Agriculture Reference
In-Depth Information
effective strategy of disease control in onions and leeks. Allium viruses are
transmitted by vegetative propagation, and traditional cultivars of garlic and
shallot are usually chronically infected with a mixture of viruses.
Information on the viral diseases of vegetable alliums was reviewed by Walkey
(1990) and, more recently, those of garlic by Salomon (2002). Much information
about different isolates of these viruses can be found at the web site of the
International Committee on the Taxonomy of Viruses (ICTVdB Management,
2006). Table 5.3 lists the viruses and summarizes some of their features.
There are different strains within these virus species that can differ in
virulence and infection range. For example, the garlic strain of OYDV cannot
infect onion, and vice versa (van Dijk, 1994). RNA sequences vary considerably
between different strains of the same virus, with up to about 20% sequence
variation within a species. The increasing volume of sequence information is
making it possible to trace patterns of genetic divergence and affinity both
between strains within a species and between species (Tsuneyoshi
et al.
, 1997;
and other references in Table 5.3 concerned with RNA sequences). Scientists
involved in conserving the world's heritage of vegetatively propagated alliums in
gene banks are aware of the danger of introducing novel virus strains to a
locality via such collections, and take steps to avoid this (Maggioni
et al.
, 2002).
Most of the viruses are symptomless and do not cause obvious disease.
However, three - onion yellow dwarf virus (OYDV), leek yellow stripe virus (LYSV)
and iris yellow spot virus (IYSV) - cause serious diseases. OYDV infection causes
onion leaves to become flattened and crinkly, with yellow striping or complete
yellowing of leaves. Infected leaves bend and lie along the ground. Bulbs from
infected plants are smaller and have a shorter dormancy than healthy bulbs, and
yield losses of up to 60% have been recorded. Infection also reduces the yield of
seed crops. OYDV has been reported in most onion-producing areas. OYDV also
produces severe disease symptoms in shallot and garlic.
LYSV infection causes leek leaves to be yellow striped and flaccid, so that
leaves are downturned and trail along the ground. Yield losses of up to 54%
can occur, and infected plants have reduced frost hardiness and a shorter shelf-
life after harvesting. Shallot latent virus (SLV) is found in virtually all shallots
and can infect the other allium crops. The influence of SLV on shallot yield, if
any, is not known but, if SLV is present along with LYSV in leeks, the severity of
disease is increased.
OYDV, LYSV and SLV are transmitted on the mouthparts of aphids on short
probing visits from plant to plant. Insecticides are not sufficiently fast-acting to
prevent spread in this way from infected plants. Therefore, disease prevention
depends on the elimination of infection sources. Fortunately, these viruses are not
transmitted in seeds, and therefore crops raised from seed are initially virus-free.
Also, the host range of these viruses is restricted to alliums. Hence, if crops are
raised from seed and isolated from infected alliums that act as a virus source,
these diseases can be avoided. LYSV built up to epidemic levels in The Netherlands
and Belgium in the 1970s when year-round leek production became common.
