Agriculture Reference
In-Depth Information
Several laboratory techniques have been developed to challenge breeding
lines with a disease or pest organism, and to measure the degree of susceptibility
or resistance. For example, Currah and Maude (1984) tested resistance to leaf
rot caused by
B. squamosa
by giving a standard dose of
B. squamosa
to small discs
of leaf on damp wadding. The discs were then incubated at a constant 15°C at
100% RH. The average time for threads of fungus mycelium to appear on the
leaf disc surface was determined. There were significant differences between
cultivars in the time for infection to develop. Using such 'screening tests' it is
possible to test for resistance in a standard way in many lines. Resistant lines
exposed by these rapid tests can then be included in larger scale and more time-
consuming tests for improved resistance in field conditions. Screening tests have
been reported for onion white rot, pink root disease, neck rot, onion fly and
others (see relevant chapters in Rabinowitch and Brewster, 1990b).
Much of the resistance to pests and diseases has been derived by mass
selection under the pressure of attack by these organisms. This still remains an
important breeding strategy, and field resistance is the ultimate test for
resistance developed by more sophisticated means. The landraces of cultivated
alliums that have been developed in the face of the worldwide diversity of pests
and diseases are probably the most important reservoir of resistance genes.
Therefore, as discussed in Chapter 1, it is vital for future resistance breeding
that these old varieties are conserved (Astley, 1990).
The breeding of edible alliums has been relatively unsophisticated compared
with many important crops, because of a comparative lack of fundamental
genetic and molecular genetic information. This is now changing, and
particularly in the area of disease resistance - for example, in the transfer of
downy resistance from
A. roylei
, new techniques are enabling the commercial
development of resistant cvs (Kik, 2002).
Genetic transformation has already produced herbicide- and insect pest-
resistant alliums, the use of which could reduce the need for pesticides and
simplify crop management. However, despite the environmental benefits from
the use of less and safer pesticides and the unlikelihood of any risks from
transgenic alliums (Eady, 2002), there remains some hostility to transgenic
crops, and this has prevented their commercialization in many countries. The
development of hybrid cultivars seems to have been the dominant trend in
onion breeding for the last 50 years, first in the USA and then in Europe and
Japan. Despite this, Dowker and Gordon (1983) pointed out that there were
little published data showing that hybrid cultivars are higher yielding, more
uniform or of better quality than the best open-pollinated cultivars.
A comparison of hybrid and open-pollinated (OP) short- and intermediate-
day cvs in New Mexico, USA found that the OP cvs performed better than the
