My Lisbon Lemon Tree has developed brown worn-looking spots on some of the leaves. It also has many blooms and small fruit that fail to mature. The fruit falls off after reaching approximately dime size. The tree is planted in a very large container in my front courtyard. I use a fertilizer designed for citrus trees. The tree does not look robust now.
Lisbon Lemon Tree Growing in a Large Container.
Certified GKH Gardening Expert
It's perfectly natural for a juvenile lemon tree to drop most of its fruit in the first few years. It's best to actually remove most of those little fruit, to allow the tree to concentrate its energy on growing nice strong roots. This article has more information: https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-fruit-drop.htm