Agriculture Reference
In-Depth Information
traditional cultivars revealed some plants that had no or very mild symptoms of
viral disease in field trials in several regions. Selection from such plants resulted in
the development of virus-free named cultivars - for example, cv. 'Thermidrome'
and some 'virus-tolerant' cvs (e.g. 'Fructidor' and tolerant strains of 'Violet de
Cadours').
In the 1970s, techniques of eliminating viruses by 'meristem tip culture'
were developed. Virus-free plants can be produced by culturing 0.5-0.8 mm-
diameter explants of shoot apical meristem in tubes of sterile nutrient agar.
Ultimately, rooted plantlets develop that can be transferred to soil (Walkey,
1990). Elimination of viruses from 'tolerant' strains of 'Violet de Cadours'
increased yields by about one-third, and the virus-free cv. 'Germidor' was
derived in this way. Meristem tip culture has been used to produce virus-free
clones of numerous garlic cvs, resulting in larger bulbs with more cloves and
25-40% increases in bulb weight (see Fig. 5.11). Meristem tip culture was also
used to develop virus-free shallot cvs 'Jermor' and 'Mikor', which have a yield
potential of 40 t/ha, as opposed to a maximum of 25 t/ha in the previous best
virus-tolerant strains (Messiaen et al ., 1993).
A system of multiplication, inspection, certification and distribution of
virus-free garlic and shallots, the first in the world, was established in France in
the 1960s (Messiaen et al ., 1993). This involves maintaining elite, disease-free
clones of the named cvs in net houses to protect them from aphid virus vectors
and multiplying the first two generations of bulbs for planting stock under
these conditions. A further four generations of multiplication for garlic - or
three for shallot - are carried out by selected growers in regions not conducive
to the spread of viruses. Fields for propagation must be isolated by at least
300 m from other alliums. During growth, plants are inspected regularly for
Fig. 5.11. Virus-free garlic bulbs (upper crescent) produced by meristem tip
culture, compared with infected bulbs of the same cultivar (lower crescent). Three
bulbs each of cvs (1) 'Rose de Var, (2) 'Printanor', (3) 'Blanc de la Drome', (4)
'Moulinin' and (5) 'Fructidor' after 1 year of growth in the field (
1/9) (photograph
courtesy of Dr D.G.A. Walkey, formerly of HRI Wellesbourne, UK).
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