172 – II.3. BRASSICA CROPS (BRASSICA SPP.)
Different agronomic practices also influence the size and persistence of volunteer
populations. In Europe, the large amount of straw remaining after harvest plus the short
time between the July harvest and August sowing dates encourages ploughing down of
residue, resulting in seed burial. In Canada on the other hand, ploughing is not practiced
and most fields are spring- sown into undisturbed stubble (minimum or zero tillage) from
the previous year’s September harvest (Hall et al., 2005). Thus, seed burial is minimised
and harvest seed losses are exposed to environmental hazards. The result is that in
Europe, old or discontinued cultivars or genotypes will persist in the seed bank for
a much longer time than in Canada. This is clearly illustrated in the changeover from high
to low erucic acid B. napus varieties. In the German oilseed rape growing province of
Schleswig-Holstein, it required ten years to reduce the commercial crop from the
traditional high erucic varieties (50% erucic) to the desired level of 2% (Sauermann,
1987). In Canada, the same results were obtained in three years (Daun, 1983).
Geographic distribution
The genus Brassica and its wild relatives are part of the tribe Brassiceae that has
its origin in the Mediterranean basin and in south-western Asia. However, the geographic
centre is thought to be in the south-western Mediterranean region (Algeria, Morocco and
Spain) where some 40 genera have been shown to be endemic or exhibit maximum
diversity (Hedge, 1976; Gómez-Campo, 1999, 1980; Al-Shehbaz, 1984; Al-Shehbaz,
Beilstein and Kellogg, 2006; Warwick and Hall, 2009). For the subtribe Brassicinae,
Hedge (1976) leaves little doubt that it originated in the Mediterranean basin. The species
distribution of the Brassicaceae family is concentrated in the northern temperate zone and
south-western and central Asia (Holm et al., 1997). Few species are found in hot, humid
tropics.
B. nigra
B. nigra or black mustard was widely grown for the sharp pungency of its seeds and
as a leaf vegetable. Prakash and Hinata (1980) placed the species origin in central and
south Europe. It is one of the oldest recorded spice crops, which undoubtedly resulted in
its early and widespread distribution across Europe, Africa, Asia and the Indian
sub-continent, and its dehiscing siliques with primary dormancy of the seed ensured
its persistence. The GRIN describes the species distribution as widely naturalised in the
following regions and countries. In Africa: countries along the south shore of the
Mediterranean as well as Eritrea and Ethiopia. In temperate Asia: Afghanistan, Armenia,
Islamic Republic of Iran, Iraq, Israel, Kazakhstan, Lebanon, Syrian Arab Republic,
Turkey and northwest China. For the Indian sub-continent: India, Nepal, Pakistan.
In Europe: all countries in western and eastern Europe as well as the Balkans and Greece.
The crop was introduced to the Americas and Australia as a spice. However, in the 1950s
it was displaced by the higher yielding, pungent B. juncea that was better suited to
mechanical harvesting. Although in many regions black mustard is now a weed of waste
places, it has never become established on the Canadian prairies, although it is present
throughout much of the United States.
B. rapa
B. rapa is thought to have originated in the mountainous areas near the Mediterranean
sea (Tsunoda, 1980). The time of domestication is unknown. Sinskaia (1928) proposed
two main centres of origin, one being the Mediterranean and the other the
Afghanistan-Pakistan region. The species appears to have attained a wide distribution
