The importance of the viruses infecting Ullucus tuberosus Caldas in Peru.
Ullucus tuberosus, an Andean tuber crop, is affected by 8 viruses. Three of these viruses (potato leaf roll luteovirus (PLRV), potato Andean latent tymovirus (PALV) and potato T trichovirus (PTV)) also infect potato. One virus (arracacha A nepovirus (AAV)) infects Arracacia xanthorrhiza. This is the first report of PTV and AAV infecting U. tuberosus. The viruses with the highest incidence in the Germplasm Bank maintained in vitro in CIP (International Potato Center, Lima, Peru) were Ullucus C comovirus (UCV), papaya mosaic potexvirus (PapMV-U) Ullucus strain, Ullucus mosaic potyvirus (UMV) and Ullucus mild mottle tobamovirus (UMMV). In a field experiment in the CIP Experimental Station, Huancayo, Peru (elev. 3280 m), plants infected with UMV, UCV and UMV+UCV+PapMV-U yielded 29, 27 and 38% less, respectively, than the healthy control plants. In Peruvian farmers' fields in La Libertad, Concepción Province, Junin, Department (elev. 3500 m), a low percentage of U. tuberosus plants from virus-free seed were infected with UMV, UCV, PALV and PLRV during 1 season in the field. It is suggested that the production and use of virus-free seed could be justified in this environment. PLRV is an important potato virus and can be transmitted by the aphid Myzus persicae from U. tuberosus to potato and vice versa. It is possible that U. tuberosus infected with PLRV could be a source of inoculum for potato fields.