Hi, why does my lemon tree have a black spot that goes deep into the fruit? Thanks.

Hi Erin, it sounds like your fruit might be infected by guava moth larvae (caterpillar). When you cut the fruit in half it will be rotten inside, this is guava moth. 

The adult moth lays eggs on the outside of fruit and when the larvae hatch, they burrow into the fruit, which makes control very difficult. Contact insecticides are not effective as the larvae are on the outside of the fruit for a very short period of time. To minimise the damage of guava moth the best thing to use are guava moth pheromone traps hung in the trees to attract the adult males and break the lifecycle of the moth, these are available at garden centres and DIY stores. Other things that can be done are picking up infected fruit fallen off the ground as the larvae overwinter in the soil, or find another host. Unfortunately guava moth have a continuous life cycle, they don't have a dormant stage and will find another host. Another thing that can be done, is placing fine mesh bags over the fruit once they are pollinated to prevent the moth laying eggs on the fruit. Light still goes through the mesh so the fruit ripen, these bags can be purchased from $2 variety stores or garden centres and DIY stores. Do not compost fallen fruit, dispose of or burn. Lianne.