SURVEY AND CHARACTERIZATION OF POTYVIRUSES AND THEIR STRAINS OF ALLIUM SPECIES

Nearly 5700 plants of 14 cultivated and 8 wild Allium species and vari eties from the Netherlands and other parts of the world, were tested f or infection with aphid-borne potyviruses by ELISA, electron microscop e decoration tests and/or inoculation onto test plants. This resulted in the detection of two known viruses, viz. leek yellow stripe virus ( LYSV) and onion yellow dwarf virus (OYDV), and the discovery and chara cterization of two new viruses, viz. shallot yellow stripe virus (SYSV ) and Welsh onion yellow stripe virus (WoYSV), and of six strains of t hese viruses. 'Garlic mosaic', 'garlic yellow streak', 'onion mosaic', 'shallot mosaic', 'shallot X', and 'shallot yellows' viruses, incompl etely described in the literature, are now reidentified as well-known viruses or as strains or mixtures of such viruses. 'Garlic yellow stri pe virus' is also a complex containing a potyvirus possibly differing from the viruses found in this survey. The symptoms of the potyviruses studied varied widely and ranged from mild to severe chlorotic to yel low striping of leaves, and they are of little diagnostic importance. LYSV was found in vegetatively propagated pearl onion (A. ampeloprasum var. sectivum) from Europe and Asia. It has decreased in leek crops ( A. ampeloprasum var. porrum) in the Netherlands since the 1970s, appar ently due to resistance in new cultivars. OYDV was common in onion (A. cepa var. cepa) from the former USSR and North Africa, and in Europea n cultivars of shallot (A. cepa var. ascalonicum), with the exception of the highly resistant 'Sante', but was not detected during this surv ey in Asian shallot. European samples of ever-ready onion (A. cepa var . perutile), multiplier onion (A. cepa var. aggregatum) and tree onion (A. cepa var. viviparum) contained OYDV. It was also found in sand le ek (A. scorodoprasum) from European gene collections. A strain of OYDV from onion and shallot in Morocco and Spain was virulent on onion and shallot cultivars resistant to common OYDV, as reported early for a s imilar isolate in the USA.Asian shallot appeared generally infected wi th the new SYSV, similar to OYDV in host range and symptoms but serolo gically distinct. It was not detected in onion and shallot from Europe or North Africa. A virulent strain of this virus caused striping in s ap-inoculated garlic (A. sativum) and Formosan lily (Lilium formosanum ). The new WoYSV, infecting Welsh onion in Indonesia and Japan, was ea rlier described in Japan as OYDV from rakkyo and Welsh onion. It appea red serologically closely related to SYSV and distantly to OYDV, but d iffered in its host range. Host-specific strains of LYSV and OYDV were detected in garlic, wild garlic (A. longicuspis), an unidentified All ium species (suffix -G), and great-headed garlic (A. ampeloprasum var. holmense) (suffix -GhG). LYSV-G and OYDV-G infected on average 45% an d 73%, respectively, of the garlic samples of worldwide origin. Sympto ms of isolates of both strains varied in severity, implying the necess ity of serological tests for disease diagnosis and health certificatio n. LYSV-GhG was the cause of yellow striping in 93% of the great-heade d garlic plants tested, mainly from the Mediterranean area. One sample was also infected with OYDV-GhG. Many samples from vegetatively propa gated crops grown from non-certified planting stock contained a few pl ants free of potyviruses, implying the possibility to obtain healthy ( and possibly resistant) selections of such cultivars avoiding meristem -tip culture. Cross-protection of garlic sets by a mild potyvirus isol ate seems to be an altemative to the use of vulnerable virus-free sets . Generally, viruses and virus strains could not be transmitted to any Allium species other than their natural host, except to the highly su sceptible crow garlic (A. vineale). This species, and other predominan tly vegetatively propagating wild Allium spp. (field garlic, A. olerac eum; ramsons, A. ursinum; sand leek), were found not to be reservoirs of viruses that might infect Allium crops in the Netherlands. Streakin g in vegetatively propagated wild leeks (A. ampeloprasum and closely r elated species) originating from the Mediterranean area and Asia was d ue to an undescribed mitebome virus. The survey confirmed that spread of potyviruses in Allium crops in the Netherlands is from planting set s, and from a neighbouring crop only if of the same species.