Cause Unknown. Possibly a physiologic problem because no insects or pathogens have been associated with this problem. Severity of the problem changes from year to year.
Symptoms Brown stain is characterized by a brownish liquid that soaks the side or apical end of the nut. Staining begins when nuts are about half grown. The first symptoms are water-soaked spots between the vascular bundles on the shell that are softer than normal. Soon spots become brown to dark brown and sunken. Internal areas are affected later, producing a water soaking around the developing kernel. The entire interior of the shell, including kernel, may turn into a soft, brown, watery mass. Kernels may also be misshapen.
Cultural control None is known.
Reference Miller, P.W. 1945. Diseases of filbert in the Pacific Northwest and their control. Oregon State College Station Bulletin 428.