If you’ve been growing vegetables for long enough, chances are pretty good that you’ve dealt with powdery mildew in the garden at some point.
It’s one of those super common problems that shows up in almost every garden eventually.
Whether it’s coating the leaves of your pumpkin vines or marring the beauty of your roses, if you thought powdery mildew was common in the garden, it’s twice as prevalent in greenhouses.
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That’s because plants grown in greenhouses tend to be crowded and the humidity is high. Plus it’s nice and warm. All the environmental conditions that powdery mildew loves.
The challenge with powdery mildew in a greenhouse environment is that it tends to spread very quickly and, since you usually have lots of plants in a contained area, it can rapidly infect multiple species.
Are you just starting out on your greenhouse-growing adventure? Check out our beginner’s guide!
In this guide, we’re going to discuss one of the most common problems in greenhouses and how to deal with it: Powdery mildew.
Here’s what I’ll cover:
What Is Powdery Mildew?
Powdery mildew is a broad term for an infection caused by multiple fungi in the Erysiphales family that cause similar symptoms.
It causes huge economic losses for commercial flower and vegetable growers. Everything from roses and poinsettias to tomatoes and herbs are at risk from this disease.
The fungi that cause powdery mildew don’t actually want to kill the plant. Their goal is to live off the live host, and they can’t do that if it’s dead.
So most of the time, the infected plant will survive, but it won’t look good and it won’t be very productive in terms of flowers or fruits.
The fungi that cause powdery mildew are broad, but those in the Erysiphe, Golovinomyces, Phyllactinia, and Podosphaera genera are most common.
Some are species-specific and some are more opportunistic.
Roses, for example, are attacked by Podosphaera pannosa var. rosea. This pathogen doesn’t target anything else but plants in the Rosa genus.
In dahlias, it’s both Erysiphe cichoracearum and E. polygoni. These pathogens will attack plants in lots of genera, including cucurbits like pumpkins and melons.
Podosphaera leucotricha attacks apples, pears, and other closely related species in the Rosaceae family.
Some plants can be host to multiple species of powdery mildew-causing fungi, which may be species specific or generalized.
Golovinomyces cichoracearum, for example, is found on chrysanthemums, cucurbits, and more. Meanwhile, G. chrysanthemi uses chrysanthemums as its only host.
Normally, it’s not super important that you know which specific pathogen is causing the issue, but it can actually matter in this case. That’s because knowing which pathogen is present can inform you as to which other plants in your greenhouse are at risk of infection.
So in the case of an outbreak, research your particular plant and understand what species of powdery mildew pathogen attacks it and what other species of plants it can also infect.
You know that if your roses are infected you don’t have to worry about your tomatoes, for example.
You can also send in a sample to a lab and they can give you the specific details.
Otherwise, just quarantine the infected plants and assume the worst, which is that whatever it has can infect everything else in the greenhouse and act accordingly.
Identification
Some diseases can be pretty hard to recognize. Or, the symptoms might look similar to a million other diseases. I mean, have you ever tried to tell leaf spot diseases apart? It’s hard work!
Not powdery mildew, though. It causes a white or gray powdery growth on the infected parts of the plant. You can wipe the “powder” off, but the disease is still there.
This “powder” is actually the fungal growth. The hyphae and conidia, as they are known, are thread-like strands and chains of fungal spores.
Typically, the fungal spores will appear on young, new growth and the upper sides of the leaves first. Then, the infection moves onto stems, older growth, and the undersides of the leaves. There can be exceptions, but this is the usual pattern.
But the characteristic powdery fungal growth isn’t actually the first sign of the presence of this disease. If you look carefully, you can often see purple, necrotic, or chlorotic lesions or spots on the leaves.
After the fungal spores take over, the leaves might become pale, and they may shrivel, dry up, or develop brown spots. Finally, they can drop from the plant.
Rarely, the plant might develop witches’-brooms or other distorted growth.
Before we go into how to control this disease, I will tell you that identifying it early on is absolutely crucial.
Your plants can go from having just a few infected leaves to the whole specimen symptomatic in a week.
Biology and Life Cycle
The fungi that cause the disease produce spores called conidia that can be transmitted via the air, but they need live tissue to survive and reproduce.
This is known as an obligate parasite or a parasitic organism that must have living tissue to reproduce.
Unlike many other fungal pathogens, they don’t need free moisture to make their way into the plant’s system. In fact, the spores can’t germinate in water.
Once the conidia find a place to land, they can begin reproducing in as little as 72 hours, which is why this disease can spread so quickly.
When the conditions are favorable, meaning temperatures between 68 and 86°F, with high humidity of over 95 percent at night and low humidity during the day, the spores can spread and reproduce rapidly.
If your greenhouse has a gentle fan to move the conidia around when conditions are right, even better!
But even without a fan, just the action of opening and closing the greenhouse door can create enough wind to help spread the spores far and wide.
Powdery mildew can be a problem in greenhouses during spring, summer.. And just as the growing season is extended for plants in the greenhouse, so is the growing season for pathogens!
Though most of the pathogen species need a living host, some develop small, black fungal structures called chasmothecia that can survive without a host for short period . These will wait in the greenhouse environment until a suitable host is nearby.
On the other hand, if you grow a plant species in your greenhouse and then skip a few months before you grow it again, the pathogenic species specific to that plant will die off in the absence of a host – that is, of course, if there isn’t another suitable host nearby.
Organic Control Methods
We’ve reached the part I know you’ve been waiting for: how to fix the problem. We have lots of options, but I highly recommend that you use more than one method of control.
If you take a multi-pronged approach and mix a few cultural controls with a biofungicide alternated with an organic fungicide, powdery mildew will be a thing of the past.
