The downy mildew of brassicas is a serious disease caused by the fungus-like water mould Hyaloperonospora parasitica.
The disease occurs and spreads rapidly wherever brassica crops are grown and infects cabbage, Brussels sprout, cauliflower, broccoli, kale, Chinese cabbage, turnip, radish, and mustard as well as cruciferous weed species.
Although it is particularly important on seedlings, it can also cause poor growth and reduced yield and quality of produce at later crop stages.
Oospores of the fungus are formed in ageing and dying leaves, and survive in debris in the soil from one crop to the next. The fungus is known to survive on seed and on several species of crucifer weeds and in seedling nurseries the fungus often survives on old, slightly-infected seedlings.
The white powdery 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.
The disease is favoured by cool temperatures with an optimum range of 8-16°C. The vegetative spores need water to germinate and can infect seedlings within three hours of contact with a leaf.
Infection of leaves and inflorescences results from sporangia produced on living hosts while secondary sporangia are spread by wind and splashing water.
Downy mildew is initially seen as a fluffy or powdery-white mass of spores on the undersurface of brassica seed leaves i.e., the cotyledons. This is followed by a black speckling and puckering of the upper surface. The infected leaves prematurely yellow and fall from the plants.
Symptoms on mature crops in the field are generally confined to the leaves closest to the ground. The fungus produces spores on the undersurface, and brown to black, sunken, angular, pepper-like lesions on the upper surface of leaves.
Infected heads deteriorate rapidly and the value of the head is ruined.
The fungus can also cause black lesions on cabbage heads and breakdown of cauliflower curds in storage.
The affected tissues become susceptible to attack by secondary rotting organisms.
Downy mildew also attacks the taproots of turnip and radish and infected organs develop a black, epidermal blotch and an internal discoloration.
The disease spreads very fast and thus a successful disease management strategy should be implemented prior to the appearance of the disease symptoms.
The following approaches are used to control the disease;
For a successful control of the disease, systemic and translaminar fungicides with active ingredients that target oomycete fungi are recommended.
These fungicides should be applied every 7-14 days depending on disease severity.
Since fungicide resistance is a major concern with downy mildew pathogens, fungicides from different classes should be alternated throughout the crop season.
The following fungicides are recommended for use in management of downy mildew of brassicas;
- CADILAC 800WP 50g/20l (for prevention purposes)
- FORTRESS GOLD 720WP 40g/20l
- GEARLOCK TURBO 250WP 25g/20l
- SACRIFIDO 125EC 20ml/20l
- TOWER 720WP 50g/20l
- ABSOLUTE 375SC 10ml/20l
- PYRAMID 700WP 50g/20l
- TRINITY GOLD 452WP 50g/20l
- KATERINA 720SC 50ml/20l
- PROPELLER 722SL 50ml/20l
- COMRADE 450SC 20ml/20l
- Practice rotation with non-brassica crops.
- Use disease free planting materials, e.g. transplants
- Remove crop debris and weed hosts as this reduces inoculum level.
- Avoid excess overhead irrigation and consider irrigating during the late morning to facilitate rapid leaf drying. Where possible, use drip irrigation.
- Manage the disease on transplants in the seedling bed by improving air circulation and irrigating early in the day.
- Plant resistant or tolerant cultivars.
- Maintain a balanced program of nutrition. For instance, deficiency of potash increases the susceptibility of seedlings to downy mildew.
- Remove any source of spores, e.g. heavily infected trays of seedlings, old infected seedlings, and weedy crucifer weeds.
- Choose planting sites with good air movement and without shading.
- Inspect crops for symptoms regularly
- Always mix the fungicide with INTEGRA 3ml/20l, a sticker, spreader and penetrant which improves the efficacy of the fungicide.
- Ensure a timely control of the disease in order to reduce or prevent losses attributed to the disease infection.
- When using a pesticide, always wear protective clothing and follow the instructions on the product label, e.g. Dosage, timing of application, and pre-harvest interval.