A large number of deficiency symptoms will appear in the crop as a consequence of a lack in the availability of nutrients. A crop that is not healthy will have poorer defence mechanisms and less tolerance against fungi, insects and abiotic stress, affecting the overall crop growth and quality. Oilseed rape shares many of the same diseases and pests with other members of the brassica family.
Sulphur influences, not only the quantity of proteins but also the quality through the synthesis of sulphur-containing amino acids (cysteine and methionine), these amino acids are also the precursors of co-enzymes and glucosinolates. The latter are essential since they contribute to the oilseed rape’s resistance to disease and parasites although too high a content renders the oilcake unsuitable for consumption by monogastric animals. The current “00” varieties have glucosinolate contents between 8 and 15 μmoles/g of seed.
Pathogens represent a threat to the crop, but the pressure from these organisms can be reduced by not too narrow crop rotation. The rate of infestations decreases considerably with rotations from 3 (17%) to 6 (33%) years.
Clubroot is an increasing problem in oilseed rape and close rotations have exacerbated this problem. Plants can wilt in hot, dry weather, be very stunted and might die out completely.
Clubroot is caused by a soil-borne fungus and affects all brassica species. It infects root tissues, stimulating abnormal growth of affected parts. This clubbed tissue significantly restricts growth and can also release spores which can be transferred to other fields.
Within the soil, these spores can last over ten years, infecting any subsequent brassica crop in the rotation. While some fungicides are available, these are limited in activity and not available in many countries. Resistant varieties are also limited in their availability to resist all clubroot variants.
Liming to maintain a pH above 7.2 as part of an integrated control programme that utilises calcium-based fertilisers will help to suppress the pathogen.
Good rotations (every 7 years) between Brassica crops (oilseed rape and vegetable brassicas); practices that minimize the spread of infected soil; better drainage and control of all brassica family weeds are all recommended methods of minimizing clubroot.
Sclerotinia stem rot also called stalk break or white mould can be one of the most destructive diseases in oilseed rape. The severity of the attacks varies from year to year depending on the weather, but it also varies from region to region or even from field to field. As the scale of production of oilseed rape has increased, sclerotinia has become a more serious problem. The reason can be more narrow rotations, and/or denser canopies. Humid weather makes sclerotinia thrive in the crop.
The infections of sclerotinia occur when the oilseed rape plants are in bloom. The fungus is soil-borne and has a large range of hosts that includes more than 75 broad-leaf plant families. The result of sclerotinia infections is premature ripening and crop lodging, leading to a general loss in production.
Excess nitrogen fertilisation will produce a lush, humid plant canopy that will favour sclerotinia in addition to delay crop maturity. Soil analyses are necessary to test for soil nutrients and apply only the necessary amount of nitrogen fertiliser.
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