The fruit of our work grows up on other people's trees.
If you aren't familiar with Drucker, you need to be. He is widely considered to be the father of modern management and has had a great influence on leadership both inside and outside of the church.
I believe that what Drucker is getting at here is similar to what the Apostle Paul was trying to get across to a young pastor named Timothy:
You then, my son, be strong in the grace that is in Christ Jesus. And the things you have heard me say in the presence of many witnesses entrust to reliable men who will also be qualified to teach others.2 Timothy 2:1-2 (NIV)
All of us have a responsibility to pass along the faith; often times the work that God calls us to is to produce fruit through others. How are you doing in growing fruit on other trees?