As a gardener, there is almost no sight more dispiriting than the appearance of little white blotches on the leaves of your plants. It usually means that you’ve got a fungus problem called powdery mildew. The problem is that it’s also very common, especially if you live where summers get hot and humid.
You don’t need to throw in the towel on your garden if you see it. It is very hard to get rid of once it sets in, but it is possible to manage it so that your plants can keep producing vegetables until fall weather sets in.
Table of Contents
- What is powdery mildew?
- Infected plants
- Preventing powdery mildew
- Treating powdery mildew
- Household treatments
It doesn’t take a master gardener to figure out when your plants have powdery mildew. You’ll just see it. You’ll see it well before your plants start to show signs of being sick.
Powdery mildew shows as blotchy white patches on the leaves. It might look a little like a spider web, but it’ll show up on the flat of the top of the leaf. It can spread quickly. Under the right conditions, a healthy green plant can become covered in white blotches and be withering in just a couple of days.
Whether this really is a death sentence for your garden or just an annoyance that requires extra time depends on how quickly you act. If you move quickly, you can address plants with severe infections and prevent it from spreading throughout your garden.
What is powdery mildew?
Powdery mildew is a very common family of fungi. It’s so common that it’s probably the first plant disease that people new to gardening will have to address.
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Within the family, different species will attack different plants. A fungus that infects cucurbits, plants that include cucumbers and squash, won’t attack grapes or dahlias. A fungus that attacks roses won’t attack beans or lilac.
Powdery mildew likes dry leaves but humid air. It also likes low-light conditions and temperatures between 65 and 90 degrees Fahrenheit.
Although it looks horrifying, powdery mildew isn’t a death sentence for most plants. The problem is that it looks bad and curtails the production of fruits and vegetables. A rule of thumb in gardening circles used to be that once powdery mildew set in, you could harvest anything that started developing before, but don’t count on anything to come after. Left unchecked, powdery mildew might just cause unsightly blemishes on ornamentals, but it might cause your zucchini to stop producing food.
Once a fungus spore lands somewhere favorable, it penetrates the surface cells of the leaf through a structure called haustorium, which begins robbing the plant of nutrients. This blending of the fungus and the plant at the leaf’s surface is a big reason why it’s so hard to eradicate once it appears. It’s not just a matter of wiping it off because it’s intermingled with the leaf itself. This is also why a big part of a control strategy is removing infected leaves.
Preventing powdery mildew
The best start to a strategy to control powdery mildew in your yard is through prevention. Do what you can to make conditions for it as unfavorable as possible. Even if it winds up infecting your plants, if conditions aren’t very favorable you can minimize the damage until autumn’s chilly winds arrive and spare your annuals death by fungus by instead killing them with frost.
Powdery mildew overwinters in leaf litter and on perennials. Controlling it starts when you winterize your garden. Remove as much plant matter as you can, especially plants that were infected with it that summer. Prune back plants, especially plants that had infections. Don’t compost leaves and stems from infected plants. The heat generated by composting might not be enough to kill the fungus.
It’s also a good idea to get your soil tested to check your nutrients. In general, plants with access to the nutrients they need to thrive do a better job surviving infections than plants that are constantly hungry. Test kits are usually pretty inexpensive, and in some states are available through extension services. Even if you do your own testing, if your state has an extension service, we suggest contacting it for a nutrient plan during the offseason. In discussing how much fertilizer to spread and at what point during the season, let them know that you’ve got powdery mildew. That could affect the plan they advise you to follow.
Another pre-season approach is to look for plants that are resistant to it. If powdery mildew is prevalent in your garden, the chances are good that there is a variety that has evolved some resistance to the disease. You can pair this knowledge with knowing what kinds of plants you have that are affected, because – again – powdery mildew is species-specific in the plants it affects.
During the growing season, an effective means to control powdery mildew is to keep up on your weeding. Prune off dead leaves and leaves at the bottom. This will create more space for air to circulate around your plants. It’ll also allow the space to dry out. While powdery mildew prefers dry conditions for its most rapid growth, the spores still like a little bit of humidity to spread.
