Sampling and Histological Procedures for Diagnosis of Ash Yellows. W. A. Sinclair, Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5908. R. J. Iuli, A. T. Dyer, and A. O. Larsen. Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5908. Plant Dis. 73:432-435. Accepted for publication 22 November 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0432.

The DAPI (4’,6-diamidino-2-phenylindole· 2HCl) fluorescence test was superior to Dienes’ stain for histological detection of mycoplasmalike organisms (MLOs), which cause ash yellows, in ash (Fraxinus spp.). MLOs were detected by the DAPI test more than twice as often in roots (2–6 mm diam) as in twigs of F. americana from which one twig and one root per tree were tested. 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. Therefore, given independent probabilities of false-negative results, the testing of two roots from a tree would result in less than 1% chance of erroneously declaring the tree free from MLO infection. MLOs were detected by the DAPI test in the roots of 68% of 108 F. 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 symptom of MLO infection.