Downy mildew in grapevines usually thrives under cool, wet conditions, rapidly infecting leaves and clusters with an olive-gray mold. Severe infections can destroy entire crops if left unmanaged.
Over the years, I've tested various integrated approaches to find the most practical and effective strategies for my vineyard. In this article, I'll share techniques I've developed for successfully minimizing downy mildew risk without over-relying on fungicides alone.
What is Downy Mildew? Characteristics, Symptom, and Bad Effect to Grapevines
To effectively combat downy mildew, we must first understand its biology and life cycle. Only then can we deploy targeted monitoring and controls. In this article, I'll provide an overview of this disease's identifying traits, Latin name, and the damage symptoms it inflicts. Armed with this knowledge, growers can more accurately detect and respond to infestations.
Fungal Description and Latin Name
Downy mildew of grapes is caused by several species of oomycete fungi in the Plasmopara viticola genus. These microbes produce asexual spores called sporangia and sexual spores called oospores that disperse via wind, rain and irrigation to initiate infections across wide areas.
Typical Damage Symptoms
Symptoms aren't always obvious, but knowing what to look for helps detect infestations early:
- Grayish-blue or purple spots on leaves undersides with fuzzy white growth.
- Leaves yellow, curl or distort as fungus saps photosynthesis.
- Premature defoliation weakens vines and reduces ripening.
- Soft, downy mold on flower clusters and young berries causes abortion.
Regular scouting helps spot these subtle cues before extensive crop damage occurs. Early detection is key to an effective response.
Disease Cycle and Peak Risk Periods
Downy mildew fungi overwinter as oospores in plant debris or in alternative hosts. When temperatures favor disease, airborne sporangia directly infect susceptible tissue, rapidly repeating cycles of spore production. Primary infection periods coincide with spring and fall under cool, wet conditions.
Physiological Damage to Grapevines
By colonizing leaves, young fruit clusters and berries, downy mildew blocks sunlight penetration and gas exchange, severely limiting photosynthesis. This starves vines of energy for growth and fruit production. Infected leaves also prematurely senesce, further weakening the plant.
Fungal toxins and cell-wall degrading enzymes cause chlorosis, necrosis and defoliation. On fruit, dense mold growth destroys clusters, rendering grapes unmarketable. Heavily infected vines are often severely stunted or killed outright by multiple infection cycles in a season.
Economic Losses for Growers
Yield reductions from downy mildew translate directly to financial losses. Research shows infections can decrease grape harvests by 30-100%, slashing potential profits significantly. Replanting failed acres drives up costs.
Effective Prevention and Pest Control for Downy Mildew in Grapevines
1. Choose Resistant Varieties
The foundation of downy mildew management starts with selecting cultivars that show some level of tolerance or resistance to the disease. While there are no varieties completely immune to downy mildew, certain options have proven more durable on my farm over the years compared to highly susceptible types. 'Traminette' and 'Vignoles' are two varieties that have performed relatively well versus other varieties that seem to always struggle, like 'Chardonel.'
These resistant varieties may still require fungicide applications in high pressure years, but they avoid the severe defoliation and crop loss that can occur on untolerant types. When purchasing new vines, I work closely with my supplier to identify varieties that have shown resistance in our region and soil/climate conditions based on university trials and grower experience.
2. Adjust Planting Densities and Pruning
Pruning and canopy management practices that promote good air flow are another crucial aspect of an integrated downy mildew program. In my vineyard, I have found that looser cordons and spur pruning that results in a more open, vase-shaped canopy significantly reduces disease pressure compared to dense, wall-trained systems. This open architecture allows drying winds to penetrate into the leaf layer more readily, hastening the drying of inoculum in wet seasons.
Appropriate vine spacing is also important, as overcrowded rows impede airflow and create a microclimate ideal for downy mildew proliferation. While high-density systems maximize production per acre in the short term, in my experience the added disease risk often outweighs the incremental yield benefit.
3. Monitor Weather Forecasts
Knowing when environmental conditions become conducive for downy mildew is crucial to properly time preventative applications. In my region, primary infection periods tend to coincide with extended leaf wetness from rain or heavy dew under temperatures between 70-80°F in the spring and fall seasons. I have found that monitoring local weather forecasts, especially from sources that provide accurate leaf wetness predictions, enables me to be proactive instead of reactive.
4. Fungicide Options
When disease pressure warrants, I have found targeted applications of protectant fungicides to be highly effective against downy mildew if properly timed. In particular, products containing phosphite or dimethomorph as the active ingredient tend to perform well and provide 2-3 weeks of protection under most conditions in my region.
To maximize efficacy, I strive to apply these materials during bloom and the early stages of fruit set, as downy mildew can cause the most damage during grape cluster development. Reapplying again before any forecasted infection periods ensures continuous protection of new growth. Additionally, fungicides with different modes of action are alternated throughout the season to avoid the development of resistance over time.
Conclusion
With diligence across these integrated strategies, downy mildew can be successfully managed on grape farms season after season. Monitoring conditions and deploying the right controls provides the best long-term success.
Post a Comment