Downy mildew of Brassicas (Brassica downy mildew)
Downy mildew of brassicas is a disease of seedlings and also mature plants.
These infections in turn release spores and spread to other brassicas by wind and rain. Expect damage in spring in young plants especially in greenhouses and other propagation environments and in summer and early autumn in maturing crops outdoors.
This disease is specific to brassicas; both vegetables, including: cabbages, cauliflowers, Brussels sprouts, radishes, swedes and turnips; and some ornamental and wild relatives, including: horseradish, Cheiranthus (wallflowers), Matthiola (stocks), Aubrieta (aubretia) and Capsella bursa-pastoris (shepherd’s purse).
You may see the following symptoms:
On seedlings: The entire plant becomes covered in a whitish coating of the pathogen and dies rapidly
On larger plants: Yellowish patches of discolouration on the upper surfaces of leaves, often angular and limited by veins. On the corresponding lower surface is a fuzzy whitish outgrowth of the pathogen. Eventually, the affected tissues die, shrivel and may drop out
On cauliflower curds and radishes: Leaf damage may be accompanied by internal browning of the cauliflower curds and radish flesh