Why is my plum setting heavy fruit but a lot drops before ripening?

Some fruit drop in plum is normal self-thinning after the heavy set, but excessive drop usually comes from poor pollination, moisture stress, poor nutrition, or heat and disease. Steps to reduce it: ensure good pollination by planting compatible pollinizer varieties and keeping bees active at bloom, since poorly fertilised fruitlets drop; maintain even soil moisture with drip especially during fruit development, because dry spells followed by sudden water trigger drop; provide balanced nutrition with adequate potassium, calcium and boron based on soil test; and control aphids, brown rot and other stress factors. Even after this, plum often sets more than it can size, so a degree of natural drop actually helps the remaining fruit grow larger. If set is genuinely poor (not just drop), the cause is more likely pollination or frost at flowering. Steady moisture, good pollination and balanced feeding give the best fruit retention and size.