Downy mildew of brassicas is a foliage disease causing whitish, fuzzy patches on the undersides of leaves and yellow discolouration on the top. It affects seedlings and mature plants.
What is downy mildew?
Downy mildew of brassicas is a disease of seedlings and also mature plants. It is caused by a fungus-like organism, Hyaloperonospora brassicae (synonym Peronospora parasitica subsp. brassicae (Gaumann)), that penetrates the tissues under wet conditions and grows out to produce fuzzy whitish patches. 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).
Brassica downy mildew often occurs together with brassica white blister (Albugo candida), another foliar pathogen.
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
- Plant brassicas with wider spacings to encourage air flow
- Remove wild host plants like shepherd’s purse
- Remove infected material promptly and destroy, to avoid contaminating the soil with resting spores
- Practice crop rotation
No fungicides are available to amateur gardeners for treating downy mildews.
Downy mildews are a large group of plant diseases caused by microscopic fungus-like organisms related to the pathogen that causes tomato and potato blight. Despite a similar name and certain similarities in symptoms, they are unrelated to the powdery mildews.
The downy mildew pathogen produces resting spores in plant debris which carry it through winter. These can infect via the roots, sometimes causing a systemic infection (one that spreads throughout the plant). However, since brassicas and their wild relatives are in leaf all year round, the pathogen is always present on green tissue and these infections provide a more important source of infectious spores whenever suitably wet conditions occur. Spores are released from the fuzzy fungal growth on the lower leaf surface and are spread by wind and water splash. Infections in mature leaves are vein-limited, giving rise to an angular shaped lesion.
Infections are favoured by warm, wet conditions. These also favour infection by Brassica white blister (Albugo candida), another foliar pathogen of brassicas and the two often therefore occur together.
The downy mildews are described as biotrophic, meaning that they can only live and grow in association with living plant tissues, during which time they disperse infective spores. When the weakened plant tissues die the pathogen also dies, or forms a dormant resting spore.
Common name Brassica downy mildew
Scientific name Hyaloperonospora brassicae synonym Peronospora parasitica subsp. brassicae (Gaumann)
Plants affected Brassicas and ornamental and wild relatives
Main symptoms Whitish, fuzzy patches on the undersides of leaves
Caused by Fungus-like organism
Timing Late spring to early autumn