Plants affected
This disease occurs on most ornamental and edible Cruciferae including brassicas and wallflowers. Seedlings and young plants are the most susceptible to attack, although all stages of plant growth can be affected.
Symptoms
Yellow blotches appear on the upper leaf surface with corresponding greyish-white fungal growth on the lower surface. The infections can be limited by the leaf veins, giving a sectored appearance.
Upper leaf symptoms of brassica powdery mildew. Photograph copyright RHS Lower leaf symptoms of brassica powdery mildew. Photograph copyright RHS
If the disease is severe there can be extensive leaf yellowing, and young seedlings may even be killed. Downy mildew can also sometimes cause brown discolouration of cauliflower curds, and black internal spotting or streaking in radishes.
Cause
Brassica downy mildew is caused by the fungus Peronospora parasitica.
The disease is favoured by mild, damp weather and humid conditions. The fungal spores are spread by the wind, and depend on the presence of a film of water on the leaves to germinate and infect the plant.
Infection of young plants can also occur through the roots, from long-lived resting spores that contaminate the soil.
Control
Remove all infected leaves and destroy promptly, to prevent disease spread and the contamination of the soil by resting spores.
Improve air circulation around the plant by increased spacing and careful weed control. Ensure that related weed hosts, such as shepherd’s purse, are not present.
Maintain as long a rotation as possible to reduce the carry-over of resting spores.
There are no fungicides available to the amateur gardener to control downy mildew on this crop.
See also pansy downy mildew and lettuce downy mildew.
This disease can be confused with powdery mildew, which is caused by a different organism.
