Efforts to understand transmission of the papaya meleira virus complex by insects

Papaya sticky disease (PSD) is an emerging disease-causing significant crop loss in some of the major papaya-growing regions of the world. The vectors of the PSD associated viruses in Brazil are still unknown. The papaya meleira virus complex comprised of a fusagra-like virus, papaya meleira virus (PMeV), and a umbravirus-like associated RNA (ulaRNA), papaya meleira virus 2 (PMeV2) is found infecting diseased papaya plants in Brazil. PMeV capsid protein packages both PMeV and PMeV2 genomes separately resulting in virions with the same morphology. Epidemiological analyses attributed fruit thinning as a mechanical mechanism responsible for the spread of sticky disease, but an aerial vector was not ruled out. Hemipteran insects have been implicated as vectors but a definitive conclusion on the biologically relevant vector has not been reached. Cicadellids have a population peak a month before the peak of papaya sticky disease incidence in the field and their ability to acquire and transmit the Mexican isolate of PMeV has been demonstrated. Whitefly (Bemisia tabaci MEAM1) is not considered a papaya pest in Brazil but has been reported to occur in plants near papaya trees and they transmit an Ecuadorian virus similar to PMeV2. In Brazil, Trialeurodes variabilis which colonizes papaya trees can acquire, but not transmit the PMeV complex. In this review, we discuss transmission assays and epidemiological analysis conducted in the last 30 years; the similarity of the PMeV complex capsid protein with viruses that infect fungi; the challenges imposed by laticifers, a well-known plant defense structure, in the acquisition of viral particles; and the presence of PMeV2. Elucidation of the PMeV complex vector would contribute to the efficient management of papaya sticky disease and increase understanding of the transmission mechanisms of plant-infecting fusagra-like viruses.