Answer: Is the fruit itself dying and turning black or is there a black substance on the skin? If the fruit itself it dead, the usual cause is a cold snap while it was developing. You should be able to pull the dead fruit off the tree - or cut it off now. If, on the other hand, there's a black coating on the skin, it's probably sooty mold which is caused by insect feeding on the tree. Sucking insects such as aphids, scales and mealybugs excrete a substance called honeydew which is quite sticky and will coat the upper sides of the leaves and the fruit that is directly beneath where they are feeding. As the honeydew ages it turns black. It can be scraped off the leaves and the skin of the fruit with a fingernail. If this is what is on your lemons, inspect the tree for aphids, scales and mealybugs. Once you control these pests, the sooty mold will weather away. Good luck with your lemon tree!
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