There are a lot of dead small branches in the tree and small fruit.
Certified GKH Gardening Expert
Multiple citrus diseases can cause dieback of twigs in a lemon tree along with poor fruit production, including Phytophthora or Anthracnose infection or Bacterial Blast. Please see the following articles for help with diagnosis:
Because diseases can vary by the region, it is best to also ask a local extension agent or other expert.