by johnah on November 9, 2020
What causes grape powdery mildew?
Grapes are susceptible to powdery mildew because they have high sugar content. Sugar makes them prone to disease and infection. For example, if the soil is too dry or there is not enough moisture in the air, then it will cause diseases like black spot and rust fungus. If there is no sunlight, then it will make the fruit rot easily.
The problem with powdery mildew is that it does not affect all varieties of grapes equally. Some types are more resistant than others. There are many factors which influence the susceptibility of a variety to powdery mildew. These include its age, size, shape, location within the vineyard and other factors such as climate conditions and even the type of soil in which it grows.
Powdery mildew is a fungal disease which affects the leaves, stems and fruit of the plant. It can spread from one part of the vineyard to another through wind and water runoff. Puffiness of the affected area indicates that spores have been carried into it by rainwater or spray during a downpour.
Symptoms of powdery mildew on grapes include yellowing, wilting and death of some parts of the fruit. These can be mistaken for signs of drought stress. Infected areas may turn brown, crack or become corky. The fungus itself appears as a white, powdery dust on the outside of the fruit and on the surface of the leaves.
Powdery mildew thrives in cool, damp conditions with high humidity such as in foggy mornings. Infection is most likely to occur between 2-7 am and again in the late evening. It is less common in dry weather. The fungus spreads by windblown spores, which can settle in moist pockets of soil.
Best Fungicide for powdery mildew on grapes
Initially, you should use a copper fungicide. This is not toxic to humans, but it can affect other wildlife that eats the grapes. Copper does not remain effective for very long, so you may need to reapply it after several weeks.
A second option is a product called Maneb. It is less toxic to humans and other wildlife than the copper-based fungicide. It can remain active on the leaves for up to two months.
Or you can switch to a different type of fungicide called chlorothalonil. It is the most effective of all the fungicides for powdery mildew. The downside is that it is very toxic to bees and aquatic life, so you don’t want to use it in an area where it can run off into rivers or streams.
Other options include mancozeb, sulfur or a copper-ARS mixture.
Organic treatment for powdery mildew on grapes
If you want to go the organic route, neem oil shows some promise. It can be applied every few weeks during the growing season. It is harmless to humans and wildlife, but it breaks down fairly quickly in sunlight. The disadvantage of a product like this is that it must be applied frequently and it does not last as long as the chemical treatment options listed above.
It can take several weeks before you see results when using an organic fungicide. In some cases, you may have to reapply it two or three times to obtain the desired effect.
Prevention of powdery mildew on grapes
The best way to avoid problems in the future is to plant vines that are resistant to powdery mildew. Unfortunately, resistant vines have only recently become available. Previously, the only option was to use chemicals or organic treatments, but now you at least have the choice of planting disease-resistant vines.
Washing down the leaves after a rainfall can also help to prevent an outbreak once an infestation has occurred.
Chemical control of powdery mildew on grapes
If you do nothing else, you should certainly use a copper-based fungicide. It can keep the fungus under control at low levels, but it isn’t very effective once an infestation has become severe. It is fairly safe to humans and wildlife when used as directed. It has to be reapplied fairly frequently, however.
One application lasts only about a month. You may need to make two or three applications per season.
Your other option is maneb, which is only somewhat effective when used by itself. It can remain on the leaves for up to two months, however, so you do not have to apply it as often. You will still need to reapply it once a month. It is also somewhat toxic to humans and wildlife, but not as much as the triadimefon/triazicide mix.
You can increase the effectiveness of either of these chemicals by mixing them with chlorothalonil, mancozeb or the copper-ARS mixture.
Organic options for treating powdery mildew on grapes
Elevating the humidity around the plants helps to reduce the spread of disease. You can do this by placing burlap sacks filled with moistened gravel on the ground under the vines. The moisture will be absorbed by the soil and raise the humidity around the plants.
