Agriculture Reference
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signs of virus disease, as well as sampled, to ensure they are free of stem and
bulb nematode and white rot. Any diseased plants found during inspection,
plus adjacent plants, are removed and destroyed. A score of fewer than 1% of
diseased plants in the final generation of multiplication is required for
certification as 'virus-free' planting stock (Messiaen et al ., 1993).
Reinfection of virus-free garlic with OYDV or LYSV or a mixture of the two
showed that OYDV caused more severe symptoms and yield loss than LYSV (Lot
et al ., 1998). Yield loss, which was associated with earlier maturity of the
infected plants, varied with cultivar, ranging 40-60% for OYDV, 17-54% for
LYSV and 60-90% for the mixture. Similar experiments on reinfection with
GarV-A and GarV-C showed smaller, but still significant, yield losses, being
about 14-32% for GarV-A and 6-11% for GarV-C, again depending on cultivar
(Cafrune et al ., 2006).
Given such immediate and large decreases in yield from artificial reinfection,
the question arises whether the expense and effort of producing virus-free
planting stock can be justified in some countries. Field trials in both Argentina
and Brazil showed that yields from virus-free planting cloves, which were more
than double those from the chronically infected local strains in the first year,
declined exponentially with successive years of replanting. Despite this, after 7
years, yields from the originally virus-free stock remained significantly higher
than from the local, chronically infected planting stock, indicating a continuing
benefit over many seasons from planting virus-free material (see Fig. 5.12).
As observed in the above-mentioned French trials, healthy plants of both
garlic and shallot not infected or not showing symptoms often occur among
symptomatic plants (van Dijk, 1994). Also, there are reports of virus-tolerant
garlic cvs that show no yield improvement after virus elimination by meristem
tip culture (Walkey and Antill, 1987). These effects may be caused by infections
with mild virus strains which 'cross-protect' their hosts and prevent infection
Fig. 5.12. Garlic yields from cloves that were virus-free in year 1 over 7 successive
years of saving cloves and replanting in Brazilian trials. The 'control' line represents
the yield from standard virus-infected cloves used by growers. The yields of virus-
free stock showed an exponential decline with time (from Melo Filho et al ., 2006.
Courtesy of the European Journal of Plant Pathology ).
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