Downy mildew on cabbage (Peronospora parasitica)
Downy mildew is caused by a fungus which mainly attacks foliage. It is responsible for substantial losses in seedling production, especially during the cooler months. Young cabbage plants are more susceptible than older plants. The powdery white spores are the major means of dispersal of the fungus, especially once it is established within seedlings crops. Spores are produced overnight and released the following morning as the air dries out. They are dispersed by wind and rain splash. Downy mildew disease development is favored by cool, moist conditions.
Downy mildew disease is first seen as a fluffy or powdery-white mass of spores on the undersurface of brassica seed leaves (cotyledons). This is followed by a black speckling and puckering of the upper surface. Leaves prematurely yellow and fall from the plants.
Symptoms on mature plants in the field are generally confined to the leaves closest to the ground. The fungus produces spores on the under surface, and brown to black, sunken, angular, pepper-like lesions on the upper surface of leaves. The fungus can cause black lesions on cabbage heads and breakdown of cauliflower curds in storage.
- Use seed treated with hot water or seedlings raised from such treatment.
- Plant disease-free seedlings.
- Do not water seedlings in the morning when spores are released.
- Keep seedlings as dry as possible. One heavy watering is preferable to a long light watering.
- Maintain a well-ventilated environment - a lower relative humidity minimises spore production. This may mean fewer plants and trays per square meter.
- Maintain a balanced program of nutrition - deficiency of potash will increase the susceptibility of seedlings to downy mildew.
- Remove heavily infected seedlings, old infected seedlings, and weedy crucifer weeds.
- Plough in field crop debris immediately after harvest and use a crop rotation of two to three years if possible.
Peronospora parasitica
