If you're noticing brown leaves on your lilies, it's likely due to a few different reasons. First, lilies are susceptible to a number of different diseases, including fungal diseases like botrytis and rust. These diseases can cause the leaves of your plant to turn brown and eventually drop off. Secondly, lilies are also susceptible to environmental stresses, like too much sun or wind exposure. This can cause the tips of the leaves to turn brown and dry out. Finally, lilies can be affected by pests like aphids and thrips, which can also cause the leaves to turn brown. If you're not sure what's causing the problem, it's best to consult with a local nursery or gardening center for help diagnosing the issue.