Submitting question from the Monroe County Extension office for resident Karl Mathe.... "My black walnut tree has been great till last year when I started to noticed problems. It is already dropping walnuts that are not ready and rotting easily. Is this a familiar problem? What can I do to get this tree back into shape?"
Monroe County Michigan
Early drop or walnuts is not unusual. There can be different reasons for early dropping of walnuts: weather, pollination, pests, diseases, et al. This year with the cool spring and excessive rain the tree may have put forth more nuts than usual and with the heat and lack of rain it adjusted its nut load and aborted several. High temps can interfere with pollination and the fruit is aborted. Lack of pollinators can also cause this problem. Diseases such as anthracnose (https://extension.psu.edu/walnut-anthracnose) can induce fruit drop. Codling Moth (https://www2.ipm.ucanr.edu/agriculture/walnut/codling-moth/) can also cause nuts to abort. Check dropped fruit to see if you see insect frass The most likely cause is weather.
I hope this helps