Large, mature lemon tree produced prolific number of lemons for years gradually fewer and now none at all….why/

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Answer 1

Have you fertilized? Good soil and water management are key factors in supporting growth, flower and fruit production. Plants use soil based nutrients and others are leached from the soil with irrigation and rains. If you rake up fallen fruit and leaves then the nutrients are not returned to the soil.
Fertilize, mulch and deep water once every week or 10 days. Don't water too frequently or root and crown rot can develop.
https://www.gardeningknowhow.com/edible/fruits/citrus