Exobasidium japonicum inhabits in node strategically during summer in Rhododendron

Pi-Han Wang, Chia-Chen Tsao & Tsung-Yu Pai

Wang P.-H., Tsao C.-C. & Pai T.-Y. (2014) Exobasidium japonicum inhabits in node strategically during summer in Rhododendron. – Sydowia 66 (2) S. 325–334. http://dx.doi.org/10.12905/0380.sydowia66(2)2014-0325.


Exobasidium japonicum (Exobasidiales, Basidiomycota) is a fungus that causes the development of galls in tender tissues of azalea (Rhododendron spp.) from autumn to spring in Taiwan. It is not known where the pathogen inhabits during summer and how it infects the tender host tissues in autumn. Healthy shoots were infected by spores in disease season. In the offseason leaves and stems inhabited by the pathogen matured and matured leaves and stems did not cause galls. We sought to determine how the pathogen reaches young, developing tissues, which will develop several months later. We designed Exobasidium-specific PCR primers from rDNA ITS sequences of E. japonicum. These primers were used to locate the fungus in host tissues. Exobasidium japonicum was detected in the shoots with gall by PCR at the end of offseason and it was most frequently detected from nodal tissues by PCR. Exobasidium japonicum infected asymptomatic shoots, nearby the shoots with galls were detected as well. Exobasidium japonicum grew into petioles and inhabited the nodal tissue under the base of the petiole of infected leaves. The pathogen inhabits quiescently mainly in the nodes during the dry, hot season. From the meristem tissue of the nodes, it extends into axillary buds or adventitious buds when the nodes sprout. Depending on the organ that develops from the infected apical, axillary or adventitious buds, the galls are formed by tissue of leaf, stem, or flower organs. Thereby, E. japonicum is able to reach young, developing tissues and starts a new disease cycle. The PCR assay successfully detected low levels of E. japonicum DNA in Rhododendron, making this assay a good tool to study the ecology of the fungus.

Keywords: Exobasidium japonicum, inhabitant, PCR-detection, Rhododendron.