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Passionfruit plantings in Vietnam increased to 10,000 ha in 2019. However, outbreaks of passionfruit woodiness disease (PWD) have become a serious threat to production. In this study, five virus isolates (DN1, DN4, NA1, GL1, and GL2) were collected from different areas of Vietnam. Their causal roles in PWD were verified by back-inoculation to passionfruit. Analyses of coat protein (CP) and genomic sequences revealed that the GL1 isolate is closely related to East Asia Passiflora virus (EAPV) AO strain of Japan (polyprotein nt and aa identities of 98.1 and 98.2%, respectively), and the GL2 isolate is related to Telosma mosaic virus (TelMV) isolate PasFru, China (polyprotein nt and aa identities of 87.1 and 90.9%, respectively). CP comparison, host range, and cytological characterization indicated that DN1, DN4, and NA1 are potyviruses but are different from EAPV and TelMV. Phylogenic analyses of their CP and genome sequences indicated that these three isolates and the passionfruit severe mottle-associated virus Fujian isolate of China belong to a distinct clade, which does not meet the threshold (76% nt identity of polyprotein) to be regarded as any of potyviral species. Thus, a new species name, Passiflora mottle virus, (PaMoV), has been proposed by the International Committee on Taxonomy of Viruses. A rabbit antiserum was produced against the CP of DN1, and it can distinguish PaMoV from TelMV and EAPV in western blotting and enzyme-linked immunosorbent assay (ELISA) without cross-reactions. Field surveys of 240 samples by ELISA and reverse transcription PCR found that PWD in Vietnam is caused mainly by PaMoV, followed by EAPV, mixed infection of PaMoV and EAPV, and rare cases of TelMV.

A survey conducted on newly cultivated lowland rice fields by direct seeding method in Hai Duong Province, Viet Nam, in March 2017 revealed high devastation of the field. In these fields, farmers used an annual crop rotation cycle of rice-scallion-rice. Investigations on the devastated fields revealed that the chemical and physical soil properties were appropriate for rice cultivation. On the other hand, observations done on the root systems showed that the dead plants have symptomatic root galls suggesting the presence of plant parasitic nematodes. Sequencing of the internal transcribed spacer (ITS) region of the rDNA genes of the nematodes showed that the root nematodes extracted from the infested fields belonged to Meloidogyne graminicola. The reproductive factor of the isolated M. graminicola population on the IR64 rice variety (Oryza sativa indica) was normal, suggesting that the impact of this plant pest was not due to the emergence of an unusual virulent population. The combination of the three factors (wrong cropping choice for rotation, using rice variety susceptible to M. graminicola and direct seeding) were obviously promoting the nematode infection and its high proliferation in the surveyed fields. Meloidogyne graminicola could parasitize and propagate in scallions of Vietnam. Since this plant is annually cultivated on a paddy field for crop rotation, preventive measures or alternative plant for crop rottion is necessary.