Lemon trees are bright, valuable additions to home gardens, and bring with them the deep green foliage, sweet blooms and heady scents of a citrus tree. Like any citrus trees, though, lemons have some extensive growing needs. Ignoring these needs may lead to a range of problems, including yellow leaves.
According to Aggie Horticulture, lemons are the most cold sensitive of the citrus trees, and require consistent warm temperatures. They cannot tolerate frosts or temperatures under 30 degrees Fahrenheit, and experience leaf yellowing and death in cold areas.
All fruit trees require full sunlight for growth and fruit production. Lemon trees that don't receive enough light--natural or artificial--will display yellow leaves as photosynthesis fails and the chlorophyll dies.
Water and Drainage
Lemon trees require rich, quick-draining soil to keep their roots from rotting. At the same time, they need frequent waterings, with a supply of 2 to 3 inches of water a week. Lack of water, or muddy conditions from too much water, cause leaf and wood dieback.
The best way to troubleshoot yellow leaves on lemon trees is to fix the problem. If leaves are yellowing, take stock of the lemon's growing situation, compare it to an ideal situation, and do what you can to amend the growing conditions.
Prune away any yellowed, dead or damaged leaves if they don't recover immediately. Although many of these leaves may recover, they will sap the tree's energy and keep it from growing. Cut the damaged leaves off to allow the tree to put its resources into new, healthy growth under its new, ideal conditions.