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Disease Management

Cultural control

Cultural practices such as field selection, stubble management, control of volunteer crop plants and other susceptible hosts such as certain weeds, and pathogen-free seed can reduce the incidence of blackleg. Because the pathogen survives on stubble, it is important to avoid planting oilseed rape in fields that have stubble from a previous susceptible crop. This requires rotating away from oilseed rape for three to four years (longer in drier climates). There should be a minimum distance of 500 meters (yards) between oilseed rape crop residue and oilseed rape crops planted in fields without a history of the disease or where a rotation is used. This distance will significantly reduce infection by windborne ascospores from stubble residues.

Stubble residues can be destroyed by burning or by deep plowing, however burning is not allowed in many areas. Incorporation of residues hastens decomposition by soil microorganisms and helps to prevent discharge of ascospores into the air. Drier regions have more carryover of crop residues from year to year because moisture is a limiting factor in decomposition.

Although the pathogen is seedborne, incidence of seed contamination is generally very low. The use of pathogen-free seed reduces initial inoculum, especially when planting in new areas or areas that have a low incidence of the disease. Most certified seed fields are inspected by scouting to determine that disease is not present rather than by direct testing of the seed. Some government labs in Canada provide a testing service to determine if seed is contaminated by the pathogen.

Fungicides

Fungicides have been used to manage blackleg, either as seed or fertilizer dressings or as foliar applications. In Australia, fungicide is used to coat super phosphate fertilizer and then to the crop in a furrow application. Application of fungicides to seeds is usually more economically feasible than foliar applications. Seed treatments with benzimidazole, dicarboximide and morpholine fungicides can almost eliminate seed transmission of the pathogen. To prevent the introduction of the pathogen to noninfested areas, the planting of certified seed treated with a fungicide is recommended. Early control of blackleg reduces the likelihood of the damaging stem canker phase. Successful control through foliar applications, primarily to seedlings to prevent initial infections, has been reported using benzimidazole, carboxamide, and dicarboxamide fungicides. Application of fungicides or herbicides to stubble for suppression of pseudothecia formation has been tested experimentally, but this approach is not used widely because of economics, erratic performance, and the planting of disease-resistant cultivars in many areas.

Genetic resistance

Genetic resistance is the most important and sustainable means of controlling blackleg. The genetic basis of much of the resistance, however, is not well understood in oilseed rape. Genetic strains of the fungus that vary in host range and in virulence on a particular host species have been observed. Some resistant cultivars are available, but cultivar choice is dependent on which strains of the fungus are present in a particular area.

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