Powdery Mildew in Grapes 2021
Typically, the first week of May begins setting the stage for powdery mildew in grapes. Yet this May, powdery mildew was unusually calm in The Central Valley of California. However, as we approached June 1 the story changed!
Some growers want to know:
- Hasn’t the weather been too hot for Powdery Mildew?
- Are my sprays working?
- What’s causing the powdery mildew?
- What can I do to “save” my crop?
To answer such questions we must understand what we are working with.
Powdery mildew (Erysiphe necator) is a fungus that overwinters inside infected buds. As the season begins these infected tissues begin to produce fruiting bodies called chasmothecia.
Growers try to suppress the chasmothecia with dormant and bud break sprays but many of these mycelia infection sites are hidden within plant tissues and thus untouched by the sprays.
As we move into the blooming cycle of our grapes, we begin to see evidence of the powdery mildew fruiting bodies expressing themselves as yellow lesions within leaf tissues and sometimes on the blooms. In wet Springs or cooler regions, the mildew activity observed may be Downy Mildew (Plasmopara viticola), however for The Central Valley California Grape Growers this is not a common fungus.
As evident in this chart, we see a powdery mildew spike show up near the end of April with a lull after May 4th. Two questions come to mind:
Question 1: What occurred on April 27th? Answer: The first grape flowers started to bloom.
Question 2: Why did these symptoms drop to zero after May 4th? Answer: Base leaves were removed to open up the canopy.
After May 4th we saw increased activity in the field such as Leafing, sulfuring, and various fungicide chemical applications which suppressed the spore pressure during most of May. The heat also played a role while the canopy was open and overhead growth was limited. However, as we approached June several events lined up:
- The canopies began to close.
- The last week of May was very cool.
- May 31st and the first week of June weather was very hot, too hot for sulfur.
- Irrigations were increased to compensate for the heatwave driving up humidity.
- Daytime shaded fruit had optimal powdery mildew conditions throughout much of the day and deep into the evenings.
As growers are faced with increasing regulations and restrictions, the grape industry is facing many challenges on such issues as powdery mildew control. For example, some of these regulations and restrictions drive growers to repeated spray applications using products that fit within the regulatory guidelines but are too similar in nature causing an increase in resistance of Powdery Mildew.
As a consumer, I like reduced pesticides. As a consultant, I see my clients making so many efforts to provide products that are healthy and as pesticide-free as possible. As growers, we know the crop and the pest or disease cycle. Many of us know and care deeply about the beneficial biological community above and below our soils. We know when and what to spray so that the bulk issues are suppressed long before the fruit is harvested and we make decisions that support bioactivity. As a field scout, I scratch my head at some regulation and marketing decisions.
When we take a closer look at the question “What can I do to save my crop?” we are faced with the realization that depends on what we can use to stop the spread of Powdery Mildew. Some mechanics must be considered.
- Spray coverage
- Spray timing
- Material choices
As a field scout, the number one issue I see is coverage. When the canopy is in a rapid growth cycle the coverage potential is reduced daily. It is critical to open the canopy and adust nozzles to maximize shaded area coverage. Spray every row, slow down, inspect rigs, and check your coverage.
Once the “coverage” is achieved, it is important to monitor infected berries. Healthy infections will appear white and show standing spore structures.
In the image above we see the Powdery Mildew spores standing and spreading across the surface of the berry. This is healthy Powdery Mildew.
In this image, we see some grey spores around the edge of the infection sites with folded white spores in the center of the infections. This is suppressed but not dead.
When the infections are suppressed but not controlled, I’ve seen grape varietals sensitive to Powdery mildew continue to spread throughout the months of June and July. As we move into August, these old infections begin to split and become infested with rot spores. Generally speaking, we need to control the Powdery Mildew within 3 to 6 days after finding berry activity. This means sulfur dust to suppress the spread every 5 to 7 days, wettable sulfur spray wash to kill infections if the weather is cool enough in the evening, night, or early morning. Leafing to expose the infections to the spray for better coverage. Contact wash with an alternative to sulfur if the weather is too hot. For some growers, this may be a product containing Potassium Bicarbonate. However, scarring can occur in sensitive varieties, especially when growers try mixing these types of costic materials with fertilizers, fungicides, or various spray mixes. It is critical that you speak with the manufacture representative and read and follow all label instructions for the product or products you choose.
The goal is to find high suppression after a fungicide spray. In the photo above the two infected berries show good kill along with the infection edge sites with high suppression to spores in the center of the infection. This is NOT a full kill. A full kill has NO white spores.
In the above image, the powdery mildew is dead, but if you look carefully you can find cracked berries with rot spores starting to form.
So to fully answer the question “What can I do to save my crop?” we must realize the fight is a week-by-week battle. If we grow tired, or distracted, we will not get a quality product in the box at harvest.
With that said, Keep Fighting My Friends!
