Mundulla Yellows disease of eucalypts: descriptors and preliminary studies on distribution and etiology

Abstract

Mundulla Yellows (MY) is a newly recognised lethal dieback disease of eucalypts in Australia. Descriptors of the MY syndrome have been compiled from observations of affected trees in South Australia over a 5-year period. Early, medium and late stages are defined. Symptoms resembling MY have been identified in over 70 species in all Australian states as well as in eucalypts introduced to southern Spain. Spatial distribution is discontinuous. No extracellular pathogens have been associated with MY. Changes were noted in the elemental composition of symptomatic leaves but these results do not separate cause from effect. No association with soil pH or herbicide use was apparent. In trials to determine whether MY is a biotic disease, we have demonstrated the graft transmissibility of specific symptoms, observed cytological changes in affected leaves, and observed novel virus-like inclusions in some cells of affected trees. Phytoplasmas or bacteria were not seen by electron microscopy. Generic RT–PCR detected phytoplasma sequences in symptomatic and asymptomatic trees with the same low frequency. The results suggest that MY is contagious, but do not implicate any previously described intracellular etiologic agent with MY. A search for non-host genetic components is suggested as a means to further test the biotic pathogen hypothesis for MY.