Agriculture Reference
In-Depth Information
with virulent viruses (Walkey and Antill, 1987; van Dijk, 1994). Exploitation of
such virus-tolerant planting material could improve yields in regions where the
finance and infrastructure for the production of virus-free stock is not available
(van Dijk, 1994). Another way of avoiding the problems of virus accumulation
in vegetative planting stock is to grow from true seed. Seed-sown shallot
cultivars are already available (Rabinowitch and Kamenetsky, 2002), and work
is in progress to develop garlic cultivars grown from true seed (see Chapter 3).
As with nearly all crop protection problems, the situation with allium
viruses is constantly evolving. In 1998 details of a newly recognized Tospovirus
that causes yellow necrotic spots on the leaves of iris plants were reported from
The Netherlands (Cortes
et al
., 1998). In 1999 it was reported that a disease
which had been causing severe yellowing and necrosis of onion leaves and
scapes in Brazil since 1994, often resulting in the death of flowers and 100%
losses of bulb and seed production, was caused by the same virus (Pozzer
et al
.,
1999). The same disease was reported from Israel and it was established the
virus vector was the onion thrips,
Thrips tabaci
(Kritzman
et al
., 2001).
The spread of this disease in onion-growing states of the western USA has
been documented by a series of reports (e.g. Gent
et al
., 2004b). In the USA the
most severe losses have been in seed crops, where necrotic lesions have caused
the collapse of seed stalks. However, losses in bulb crop yield - particularly of
the valuable, larger size-grade bulbs - are of concern (Gent
et al
., 2004b). The
host range of the virus is narrow (Cortes
et al
., 1998). Research on disease
control has focused on avoiding disease carry-over in infected volunteer plants
and on trying to control the thrips vectors of the virus. The disease can be
particularly devastating in hot onion-growing regions like the Sao Francisco
valley of north-eastern Brazil, probably because thrips proliferate very fast
under the warm conditions. Observational evidence indicates there may be
differences between onion cultivars in susceptibility to the disease, which may
be linked with their attractiveness to thrips.
A virus-like disease of onions and leeks is caused by the phytoplasma aster
yellows (Schwartz and Mohan, 2008; Bertaccini
et al
., 1999). This can infect
many vegetable species. In onion it produces flat, yellow-streaked leaves and
distorted, sterile flowers. The disease is transmitted by a leaf hopper and can be
controlled by insecticidal treatments against these vectors, the isolation of onions
from other infected crops and the removal of weed hosts of the phytoplasma.
Bacterial diseases of onion were reviewed in depth by Mark
et al.
(2002), and
useful illustrations and information can be found in Snowdon (1991) and
Schwartz and Mohan (2008). Bulb rots caused by bacteria are the most widely
reported problem, but bacterial leaf blights can be devastating in some regions.
Details of the main bacterial diseases are outlined in Table 5.4.
