The objective of this research begun in 2003 is to explore scion/rootstock incompatibility as a possible cause of tree decline in Fukumoto navel. Tree decline in Fukumoto navel has been characterized by poor growth – especially of the scion – profuse sucker proliferation at the graft union, and sparse, yellow leaf canopies. The decline of some trees is common in almost all Fukumoto orchards on trifoliate or trifoliate hybrid rootstocks (i.e. citrange rootstock such as Carrizo and C-35), however the severity varies widely among orchards. Tree death has occurred in orchards, especially in some orchards planted on C-35, Carrizo or trifoliate rootstocks.