You can also spray the vines with a mix of one gallon water, three tablespoons of liquid soap and two tablespoons of vegetable oil. The soap and oil break the surface tension of the leaf, which prevents the spores from landing and the water washes them off the leaf. Make sure that the soap does not contain any toxins itself. It is best to use biodegradable products whenever possible.
Neem oil has also proven to be an effective treatment for powdery mildew. You can purchase it in either a ready-to-use spray or a concentrate that you have to mix with water. It is non-toxic to humans and wildlife and has a residual effect, which means that it continues to protect the plant for several weeks.
You can also try increasing the humidity around the plants by covering them with burlap sacks that have been moistened. You can spray the vines with a mix of one part milk and nine parts water. The milk contains protein molecules that bind to the surface of the leaf and prevent the spores from landing. This is most effective in the late afternoon, since the spores land and begin to grow earliest at this time.
Once an infestation has become severe, none of these methods is likely to improve the situation. You can try mixing corticosteroids or immunosuppressants in with any of the sprays listed above. This helps to reduce the stress of the plant and allows it to fight off the disease better.
More than likely, once powdery mildew has taken hold it will destroy most of your crop. The only way to prevent it is to screen the greenhouse or vineyard and keep the flies out. Even if you are able to prevent an infestation that year, the fungus can still be present in the soil and infect future crops.
Organic options for treating black spot on grapes
The best way to prevent black spot is to remove all the leaves from the lower part of the plant, especially if there is more than one row. This reduces the amount of moisture around the base and makes it less attractive to the flies that carry the spores. It also gives more direct sunlight to the fruit, which helps it to ripen better.
You can also try to screen the plant. Black spot usually only shows up on the unripened berries, which means that by the time you see it there is not much you can do. You can place plastic sheeting around the base of the plant and secure it well so that no insects or birds can get in.
Sprays made of potassium or sodium silicate can be used to treat black spot once it has become severe. They are available in most nurseries and garden stores. They work by drying out the fungus and allowing the plant to heal. More than likely, though, by the time you see the infection it is too late for this method to help.
There is no organic cure for oidium, the cause of brown rot. Once it is spotted you can try the same measures as for black spot. Since it usually shows up after the fruit has ripened you can try leaving it on the vine and see if it ripens anyway, then pick and treat it yourself.
Sometimes, even with the best care, a few grapes in the bunch are going to become infected. Don’t worry, this doesn’t mean your wine is going to be ruined. Many types of wild yeast also live on the grapes. As long as there is some yeast present, this can help out if the infection takes over.
Some types of mildew and rot are actually a positive thing for certain wines. The moisture content gets too high and a type of bacteria called lactobacillus starts to thrive. This is the same type of bacteria that makes yogurt thick and creamy. The rot starts to give a buttery taste to the wine and if it gets bad enough, even turns it into a form of vinegar.
When making wine, you always want some bad grapes in with the good ones. This is what keeps the wine from getting stale so fast and allows it to age. Many times, people don’t even realize that their bad grapes have gone to rot, and include them in the final product. In fact, many famous wines, including the famous and expensive bottles of Romanee-Conti are made with a few rotten grapes.
If you are careful and know what you are doing, you can try replicating this at home!
The longer you leave the grapes on the vine after picking, the sweeter they taste. The reason for this is because as the grapes stay on the vine, they start to break down and their natural sugars become more concentrated. Of course, not all types of grapes get sweeter as they ripen, so you need to be careful which types you leave on the vine for too long. Leaving the grapes on also greatly increases your chance of rot.
Some types of rot, like the one mentioned above where lactobacillus bacteria is formed, is actually good for making a sweet dessert wine. Other types of rot give off a foul odor and make the wine taste bad. Others can actually poison you. Never eat any grapes you haven’t inspected yourself!
As the sugars in the grapes become more and more concentrated, a process called pasteurization takes place. This kills off some of the yeast and other microorganisms, making the wine healthier to drink (at least in small quantities).
