Fluorescence microscopy of cross sections of young twigs from sandal trees, stained with aniline blue, showed a marked difference in fluorescence in the phloem area of healthy and spike-diseased trees. In sections of twigs from healthy trees fluorescence was restricted to the outer zone of the phloem whereas the phloem zone in spike-diseased trees fluoresced over its total area. Older twigs and leaves, but not roots, showed a similar phenomenon. The diagnostic value of this method is discussed.