|Why are our lemons rotting on the tree? The tree is ten years old and this has never happened before. There are no sign of insect bites.|
|Citrus trees are extremely sensitive to frost. Is it possible the temperature dipped into the 30's? Citrus trees are also shallow-rooted and can benefit from 3-4 inches of mulch to retain moisture and help protect the roots.
If the lemons are small-sized, the flowers may not have been pollinated and the fruit will drop off.
If the lemons are mature-sized, they may have been affected by frost, or they may be over-ripe. Harvest when the green tint has just disappeared.
Since you find no signs of insects, and if the tree appears healthy, then the only other explaination is cultural. I'd harvest the lemons and destroy them. Then treat the tree to plenty of moisture, lots of sunshine, and some low-nitrogen, high phosphorus, high potassium fertilizer in the fall. A 5-10-10 or 10-20-20 commercial fertilizer will help your tree grow and bloom.