As the grapes dry out, their texture hardens and they become easy to crush. At this point, the alcohol content is at its highest.
The longer you leave the grapes to rot, the less sugar remains in them. The alcohol content is so high at this point that you can get a mild buzz just from breathing the air inside the vat!
Some people like to drink their wine on the day it is pressed. If you want to make wine that is going to last for years, though, you need to keep it around for a while before drinking. Otherwise, much of its flavor will disappear.
Most wines get better as they age. This is because as the water in them evaporates over time, more of the sugar and alcohol is left behind, making for a stronger, more flavorful drink.
The length of time that a wine needs to age varies from type to type. Light varieties like Riesling only need to age for a year or two before they reach their prime, while heavy varieties like Port can take decades to reach their ideal flavor.
Most wines that are aged for long periods of time are put into wooden barrels to do so. This helps to keep them from being damaged by light.
Wine is quite low in nutrients, but can still be a decent way to get liquids into a person who is sick and can’t keep anything else down. Drinking too much wine, though, can lead to a condition known as Wine Sickness, which consists of a headache, hallucinations, and vomiting.
If you drink too much wine too quickly, you can get yourself drunk. This lowers your ability to think and function and greatly harms your motor skills. It also makes you vomit and greatly dehydrates you. While this may sound like a fun thing to do on a weekend, it can actually kill you!
You know what they say: “The cobbler’s children have no shoes.” This is true in the case of winemaking. Many vintners and winemakers neglect their own health in the interest of getting their product out to the masses. As a result, drinking is one of the fastest ways for them to get quickly sick and die early deaths.
The reason why they drink on the job is because it is important for them to keep an eye on their products at all times. The way people store and drink wine can have a huge effect on how it tastes. Most of the time, vintners and winemakers drink as a way to keep their palates trained so that they can identify flavors in the wines they make.
So remember: while it may be important to have a wine expert helping you with your winemaking, it’s equally important to have a physician on hand to help you with your winetasting!
You’ve made it through another year of wine and winemaking! If you’re curious about the growth of your vineyard, you can go there anytime, but it probably makes the most sense to go right now, while the grapes are ready for harvest.
If you’re new to all this, don’t worry; we’ll go into more detail about how to do everything next year! (Or you can reference the previous years’ entries if you want to learn more now.)
I don’t know about you, but it’s been fun going on this little adventure with you. I hope you had as much fun as I did. If you want to try your hand at winemaking again, the tools will still be here next year. Happy Holidays!
“Wow, you really went all out this year. This is the best house on the block.” your mom says as she steps inside with a large bag of groceries.
You smile. “Thanks, Mom.”
What did you want to show me?”
“I managed to get a bunch of good deals from all the vineyards that were going out of business.” You explain.
“They’re in the basement.”
She asks, surprised. “
What did you do, rob the place?”
You laugh. “No, but they had huge sales. I got this for Grandpa.” You say and pull out a bottle of wine. The brand is unfamiliar to you, but it’s clear that it’s high quality stuff.
You rush down into your wine cellar with your Mom to show her your new wines. You lead her over to the stack of cases that you bought.
They’re all good years. And most of them are prices pretty low, even for normal people like us.”
Your Mom asks. “
Could you drink this stuff?”
You scrunch up your face. “I dunno…” You admit.
“Well, don’t drink it then. Just set it up with the rest.”
Sources & references used in this article:
Biocontrol of powdery mildew of grapes using culture filtrate and biomass of fungal isolates by PN Singh, SK Singh, SP Tetali… – Plant Pathology …, 2017 – plantpathologyquarantine.org
The epidemiology of powdery mildew on Concord grapes by DM Gadoury, RC Seem, A Ficke, WF Wilcox – Phytopathology, 2001 – Am Phytopath Society
Effects of powdery mildew on vine growth, yield, and quality of concord grapes by DM Gadoury, RC Seem, RC Pearson… – Plant …, 2001 – Am Phytopath Society