Downy Mildew of Grapevine
Symptoms
- Initial Symptoms: Yellowish, translucent, or oily spots (often called "oil spots") appear on the upper surface of young leaves. These spots later turn brown.
- Advanced Symptoms: Infected areas on leaves develop a white, downy mildew on the underside, consisting of fungal sporangia. This white growth is most prominent under humid conditions.
- Other Plant Parts: Downy mildew can also affect young shoots, inflorescences, and berries. Infected shoots may curl and become distorted. Infected berries become soft and eventually shrivel, showing a grayish-brown appearance.
- Defoliation: Severe infections lead to premature defoliation, reducing the plant’s ability to photosynthesize and weakening the vine.
Etiology
- Causal Agent: Downy mildew is caused by the oomycete *Plasmopara viticola*.
- Host Range: Primarily affects grapevines (Vitis spp.).
- Environmental Conditions: Favorable conditions for infection include warm temperatures (optimal at 20-25°C) and high humidity or wet conditions. Rainfall and heavy dew are particularly conducive to the spread of the disease.
Disease Cycle
1. Primary Inoculum: The pathogen overwinters as oospores in fallen leaves and other plant debris on the ground.
2. Germination: In spring, oospores germinate to produce zoospores when temperatures rise above 10°C and there is sufficient moisture.
3. Primary Infection: Zoospores are dispersed by rain splash to young grape leaves, where they penetrate through stomata.
4. Secondary Inoculum: Inside the leaf, the pathogen develops intercellular mycelium and forms sporangia, which are dispersed by wind and rain to infect new tissues. This can lead to repeated cycles of infection throughout the growing season under favorable conditions.
5. Disease Progression: Under ideal conditions (warm and humid), the pathogen can produce new generations of spores every 5-10 days, leading to rapid disease buildup and spread.
Management
- Cultural Practices:
- Sanitation: Remove and destroy fallen leaves and pruned debris to reduce overwintering inoculum.
- Canopy Management: Prune and train vines to improve air circulation and reduce humidity within the canopy, making conditions less favorable for disease development.
- Irrigation Practices: Avoid overhead irrigation to reduce leaf wetness duration.
- Chemical Control:
- Fungicides: Use protective fungicides (e.g., copper-based compounds, dithiocarbamates) and systemic fungicides (e.g., fosetyl-Al, metalaxyl) according to a regular spray schedule, particularly during periods of high risk (wet and warm conditions).
- Resistance Management: Rotate fungicides with different modes of action to prevent the development of resistant strains of *Plasmopara viticola*.
- Biological Control:
- Biocontrol Agents: Research into biocontrol agents such as antagonistic fungi and bacteria is ongoing, though not yet widely adopted.
- Resistant Varieties:
- Breeding and Selection: Plant resistant grapevine varieties if available. Many modern cultivars are being developed with improved resistance to downy mildew.
- Monitoring and Forecasting:
- Disease Forecasting Systems: Use disease forecasting models and weather monitoring to predict infection periods and optimize the timing of fungicide applications.
- Regular Scouting: Monitor vineyards regularly for early signs of disease to initiate control measures promptly.
Powdery Mildew of Grapevine
Symptoms:
1. Early Symptoms:
- White, powdery fungal growth appears on the upper surface of leaves.
- Leaves may show chlorotic spots (yellowing).
2. Progressive Symptoms:
- The powdery growth can cover both sides of the leaves.
- Infected areas may turn brown or black, especially on young shoots.
- Shoots and tendrils can become distorted and stunted.
- Infected young berries become covered with white mycelium, may split, and shrivel.
- Older berries develop a netlike scarring, known as "russeting."
3. Advanced Symptoms:
- Severe infections lead to defoliation.
- Fruit quality and yield are significantly reduced.
- Affected vines exhibit reduced vigor and productivity.
Etiology:
- Causal Agent: Powdery mildew of grapevine is caused by the fungus *Erysiphe necator* (formerly known as *Uncinula necator*).
- Fungal Characteristics: The fungus produces asexual spores called conidia on the surface of infected tissues, which give the characteristic powdery appearance.
Disease Cycle:
1. Overwintering:
- The fungus overwinters in dormant buds as mycelium or as chasmothecia (sexual spores) on the bark of the vine.
2. Primary Infection:
- In spring, as temperatures rise (above 10°C or 50°F), the overwintering mycelium or chasmothecia release conidia, which are spread by wind and rain to young, susceptible grape tissues.
3. Secondary Infection:
- Conidia germinate on the surface of leaves, shoots, and berries, forming new mycelium.
- These infections produce more conidia that can further spread the disease throughout the growing season.
4. Reproduction:
- The fungus reproduces both sexually and asexually. Asexual reproduction via conidia is responsible for rapid disease spread during the growing season.
- Sexual reproduction results in chasmothecia, which contribute to the overwintering of the fungus.
Management:
By integrating cultural, chemical, biological, and monitoring strategies, growers can effectively manage powdery mildew in grapevines, ensuring healthy vines and high-quality fruit production.
1. Cultural Practices:
- Canopy Management: Ensure good air circulation by proper pruning and training of vines to reduce humidity within the canopy.
- Sanitation: Remove and destroy infected plant debris and prune out diseased shoots.
- Variety Selection: Plant resistant or less susceptible grape varieties if available.
2. Chemical Control:
- Fungicides: Regular applications of sulfur, potassium bicarbonate, or synthetic fungicides (e.g., myclobutanil, triadimefon) can effectively manage powdery mildew.
- Spray Timing: Begin fungicide applications at bud break and continue at regular intervals, especially during periods of high humidity and moderate temperatures.
3. Biological Control:
- Biological Agents: Use of biocontrol agents such as *Ampelomyces quisqualis*, a mycoparasite of powdery mildew, or other beneficial microorganisms that inhibit the growth of *Erysiphe necator*.
4. Monitoring and Forecasting:
- Disease Prediction Models: Use weather data and prediction models to determine the optimal timing for fungicide applications.
- Regular Scouting: Inspect vineyards regularly for early signs of powdery mildew to ensure timely intervention.