It’s also important to regulate when you water. Rather than a quick soak every few days, give your plants one long, deep soak every week. This will reduce the amount of moisture there is in the air. Use a drip line at the base of the plant if you can. Otherwise, water first thing in the day. This will allow the sun to dry your plants. Don’t water in the evening, which allows the moisture to stay put all night long. Remember, powdery mildew might like dry leaves, but it also likes humid air.
Prune infected plants
Immediately remove leaves that start to develop powdery mildew. Don’t try to cure them. Just get rid of them. If your plant is so frail that the removal of infected leaves will kill it, it’s not going to produce blooms or vegetables and is wasting your time. If that’s the case, save your garden by getting rid of the entire plant.
Throw the leaves in a garbage receptacle far away from your garden. Don’t toss them in the compost pile. Feel free to burn them. The cleansing power of fire can in some cases be your friend.
If you continue to fertilize, cut down on the amount of nitrogen. Young plant growth is especially susceptible to powdery mildew, and the nitrogen in your fertilizer stimulates the production of foliage.
Treating powdery mildew
Prevention also means treating your plants to prevent the disease from developing. There are chemical fungicides that are very effective in controlling it. We’re going to look at a few things you can use if you are growing organic. One thing to keep in mind is that because we’re dealing with a family of fungi rather than a single species, some controls will work better than others on different plants. You may need to experiment to see which one is the most effective on your outbreak. Keep applications of different treatments a week or so apart so that you don’t risk damaging your plants.
Copper is one of the most effective organic ways to treat powdery mildew. In our field tests, we found that it was a good preventative on the cucurbits in our garden, cucumbers and summer squash. A great benefit is that we also found it an effective way to prevent and minimize outbreaks of fungus like late blight in our heirloom tomatoes. For those reasons, it’s our front-line anti-fungicide.
It works best if you apply it to plants every couple of weeks before the hottest, most humid part of the summer arrives. While in general a couple of tablespoons per gallon is pretty standard, follow the directions on whatever copper fungicide you buy, because they will vary.
There are two cautionary notes. The first is that copper can kill plants. While you’ll want to apply it liberally during your treatment cycle, don’t apply it in between to control a nasty outbreak. The second is that you’ll need to find a source for it. It is carried at most big box home goods stores and gardening centers.
Neem oil, made from an evergreen tree, is not only an effective anti-fungal agent, but a good organic way to kill soft-bodied pest insects like aphids. Like the copper fungicide, you’ll want to follow the instructions on the bottle of whatever kind you buy. That includes safety steps. Be aware that as an insecticide, it’ll affect good insects, too. Don’t use it when bees are around. The benefit of using neem oil is that it’s not only effective, but it’ll put a nice shine on your leaves.
In field tests, we combined neem oil and aggressive pruning on some dahlias that were close to succumbing to some nasty powdery mildew. Within a week of starting our control program of pruning off infected leaves and spraying uninfected leaves every week, new foliage started to pop out and so did new blooms.
A mixture of potassium bicarbonate, oil, dish soap, and water will raise the pH of your plant leaves and make them alkaline. Powdery mildew thrives in a lower-pH environment, and if you alter the surface pH of your leaves, you create a less favorable surface for the spores to land on and begin to spread.
If you go this route, potassium bicarbonate is something else you’ll probably need to buy while planning your garden.
For every gallon of water, mix in three tablespoons of potassium bicarbonate, three tablespoons of vegetable oil, and half a teaspoon of dish soap. The soap will help the other ingredients cling to your plants. Apply it every couple of weeks, starting before high summer.
We have not tested potassium bicarbonate, although we have some in our gardening shed and it is our intention to test it in the future.
If you don’t have a copper fungicide, neem oil, or potassium bicarbonate, there are a couple of treatments you can whip together using common household items.
A solution of baking soda, dish soap, and water will also raise the pH level on your plants to where it kills powdery mildew spores. The active agent is the baking soda. The soap helps the soda cling to the leaves. Mix a solution of one tablespoon of soda and half a teaspoon of soap per gallon of water. Apply with a sprayer first thing in the morning every couple of weeks. Like copper, this is best applied in a 14-day cycle starting before the most humid weather of the season arrives.
