Can Hostas be planted under fruit trees?
I planted Hostas under my pear tree and apple tree. After 3 years the leaves on the pear tree are turning black.
I've never heard of hostas having a bad interaction with pear. The most likely cause is sooty mold. If you can wash/rub the black off the leaves, that's what you've got. It is a mold that grows on the sticky exudate from aphids and scale insects. You can control them with simple spray. Read on: https://www.gardeningknowhow.com/plant-problems/pests/insects/aphids-tree-disease.htm
If sooty mold is not the problem, your tree might have a bacterial or fungal disease. To find out what it is, take some leaf and twig samples to the Extension Service for analysis and recommendation. This link will help you locate one: https://www.gardeningknowhow.com/extension-search/