How to Get Rid of Powdery Mildew During Flowering

Kristen Klepac Pathogens Sanitation

Are you familiar with that dusty, powdery appearance of sifted flour? That's how symptoms of powdery mildew, or PM, often appear on plant leaves. This common and rapidly growing fungal disease can wreak havoc on your plants if left unattended. It often leads plant growers pondering how to get rid of powdery mildew during flowering.

With over 900 species, located worldwide and each with a limited host range, it is essential to familiarize yourself with this plant parasite. 

We have the details on what powdery mildew is, how to get rid of it, and why it's crucial to follow preventative measures. Plus, we found the best advice for how to get rid of powdery mildew during flowering is to be vigilant and proactive. If that fails, both time tested practices and scientific innovations give us a plethora of ways to battle and prevent PM while maintaining safe environmental practices.

What is Powdery Mildew? 

Powdery mildew is a highly transmissible fungus that belongs to the Erysiphales order of fungi. Erysiphales are obligate parasites.

Mycelium is what grows on the surface of infected plants. As it grows, threads of vegetative, root-like structures reach out to penetrate host cells. They take nourishment directly from the plant.

The part of powdery mildew that is first spotted by the naked eye are the tell-tale white, powdery mildew spots, hence the name. Depending on your plant, these spots might appear on younger or older leaves first. For example, on lettuce, the disease usually appears on older, lower leaves. Whereas on roses, younger leaves often display first signs. 

Complications Caused by Powdery Mildew

Complications caused by powdery mildew are numerous. While plants do not usually die, although they can, typically, the entire crop can quickly become infected and impact your harvest or the plant aesthetic. Some of the common effects and damages of a powdery mildew infection include:

  • Reduction in photosynthesis: leaf surface is taken over by the mildew thus reducing the efficiency of the plant structure and ability to take in sunlight
  • Inhibited plant respiration: the CO2 gas exchange process is complicated
  • Damage in harvest yield 
  • Defoliation and cosmetic damage
  • Weakened plants and poor growth

Complications of Powdery Mildew for the Flowering Stage

One primary concern for farmers is how to get rid of powdery mildew during flowering. Often it is during this stage that plants start to develop clear signs of infection. At the same time, this is when you don't want to be spraying your plant with fungicides or disrupting their growth.

If you do not contain or address PM, the plant infection will take over the leaves, stem, and flowers. For many plants, this can ruin the crop or overall aesthetic. For example:

  • Squash: if you are growing squash, infected plants could lead to an inedible crop. 
  • Roses: flower buds often fail to open, or the plant will produce deformed flowers

How Can You Get Rid of Powdery Mildew Before It Takes Over?

crowded humid plants

There are many solutions for how to get rid of powdery mildew once you've spotted an infection. However, one of the most widely held practices is that prevention should come first. It is the surest way to be one step ahead of this problematic disease. 

Many horticulture experts will refer to building a healthy plant management system for your plants. With any kind of garden or farming, this type of strategy can help ensure your plants thrive.

Do your homework about what environment is best for your plant and keep the following in mind:

  • Use powdery mildew resistant plant varieties when available.
  • Identify the right site or location: the plant is a good fit for the type of soil, sun, and water conditions.
  • Practice strategies that keep your plant healthy and avoid pest and mildew problems by maintaining site sanitation, fertilization, and pruning practices.

Using pesticides or fungicides is undoubtedly an option, but it shouldn't be your first course of action.

Knowing the Causes Can Help You Prevent Infection

The powdery mildew spores that land on your plants and infect them have particular conditions in which they thrive. So, building a plant environment that doesn't invite them is one of the best prevention methods. Monitoring humidity and temperature is very important. 

Poor Airflow: An Invitation to Spores

A spore can infect a leaf, either airborne or through direct contact. Once it has access to the plant tissue, it adheres to the surface to begin robbing it of nutrients. 

Poor airflow limits the movement of air, allowing time for the spores to drop in and grow on your plants. Poor airflow can be caused by:

  • Dense foliage
  • Poor ventilation
  • Inadequate circulation 

High Humidity at Night: A Recipe for Mildew Growth

H2O and sunlight are keys when growing plants. However, often the two combined can cause a level of humidity where many other organic species thrive. 

In fact, powdery mildew prefers conditions in which many plants thrive, which is why it is a tricky disease to evade. The fungal spores thrive in warm, moderate temperatures (70-80 F), with some humidity and shade. 

This paradox is one of the biggest hurdles with facing powdery mildew. With only small amounts of moisture and typical spring weather, you can easily have a recipe for disaster. Those warm, dryer days and cool, moist (humid) nights, which are great for your plants, also invite PM to spread and multiply.

