DM is caused by the fungus Peronospora parasitica, which infects seedlings, leaves, flowering stems and seed pods.
On seedling leaves, DM shows up as black dots and leaf yellowing and reddening, with purplish grey sporulation on the underside of leaves. On older leaves, DM is shown as lesions with leaf yellowing, often accompanied by black speckling. Affected leaves die off prematurely.
DM can greatly affect feed quality and yield.
DM can remain in the soil as free living spores or in trash. In spring, these spores infect leaves, with rapid build up in mild, moist weather conditions. DM can also be introduced via infected seed.
Prevention and management
Sowing clean seed minimises infection, along with thorough cultivation and good weed control. Good crop rotations are also essential, especially in areas with high levels of DM present. If infection does occur, grazing affected plants does limit the spread of the disease within the crop.