Downy mildew of Cucurbits.
Causal Organism
Scientific Name: *Pseudoperonospora cubensis*
Classification:
- Kingdom: Chromista
- Phylum: Oomycota
- Class: Oomycetes
- Order: Peronosporales
- Family: Peronosporaceae
Symptoms
1. Foliar Lesions:
- The disease primarily affects the leaves.
- Initial symptoms are small, yellow, angular spots on the upper leaf surface, often confined by the leaf veins.
- As the disease progresses, these spots enlarge, become brown or necrotic, and may merge, leading to large areas of dead tissue.
2. Underside of Leaves:
- A distinguishing characteristic is the presence of a downy, grayish to purplish fungal growth on the underside of the leaves beneath the yellow spots.
- This downy growth consists of the sporangia and sporangiophores of *P. cubensis*.
3. Leaf Curling and Death:
- In severe cases, leaves can curl, die, and drop prematurely.
- Defoliation can lead to reduced photosynthetic capability and poor fruit development.
Etiology
Pathogen :
- *Pseudoperonospora cubensis* is an obligate parasite, meaning it requires living host tissue to survive.
- It produces sporangia (asexual spores) that can be dispersed by wind or water.
Environmental Conditions:
- Optimal conditions for the development and spread of downy mildew include high humidity (above 85%) and moderate temperatures (15-25°C or 59-77°F).
- Wet conditions, such as those caused by dew, rain, or overhead irrigation, facilitate the germination of sporangia and infection of host plants.
Disease Cycle
1. Primary Inoculum:
- The primary source of inoculum is typically sporangia produced on infected plants from the previous growing season.
- These sporangia can be spread by wind over long distances or by water splash.
2. Infection:
- Sporangia land on susceptible leaf surfaces, germinate, and penetrate the leaf tissue, often through stomata.
- The pathogen then grows intercellularly within the leaf tissue, extracting nutrients from host cells.
3. Sporulation:
- Under favorable conditions, new sporangia are produced on the undersides of infected leaves within 4-12 days after initial infection.
- These sporangia can then disperse to new leaves, repeating the cycle.
4. Survival:
- *P. cubensis* overwinters in regions with milder climates where cucurbits or related wild hosts remain green year-round.
- In cooler regions, the pathogen may survive in greenhouse settings or be reintroduced each growing season via contaminated seed or transplants.
Management
1. Cultural Practices:
- Use resistant or tolerant cultivars when available.
- Implement crop rotation with non-cucurbit crops to break the disease cycle.
- Avoid overhead irrigation to minimize leaf wetness and humidity.
- Ensure good air circulation within the crop canopy by proper spacing and pruning.
2. Chemical Control:
- Fungicides are a key component in managing downy mildew. Effective fungicides include chlorothalonil, mancozeb, and copper-based products.
- Systemic fungicides, such as those containing active ingredients like azoxystrobin, can also be effective.
- Follow local guidelines for fungicide use, including rotation of fungicides with different modes of action to prevent resistance development.
3. Monitoring and Early Detection:
- Regularly scout fields for early signs of downy mildew.
- Implement a disease forecasting system if available, which can help predict disease outbreaks based on weather conditions and guide fungicide application timing.
4. Sanitation:
- Remove and destroy infected plant debris after harvest.
- Clean and disinfect tools and equipment to prevent the spread of the pathogen.
5. Biological Control:
- Research on biological control agents is ongoing, and some beneficial microbes may help suppress *P. cubensis*.