Lemon trees are vulnerable to several diseases. Huanglongbing (citrus greening disease) is caused by a bacteria and produces blotching and spotting on the leaves. Although it sounds like the leaves turn green, they actually develop yellow marks in the early stages of infection. As the disease progresses, brown blotches appear. However, the lemons that grow on infected trees are greener in colour towards the end furthest from where the fruit is attached to the tree. The oddly-named citrus stubborn disease is also bacterial and produces symptoms similar to those of huanglongbing: blotchy yellow areas on the leaves and misshapen fruit. Viral diseases that infect lemons include tristeza, exocortis and psorosis. Tristeza damages the stem, branches and leaves and can be fatal to lemon trees, while exocortis can lead to stunted growth but doesn’t typically kill the tree. If your tree is starting to lose its bark, has yellow spots on the leaves and has discoloured fruit, though, it might be suffering from psorosis.