We have tested baking soda in the past. Our testing produced inconclusive results, but we do buy the explanation for why it works.
Some people swear that a bowl of sulfur next to your plants can prevent powdery mildew from spreading (no). Some people swear that powdered sulfur brushed onto it can kill it on contact (no). Some people swear that sulfur dusted or sprayed onto uninfected leaves can be an effective means of preventing its spread (yes). The great thing is that you can get a lot of sulfur from most pharmacies for just a little money. You might also have a duster already on hand to spray it onto dry leaves. If you get sulfur that can be made into a liquid spray, follow the instructions on the container.
If you use sulfur, be aware that it can damage plants, especially if used in conjunction with neem oil. While you can stagger their use to keep your plants safe, there are enough alternatives available that you can probably just pick one or the other. You don’t want to destroy plants in the process of saving them.
Be careful when people tell you that milk is the best control available. Not only is there no consensus on what makes milk effective, just about everyone who says to use milk has their idea about why it works. If someone can’t tell you why something is good, take it with a grain of salt.
We have reason for skepticism. We’ve tried aggressively spraying it on infected plants and uninfected plant leaves. It seemed to have very little effect on cucurbits, which all died.
If you go this route, use skim milk so your sprayer doesn’t clog up with milk fats. Use about a cup and a half per gallon of water. Most people say to spray every few days for it to be the most effective.
The acetic nature of vinegar can be effective in preventing powdery mildew from spreading by killing spores before they can infect leaves. You will want to be careful in using vinegar, however, because the same thing about it that kills fungus will also kill plants. It’s probably a good idea to leave vinegar as a last-ditch treatment. It might address your powdery mildew, but it might also kill your plants.
Based on our testing, we’d rate this as moderately effective. We tried this as a control on summer squash. Plants that we sprayed on a rigorous schedule appeared less affected than plants that we didn’t. We also missed a couple of scheduled sprays. Plants that we treated seemed to survive better than plants right next to them that we didn’t. Since the only variable was our application of vinegar as a control, we have to attribute the difference to that.
Mix five tablespoons with a gallon of water. Do that directly so that it runs off the plant after treatment. You don’t want something that is a poison to the plant to remain persistent on its leaves.
Some people think garlic works the same way that people think garlic will cure everything, from the common cold to high blood pressure. Be very aware of that, since preparing garlic for use will take a long time and will produce questionable results.
We haven’t tested this, and frankly suspect that it’s woo hokum. Garlic gets used as a stand-in cure for everything, and you’d be forgiven if you thought that people pushing it substituted the words “powdery mildew” for “vampires.” The “science” behind it is that garlic is high in sulfur, and that sulfur applied to infected leaves makes it easy to wipe off the fungus. It’s not actually easy to wipe powdery mildew off anything because of the way it infects leaves. Anyway, there’s something with an even higher concentration of sulfur, and that is sulfur. Buying and applying it takes a lot less time and trouble.
If you insist on trying this, crush a couple of whole cloves of garlic and soak it in rubbing alcohol and vegetable oil. Let it sit for at least a day. Strain it and set aside the liquid. Soak the garlic for another day in a cup of water. Strain it again and mix the water and alcohol mixture with a gallon of water. Apply that.
The appearance of patches of blotchy white on plants will cause some gardeners and landscapers to go into fits because it means they’ve got powdery mildew. It’s not quite the death sentence you might think, but at the very least it can curtail the production of fruits and vegetables in those plants and can cause unsightly blemishes on ornamentals.
Part of conquering this common problem is knowing a little about powdery mildew, what it is, and how it infects plants. Know all that and you’ll start to understand why the best solution starts with soil preparation and basic garden maintenance. Treatment of infected plants is a little more complicated, involving the removal of infected parts and making conditions unfavorable for it to thrive. Some, like the ambient temperature, are outside your control, but you can use various sprays to make plants unattractive to spores floating around in the air.
Above all else, keep your cool. It’s not necessary to let a little mildew rob you of the pleasure of keeping a garden.