What is downy mildew?

Downy mildew of brassicas is a disease of seedlings and also mature plants. It is caused by a fungus-like (Oomycete) organism, Hyaloperonospora brassicae (synonym Peronospora parasitica subsp. brassicae), that penetrates the tissues under wet conditions and grows out to produce fuzzy whitish patches of growth on the underside of the leaf. This growth releases spores that are carried by wind and rain to infect other brassicas. Expect damage in spring in young plants (especially in greenhouses and other propagation environments) and in summer and early autumn in maturing crops outdoors.

Hyaloperonospora brassicae affects vegetables including cabbages, cauliflowers, Brussels sprouts, radishes, swedes and turnips. Some ornamental and wild relatives, including horseradish, Cheiranthus (wallflowers), Matthiola (stocks), Aubrieta (aubretia) and Capsella bursa-pastoris (shepherd’s purse) are also affected by downy mildew, although in some cases this has now been shown to be caused by different Hyaloperonospora species.

Brassica downy mildew often occurs together with brassica white blister (Albugo candida), another foliar pathogen.

Symptoms

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

Control

Non-chemical control

• Plant brassicas with wider spacings to encourage air flow
• Remove wild/weed host plants to reduce any chance of cross-infection
• Remove infected material promptly and destroy, to avoid contaminating the soil with resting spores
• Practice crop rotation

Chemical control

No fungicides are available to amateur gardeners for treating downy mildews.

Biology

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 there will be brassicas (or their wild relatives) of some type in leaf throughout the year, 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.