Sampling and histological procedures for diagnosis of ash yellows

Repetitive sampling of the root systems of 30 F. americana revealed that, within limits of the DAPI technique, the testing of one root per tree would give a false-negative result for the tree as a whole in fewer than 10% of cases. MLOs were detected by the DAPI test in the roots of 68% of 108F. americana that were growing slowly or showing dieback but lacked witches'-brooms or deliquescent branches. Deliquescent branching was a strong indicator but not a definitive diagnostic sympton of MLO infection