also be a build up of salts in the soil.
This can happen from watering with conditioned water.

Here are some links with more information.

https://www.gardeningknowhow.com/houseplants/hpgen/leaching-indoor-plants.htm
https://www.gardeningknowhow.com/plant-problems/disease/plant-leaf-spots.htm
https://www.gardeningknowhow.com/houseplants/ficus/ficus-tree-care.htm