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Many strains of the virus induce conspicuous mosaic and mottling of leaves, the severity of which is dependent on the varietal tolerance of susceptible species and environmental conditions (Broadbent, 1957). However, numerous symptomatologically-distinct strains of the virus have been identified (for example, Lung and Pirone, 1973; Hull and Howell, 1978; Tomlinson and Shepherd, 1978; Hull, 1980; Al-Kaff and Covey, 1994, 1995). Symptoms are usually much more severe in plants also containing Turnip mosaic virus or Broccoli necrotic yellows virus (Broadbent, 1957; Feldman et al., 1978; Shepherd, 1981; Henriques and Henriques, 1984; Spak and Novikov, 1994).
The most important control procedure is to limit or prevent virus infection of seedlings in seedbeds, because the effects of infection are greater when seedlings are infected at a young stage, and infected seedlings are the most important source of infection within a crop (Pound, 1946; Broadbent, 1957). Infection can be minimized by establishing seedbeds in isolation from susceptible crops or, if this is impractical on small farms, by surrounding the seedbeds with a barrier of cereals (mixture of barley and oats) or, if the costs are justifiable, by growing the seedlings under insect-proofed conditions such as fine-meshed cages. It is suggested that seedbeds should be no wider than 12 rows if barriers are used and, if wider, should have barriers at 12-row intervals (Broadbent, 1957).
Seedbeds should be sprayed with insecticide when aphids are numerous to minimize spread.
When seedlings are transplanted in the field, those from the outer rows of the seedbed and any with virus-like symptoms should be discarded (Broadbent, 1957).
Early roguing of any infected plants can reduce subsequent spread significantly (Caldwell and Prentice, 1942b).
In areas where CaMV is endemic and causes severe annual epidemics, the culture of an intervening immune crop can sometimes break the infection cycle.
Brassica cultivars that are resistant and/or tolerant to infection are available. If valuable Brassica breeding material becomes virus-infected, virus-free plants can be readily obtained by meristem tip culture (Walkey et al., 1974).
Although not yet available for commercial use, transgenic virus-resistant plants have been produced for experimental use.
Mild Strain Cross Protection
Mild strains of CaMV have been produced and shown to be potentially useful in protecting inoculated plants from the effects of severe virus strains (Tomlinson and Shepherd, 1978).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
When prevalent, CaMV can cause severe losses of quality and yield of Brassica crops (Caldwell and Prentice, 1942b; Broadbent, 1957); for example, the virus can cause 25-59% loss of marketable cauliflower heads in Brittany, France (Kerlan et al., 1989), and up to 60 and 90% of turnip and turnip rape fresh weight, respectively, in the Czech Republic (Spak, 1989a). The virus is considered to be of major importance in many countries worldwide (Tomlinson, 1987).