Papaya 'sticky' disease caused by virus 'couples': a challenge for disease detection and management.

Papaya sticky disease (PSD) is a major virus disorder of papaya (Carica papaya). The disease is characterized by fruit damage caused by the oxidation of spontaneously exuded latex. In Brazil, PSD is caused by the co-infection of two viruses, papaya meleira virus (PMeV), a toti-like virus, and papaya meleira virus-2 (PMeV-2), an umbra-like virus. The disorder has also been reported in Mexico and more recently in Australia, but the presence of both PMeV and PMeV-2 in symptomatic plants has been documented only in Brazil. In 2021, two-year-old papaya plants (cv. Passion Red) exhibiting PSD-like symptoms were observed in Santa Elena province, Ecuador. Molecular tests of leaf tissue and fruit latex from symptomatic plants failed to detect PMeV. However, papaya virus Q (PpVQ), an umbra-like virus related to but distinct from PMeV-2, and a novel virus, tentatively named papaya sticky fruit-associated virus (PSFaV), were found in the symptomatic samples. PSFaV shares 56% nucleotide identity with the genome of PMeV, suggesting that PSD symptoms can be caused by 'couples' of viruses related to but distinct from PMeV (a toti-like virus) and PMeV-2 (an umbra-like virus). This review discusses the history and epidemiology of PSD and the genomic features of newly discovered virus 'couples' involved in this syndrome. Given the unusual etiology of PSD which involves distinct virus species, the importance of implementing proper diagnostic approaches for PSD is highlighted.