VIRUS DISEASES OF ALLIUM SPECIES AND PROSPECTS FOR THEIR CONTROL

Allium crops are usually virus infected, especially when vegetatively propagated. During an extensive study of allium viruses at the DLO Research Institute for Plant Protection, Wageningen, The Netherlands, nematode-borne nepo- and tobraviruses, miteborne rymoviruses, and aphid-borne carla- and potyviruses were identified.

The potyviruses are the most important because they induce yellow striping which may result in large yield reductions. Their transmission takes place efficiently by various aphid species when briefly probing on plants. These viruses and their host-specific strains are usually limited to a single Allium species. Onion yellow dwarf virus (OYDV) is known from onion (A. cepa) and shallot (A. cepa var. ascalonicum) in the USA and Europe. Shallot crops in Asia were found to be infested by the new shallot yellow stripe virus (SYSV). Work at the Lembang Horticultural Research Institute in Lembang, Indonesia, showed that OYDV is now spreading in Java. Leek yellow stripe virus (LYSV) occurs in leek (A. ampeloprasum var. porrum) in Europe and in pearl onion (A. ampeloprasum var. sectivum) in Indonesia. Garlic strains of OYDV (OYDV-G) and of LYSV (LYSV-G) affect garlic (A. sativum) worldwide. Striping in Welsh onion (A. fistulosum) is due to the new Welsh onion yellow stripe virus (WoYSV), which probably also causes the virus disease reported in Japan in rakkyo (A. chinense).

Control of OYDV in shallot crops and of OYDV-G in garlic crops in some European countries is by strict certification of bulbs for planting, but this is expensive and requires a well-functioning inspection service. Shallot and garlic crops in other countries often contain some plants not infected by potyvirus or not showing symptoms of potyvirus infection. Such plants may carry genetic resistance or may be cross-protected by a mild virus strain. Propagation and distribution of planting stock from healthy plants selected in the field or obtained by resistance breeding seem promising for improving yields of vegetatively propagated allium crops in the tropics.