232 – II.3. BRASSICA CROPS (BRASSICA SPP.)
found the following spring (Dunfield and Germida, 2004, 2003, 2001: Siciliano and
Germida, 1999).
Allelopathy
There have been numerous reports of inhibitory affects by Brassica residues on the
following planting of pasture, cereal and oilseed crops (Campbell, 1959; Bell and Muller,
1973; Rice, 1984; Mason-Sedun, Jesspo and Lovett, 1986; Horricks, 1969; Vera,
McGregor and Downey, 1987). The allelopathic effects include germination inhibition,
reductions in root growth, plant height, dry weight, tiller number and seed yield. Species
involved in the inhibition included marrow stem kale (B. oleracea), oilseed and turnip
rape (B. napus, B. rapa) and condiment and black mustard (B. juncea and B. nigra).
The inhibiting compound(s) are leached by water from dead or decaying stems and leaves
of Brassica vegetation. The compound(s) appear to reside in the upper soil layer for a
short period and then dissipate. Mason-Sedun, Jesspo and Lovett (1986) compared the
effect of water extracts from dry residues of four Brassica species on coleoptile growth of
common wheat (Triticum aestivum). All residues significantly reduced grain yield, plant
dry weight, plant height and tiller production, with the greatest level of inhibition
resulting from B. juncea residues followed by B. nigra, B. napus and B. rapa.
Laboratory studies indicated that when stored, dry residues became less toxic over
time. Waddington and Bowren (1978) found that rapeseed residue was no more toxic to
barley, bromegrass or alfalfa than comparable amounts of wheat residue. Normally
Brassica residue will have been rained on well before seeding, resulting in no inhibition.
Indeed, there is good evidence that cereal crops are more productive following oilseed
rape than another cereal (Almond, Dawkins and Askew, 1986). Vera, McGregor and
Downey (1987) suggested that the primary cause of the observed inhibition in western
Canada may be the release of a chemical compound from volunteer oilseed rape seedlings
that are killed by cultivation at seeding time. The chemical was thought to be the indole
glucosinolate, glucobrassicin, present in high concentrations in tissues of young seedlings
(Röbbelen and Thies, 1980).
Pathogens
The Brassica crops and their wild allies are subject to a broad range of pathogens and
adverse conditions or disorders associated with non-infectious causes. Although many of
the Brassica species have many diseases in common, there are also significant differences
in susceptibility among and within species. The Compendium of Brassica Diseases
(Rimmer et al., 2007) provides an authorative and practical reference guide to disease
problems in Brassica crops the world over. Colour plates and text describe the infectious
diseases caused by fungi, oomycetes, bacteria, mollicutes, viruses and nematodes.
In addition, non-infectious disorders such as those related to environmental effects,
herbicide injury and nutritional deficiencies are also described. The American
Phytopathological Society (APS) also provides a listing by common and scientific name
of known Brassica diseases and conditions at its website as reproduced in Table 3.A1.2 in
the annex (APS, 2001).
Of the many Brassica field crop diseases listed in Table 3.A1.2, three stand out as
particularly troublesome as they are pandemic and have the potential to cause major crop
injury: blackleg or stem canker (Leptosphaeria maculans); Sclerotinia stem rot
(Sclerotinia sclerotiorum); and clubroot (Plasmodiophora brassicae). To date there are
few control measures for these pathogens that are fully effective and economical.
