Downy Mildew: A Serious Disease of Brassicas
The downy mildew disease affecting brassicas is caused by a fungus-like water mould called Hyaloperonospora parasitica. It poses a significant threat to crops like cabbage, Brussels sprout, cauliflower, broccoli, kale, Chinese cabbage, turnip, radish, and mustard, as well as cruciferous weed species. This disease is particularly problematic for young plants, but it can also lead to stunted growth and reduced crop yield and quality in later stages.
Understanding the Biology
The fungus produces oospores that form in aging and dying leaves. These oospores can survive in soil debris, allowing the fungus to persist from one crop to another. The disease can also survive on seeds and certain weed species. In seedling nurseries, the fungus often remains on slightly infected seedlings.
The Spread of the Disease
The primary mode of dispersal for the fungus is through its white powdery spores. Once established within seedling crops, these spores play a crucial role in spreading the disease. They are produced overnight and released the next morning as the air dries out. Wind and rain splash facilitate their dispersal.
Favorable Conditions for Infection
Cool temperatures between 8-16°C provide an optimal environment for the disease to thrive. The vegetative spores require water to germinate and can infect seedlings within three hours of coming into contact with a leaf. Infection occurs when sporangia, produced on living hosts, come into contact with leaves and inflorescences. Secondary sporangia, on the other hand, spread through wind and splashing water.
The initial signs of downy mildew appear as fluffy or powdery-white masses of spores on the undersides of brassica seed leaves, specifically the cotyledons. This is followed by black speckling and puckering on the upper surface of the leaves. Infected leaves turn yellow prematurely and fall off the plants. Mature crops in the field usually exhibit symptoms on the lower leaves, where the fungus produces spores on the undersides and brown to black, sunken, angular lesions on the upper surfaces. Infected heads deteriorate rapidly, rendering them valueless.
Additionally, cabbage heads may develop black lesions, and cauliflower curds can break down during storage. Infected tissues become vulnerable to attack by other rotting organisms. Taproots of turnip and radish can also be affected, resulting in black epidermal blotches and internal discoloration.
Effective Disease Management
To successfully manage the disease, it is crucial to implement preventive measures before symptoms appear. Here are some approaches for controlling downy mildew:
Systemic and translaminar fungicides that specifically target oomycete fungi are recommended for effective disease control. These fungicides should be applied every 7-14 days, depending on the severity of the disease. To prevent the development of fungicide resistance, it is advisable to alternate fungicides from different classes throughout the crop season.
Recommended fungicides for downy mildew management in brassicas include:
- 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)
Implementing non-chemical methods alongside chemical control can enhance disease management. Consider the following practices:
- Rotate crops with non-brassica species.
- Use disease-free planting materials, such as transplants.
- Remove crop debris and weed hosts to reduce the level of inoculum.
- Avoid excessive overhead irrigation and opt for drip irrigation if possible.
- Improve air circulation and water seedlings early in the day to manage the disease in seedling beds.
- Choose resistant or tolerant cultivars.
- Maintain a balanced nutrition program, as deficiency in certain nutrients can increase seedling susceptibility to downy mildew.
- Eliminate potential sources of spores, such as heavily infected trays of seedlings, old infected seedlings, and weedy cruciferous weeds.
- Select planting sites with good air movement and minimal shading.
- Regularly inspect crops for symptoms.
To enhance the effectiveness of fungicides:
- Always mix the fungicide with INTEGRA (3ml/20l), a sticker, spreader, and penetrant that improves efficacy.
- Timely disease control is essential to minimize or prevent losses attributed to infection.
- When using pesticides, follow the instructions on the product label regarding dosage, timing of application, and pre-harvest interval. Additionally, wear appropriate protective clothing.
Downy mildew poses a serious threat to brassica crops, but by implementing effective disease management strategies, such as using recommended fungicides and practicing non-chemical approaches, it is possible to minimize its impact. Early prevention and timely control measures are crucial to safeguard the health and productivity of brassica crops.