Sweetpotato virus disease (SPVD), the most important disease affecting sweetpotato (Ipomoea batatas (L.) Lam), is caused by the synergistic interaction of the aphid-transmitted Sweetpotato feathery mottle virus (SPFMV) and whitefly-transmitted Sweetpotato chlorotic stunt virus (SPCSV). In this study, SPVD was the main disease in the Cañete Valley, the major sweetpotato-producing area in Peru. Studies on virus incidence showed that SPCSV and SPFMV were the most frequently identified viruses in Cañete Valley. Symptoms of different severity were associated with isolates of both viruses involved in the SPVD. Over 80% of plants infected with both SPFMV and SPCSV showed the symptoms (leaf reduction and deformation, vein clearing or mosaic, and stunting) typically attributed to SPVD elsewhere. SPFMV did not significantly affect the yield of the sweetpotato cultivars Jonathan and Costanero, but infection of these cultivars by SPCSV was associated with significant yield reduction. Double infection by the two viruses resulted in SPVD and greater yield reduction than for either alone. These results demonstrate that SPFMV and SPCSV interact synergistically and that the severity of SPVD symptoms also depends on the particular isolate of each virus.

SUMMARYPotato virus T (PVT) infected nine species of tuber‐bearing Solanum, most of them symptomlessly, and as a rule was transmitted through the tubers to progeny plants: two genotypes of S. tuberosum ssp. andigena were not infected. The virus was also transmitted by inoculation with sap to 37 other species in eight plant families. Chenopodium amaranticolor is useful as an indicator host, C quinoa as a source of virus for purification, and Phaseolus vulgaris as a local‐lesion assay host; the systemic symptoms in Datura stramonium, Nicotiana debneyi and in these three species are useful for diagnosis. Attempts to transmit PVT by aphids failed, but the virus was transmitted through seed to progeny seedlings of four solanaceous species, and from pollen to seed of S. demissum.PVT was purified by clarifying sap with n‐butanol or bentonite, followed by precipitation with polyethylene glycol, differential centrifugation and sedimentation in a sucrose density gradient. Purified preparations had an E260/E280 ratio of 1.18 and contained a single infective component with a sedimentation coefficient of 99 S. This component consisted of flexuous filamentous particles of about 640 times 12 nm that showed a characteristic substructure when stained with uranyl acetate. The virus particles contained a single species of infective single‐stranded RNA, of molecular weight 2–2 times 106 daltons, and a single species of polypeptide of molecular weight about 27 000 daltons.PVT is serologically related to apple stem grooving virus but not to four other common potato viruses with flexuous filamentous particles. Apple stem grooving virus and PVT cause similar symptoms in several hosts, but also differ somewhat in host range and symptomatology. Apple stem grooving virus did not infect potato, caused additional symptoms in C. quinoa also infected with PVT, and its particles did not show the structural features specific to PVT. The two viruses are considered to be distinct. The cryptogram of PVT is R/1:2–2/(5): E/E: S/C.

The aphid Myzus persicae (Sulz.) was shown to transmit potato spindle tuber viroid (PSTVd) to potato clone DTO-33 from source plants doubly infected with potato leafroll virus (PLRV) and PSTVd. Transmission was of the persistent type and did not occur when the insects were allowed to feed on singly infected plants. Only low levels of PSTVd were associated with purified PLRV virions, but its resistance to digestion with micrococcal nuclease indicates that the viroid RNA is encapsidated within the PLRV particles. Epidemiological surveys carried out at three locations in China revealed a strong correlation between PSTVd infection and the presence of PLRV, suggesting that PLRV can facilitate PSTVd spread under field conditions. Like all known viroids, potato spindle tuber viroid (PSTVd) is an independently replicating agent which completes its infection cycle without generating either a capsid or other viroid-specific proteins. Its genome is a small (359 nt), singlestranded, covalently closed circular RNA molecule whose extensive regions of intramolecular complementarity are responsible for its unusual stability in vivo (Sa$ nger et al., 1976 ;Riesner, 1987).In potato, natural spread of PSTVd has been repeatedly shown to occur either by foliar contact (Goss, 1926 ;Merriam & Bonde, 1954) or botanical seed (Hunter et al., 1969). Early reports (Goss, 1930) suggested that PSTVd might be transmitted by chewing-insects, but these reports have not been confirmed. De Bokx & Piron (1981) reported a low rate of transmission by the aphid Macrosiphum euphorbiae (Thomas) but no transmission was obtained with either Myzus persicae (Sulz.) or Aulacorthum solani (Kaltenbach).In contrast to PSTVd, potato leafroll virus (PLRV ; a member of the genus Luteovirus) is readily aphid transmitted, M. persicae being the most efficient natural vector (Goss, 1930). Like all luteoviruses, PLRV is restricted to the phloem tissue of Author for correspondence : Maddalena Querci.Fax j51 1 435 1570. e-mail m.querci!cgnet.com infected plants (Casper, 1988), and aphid transmission is both persistent and nonpropagative (Eskandari et al., 1979). PLRV has a narrow host range, and the virus is commonly found wherever potatoes are grown. Salazar et al. (1995) have recently reported that M. persicae could transmit PSTVd to potato and other test plants only when the source plant was doubly infected with PLRV and PSTVd. In some experiments, the percentage of PSTVd transmission reached 100 % ; no transmission was observed from source plants infected with the viroid alone, however. Here, we present results of experiments designed to elucidate the mechanism responsible for aphid transmission of PSTVd and discuss the possible epidemiological implications of our findings. A preliminary report of this work has been published elsewhere (Querci et al., 1996).As described earlier , occasional PSTVd contamination of PLRV isolates maintained at the International Potato Center (CIP), Lima, Peru provided the first indication that PLRV might facilitate aphid transmission o...

SummaryAll 26 accessions of Solanum brevidens, one accession of S. etuberosum and one accession of S. fernandezianum tested were all extremely resistant to potato leafroll virus (PLRV) and potato viruses Y (PVY) and A (PVA). S. brevidens and S. etuberosum were also resistant to Andean potato mottle virus (APMV) and moderately resistant to potato virus X (PVX), whereas S. fernandezianum was susceptible to these viruses. Additionally, S. brevidens was resistant to sap‐inoculated potato viruses M (PVM) and S (PVS). All the Etuberosa accessions were susceptible by graft‐inoculation to PVM, PVS, potato virus T (PVT) and Andean potato latent virus (APLV). Infections by the above mentioned viruses were symptomless in all of the Etuberosa spp. S. etuberosum and S. fernandezianum were infected by mechanical inoculation with potato spindle tuber viroid, S. etuberosum developing severe stunting and leaf‐curl symptoms, but S. brevidens was infected only by graft‐inoculation.The genes conferring resistance to PVY and PVX in S. brevidens and S. etuberosum appeared to be different from those currently utilised by plant breeders.