Alternaria
Alternaria brassicae and Alternaria brassicicola
Pathogen
Fungus
Hosts
Oilseed Rape, brassica crops and cruciferous weeds
Symptoms
Leaf symptoms of Alternaria can be found from the seedling stage onwards. Initial symptoms are small black spots about 1mm in diameter. These lesions develop into characteristic 'target-spots', 5mm to 15mm in diameter, with light and dark concentric rings. Secondary spotting occurs around the target spots and if left unchecked can cause leaves to turn brown and fall off.
Development
Infections arise from infected seed or air-borne spores from infected stubble or neighbouring brassica crops throughout the spring and summer.
In the spring and summer disease development becomes more active. There will often be reinfection and transfer to uninfected plants via airborne spores released from 'target-spots'. Infected pods may be smaller, ripen earlier and shatter before harvest. This often occurs in patches scattered throughout the crop. Very rapid spread can occur during thundery weather.
The greatest impact on yield is caused when Alternaria spreads to pods causing premature ripening, pod splitting and loss of seed. The pathogen can penetrate the pods and infect the seed within. Problems are often associated with early lodging.
Favourable Factors
Optimum conditions for infection are 17 to 25°C and spores can infect within four hours of landing on a wet leaf at 22°C
Importance
Compared to the 1980`s, damaging attacks of Alternaria have become less common in recent years. The widespread use of triazole fungicides in autumn and winter gives incidental control and has made it a relatively minor disease. However, if warm wet weather coincides with the flowering period, it can still be economically damaging.
Yield losses of up to 50 % have been recorded as a result of severe disease pressure.
Control
Separate oilseed rape crops from other brassica crops
Destroy or dispose of stubble as soon as possible after harvesting.
Seed treatments
Correct choice and timing of foliar fungicides
There is no known varietal tolerance