When you notice that a plant is infected, start a daily inspection routine of all other plants in your greenhouse. Early action will give you the best chance of getting the disease under control.
The further along this disease progresses and the more it spreads, the less likely you’ll be able to completely eradicate it.
Cultural
Start by doing your best to prevent the pathogens from finding their way into your greenhouse to begin with.
Easier said than done, I know, but it helps to inspect your plants before you bring them into the greenhouse. Look for any of the signs that we discussed above and if you see them, quarantine the plants.
You should also always clean your tools with hot, soapy water or a 10 percent bleach solution (one part bleach to nine parts water).
If you use your pruners on your rose bushes and then bring them into the greenhouse and start snipping newly-started rose cuttings, you could unwittingly introduce powdery mildew or a different disease.
Controlling the relative humidity in your greenhouse is absolutely crucial to managing powdery mildew. When the relative humidity drops below 93 percent, the fungi have a harder and harder time reproducing and spreading.
There are a variety of methods you can use to help lower the humidity around your plants. Fans, in addition to open venting, or fans that move air into the greenhouse from the exterior, are extremely helpful.
You should also ensure appropriate spacing – when plants are clustered very close together, it raises the humidity surrounding them.
Also, avoid overfertilizing. It’s not the fertilizer itself that increases the chances of powdery mildew, it’s the lush, overabundance of growth that it causes. This young, new growth tends to attract powdery mildew spores.
Depending on what you’re growing, there might be a few cultivars available that have been bred to be resistant to this disease.
Look for these when shopping if you generally struggle with powdery mildew. There are resistant cultivars available of African violets, begonias, roses, pansies, and zinnias, for example.
If you ever have a time when you aren’t growing any plants, use the opportunity to clean your greenhouse thoroughly and spray it down with a 10 percent bleach solution.
Physical
There’s an ongoing debate among scientists about whether watering on the foliage helps to reduce symptoms or not. It seems to both help and hurt, depending on the study and situation.
According to the experts at PennState Extension, there is good evidence to suggest that pouring water on the foliage of susceptible plants will prevent the spores from reproducing. But there are a few species that can thrive in moisture.
Also, keep in mind that other disease-causing pathogens thrive in lots of moisture, so it’s a risky move and should be reserved as a method for plants that can’t tolerate fungicides.
It never hurts to wipe the fungal spores off of the leaves, if you want. Remember, this won’t kill the disease, but it will certainly help prevent it from spreading further.
You can also prune out any infected foliage, assuming there is enough leftover to support the plant, meaning at least two-thirds of the foliage left.
If more than two-thirds of the plant is infected, you’ll need to use other methods of control in addition to heavy pruning.
Biological
Powdery mildew is extremely common, which is a good thing in that experts have had lots of time and opportunity to figure out how to deal with it.
Part of our available arsenal now includes biological fungicides, also known as biofungicides.
These include products that contain beneficial bacteria such as Bacillus amyloliquefaciens, B. subtilis, and Streptomyces lydicus.
All of these work better as a preventative or when alternated with an organic fungicide. They aren’t as good at eliminating an existing infection, especially if it’s extensive.
Still, don’t overlook biofungicides as an option, especially because they don’t have a negative impact on your plants or the growing environment.
I prefer to use a product that contains B. subtilis, because it has worked so well for me in the past.
CEASE Biofungicide
CEASE biofungicide is my product of choice, and you can find it at Arbico Organics in one or two and a half gallon containers.
Organic Fungicides
We’ve covered a lot of bad news, so if you’re looking for a little positive news, here it is. We know that the fungi can only colonize the living plant tissue.
That means the pathogen doesn’t want to kill its host because it needs to keep it alive. We can exploit that characteristic to control and eradicate it with organic fungicides.
Neem oil, potassium bicarbonate, sodium bicarbonate, copper, and milk, believe it or not, are effective at eradicating powdery mildew.
Sulfur can also be effective, but some plants are sensitive to sulfur, so check to see if the species you are growing is known to be susceptible to damage when using this product.
To use milk, it’s best to start in the early stages of the infection. If you don’t catch it until the damage is extensive, milk might not be enough on its own.
Mix the milk with water at a ratio of one to 10 or one to five. For serious infections, you can mix half milk and half water. Saturate the leaves thoroughly.
If you choose to use neem oil or another organic fungicide, follow the manufacturer’s directions for use.
You can learn more about how to control powdery mildew naturally in our guide.
Chemical Control
There are so many chemical fungicides available that claim to control powdery mildew, but since it’s pretty easy to manage with organic products, I see no reason to use harsh and potentially harmful chemicals.
But if you find yourself wanting to reach for chemical control, look for products with the active ingredients azoxystrobin, pyraclostrobin and fluxapyroxad, polyoxin D, myclobutanil, trifloxystrobin, or thiophanate methyl.
Again, there is usually no benefit to using these products over organic methods. Organic treatments are just as effective without having the same negative impact on human, animal, and environmental health.
If you have an extremely serious case in your greenhouse, you might need to use something stronger and faster acting than organic can provide. It’s nice to know that you have these in your back pocket if you need them.
Keep in mind that repeated use of fungicides can lead to resistance, and we don’t want that. So try to only use these once and only as a last resort.
Powdery Mildew is the Pits
Greenhouse growing comes with its own unique set of challenges and powdery mildew is one of the most common diseases you’ll encounter.
The conditions in a greenhouse are perfect for this disease to take hold and spread like wildfire.
Whether you have a tiny backyard number or a huge commercial structure, any greenhouse is susceptible.
But now, I hope you feel equipped to know what to watch for and how to deal with it if it arrives. If you’re still facing problems, let us know what’s going on in the comments section and we’ll see if we can help.
If you’ve tackled your powdery mildew problems, why not turn your attention to other greenhouse-growing topics, like these next:
