Abstract
Trunk phloem necrosis (TPN) is currently a main constraint in rubber (Hevea brasiliensis) plantations. The apparent spread of the disease, from tree to tree along the planting line, strongly supported the implication of a pathogen that could be transmitted mechanically via the tapping knife. In order to detect a causal agent of the disease, studies focusing on characterization of the known mechanically transmitted pathogens (e.g. viroids, cryptic viruses or phytoplasma) were initiated. RNA strands of low molecular weight (200-400 and > 500 bp) displaying structural similarities with viroids and viral dsRNAs were observed in various tested samples. However, attempts to show the potential role of these RNA molecules in the spread of the disease failed. First of all, there was no significant or reproducible correlation between the health status of the rubber trees sampled and these RNA molecules. Moreover, no sequence homology with known pathogens could be found when randomly amplified cDNA fragments isolated from trees presenting the disease symptoms were sequenced. In conclusion, the aetiological investigations, in order to show the presence of a pathogen responsible of the TPN disease, were non-conclusive, which tends to disprove the hypothesis of a biotic causal agent.