Preventative Measures to Discourage Powdery Mildew Growth

new garden growth

Now that you know what environment powdery mildew likes, what can you do to discourage its growth? Here are a few simple tips:

General:

  • Select plants that are known to be resistant to powdery mildew.
  • Maintain plant to plant distance, don't overcrowd.
  • Water plants during the day, so they have plenty of time to dry off.
  • Prune excessive foliage.
  • Give plants plenty of sun.
  • Prevent pest infestations as the sticky secretions from them cover the leaves and make it easier for spores to attach to leaves.

Outdoor:

  • Do not grow plants too close together or close to damp areas.
  • Remove weeds that may create density and restrict airflow.
  • Avoid the use of nitrogen-rich manure or fertilizers. It encourages young plant growth where powdery mildew thrives.

Indoor:

  • Ensure your space has an effective air purification system.
  • Maintain proper ventilation and climate in your indoor space.

What Plants Commonly Suffer from Powdery Mildew Infection?

According to a report from the Mid-Florida Research & Education Center, fungi cause about 85% of plant diseases. Powdery mildew is just one of many types of fungal infections. However, of these diseases, it is one of the most commonly encountered and easily identifiable. 

Here are just a few plants species that are known to be susceptible to powdery mildew infection: 

Flowering Plants: Roses, dahlias, chrysanthemums

Cucurbits: Cucumber, melons, squashes

Trees: Apple, Ash, Azalea, Birch, Lilac, Maple, Magnolia, Oak

Humulus lupulus (hops) 

Of course, there are long and extensive lists and tables full of plants susceptible to powdery mildew. These are just a few of the noteworthy and frequently encountered plant species commonly researched and discussed. 

Methods for Getting Rid of Powdery Mildew 

fungicides to battle powdery mildew

Of course, even if you've done all you can to be on guard against PM, sometimes infections still occur. Taking care of your plants with the highest quality and safest solutions is critical. 

Ultra-pure Chlorine Dioxide to Completely Eradicate Powdery Mildew

Most of us are familiar with the chlorine element by that summertime blue pool and the gratifying smell of sparkling disinfected water. But it's become so much more than just an incredibly effective way to ensure our water is germ-free. 

In the evolved form, chlorine dioxide or ClO2 is a pure gas form that can dissolve in water without reacting. Since it's regular usage began in the 1940s, it has become an impressive tool for a number of industries. It is the newer, higher-quality upgrade to the standard chlorine application. It's used in water treatment facilities, post-harvest cleaning of fruits and vegetables and in toothpaste, face creams, contact lens cleaners, and much more.

It is composed of one chlorine atom and two oxygen atoms. The gas that is created becomes sporocidal (able to kill spores) so it is one of the few things that can actually actively destroy powdery mildew in its path.

Usually ClO2 is made on-site. However, luckily with new innovation in products like Gard’nClean's 99.9% ultra-pure chlorine dioxide, a membrane-based microreactor can provide farmers and gardeners a low-cost, just-add-water solution to a PM infestation. 

BRe³ Light Energy Technology - Reductive

New technology has enabled farmers to utilize light energy to suppress powdery mildew. This news means we now can avoid the problems that may arise if the wrong applications or percentages of fungicides is employed. 

This discovery is great news for organic growers who prefer to avoid using certain fungicides. It could help accelerate and expand organic production without harming the plant or plant environment. 

BRe³ light works by emitting combinations of light wavelengths produced by the sun. These wavelengths kill harmful pathogens and enhance plant photosynthesis and cellular repair. So not only does it attack powdery mildew, your plant will benefit from regular usage.

It's important to note, however, that this is a tool that works to reduce, not get rid of, this highly contagious fungal growth. So, you would be required to use this light energy therapy often. 

UV Light Technology

UV light can present dangers to your existing plants. However, the use of UV light is extensively studied and researched. According to Cornell AgriTech's senior researcher David Gadoury, UV light applications killed 95% of PM in field strawberries, with similar results in both field and greenhouse trials for basil, roses, grapes, rosemary, and cucumbers. 

The difficulty with fungi is that they have evolved a system that allows them to repair damage caused by exposure to UV light rapidly. However, at night, powdery mildew shuts down this organic mechanism. So, UV light exposure at night can kill the pathogen without harming the plant.

However, using something that could potentially harm your plant is a risk. On top of that, UV Light Technology is a hefty investment, must be installed properly and carefully used. 

Organic Fungicide Solutions to Get Rid of Powdery Mildew

There is a slew of age-old remedies that have been time tested. On the other hand, there are new innovative discoveries that also help to address powdery mildew infections. Often those caring for plants care deeply about the products they use and want to treat pests and disease safely. 

With all the information across the internet, it's hard to know which solutions are the safest and the most effective. So, we found recommendations from Janna Beckerman of the Department of Botany and Plant Pathology at Purdue University. She advises first that before you use any type of fungicide, you ensure you have correctly diagnosed the disease. 

Sulfur

One of the most ancient fungicides, sulfur has been used for more than 2,000 years. It works by preventing fungal spores from germinating, so it is a preventative method.

  • Must be applied before evidence of a diseased plant.
  • Can be purchased as dust, powder or liquid
  • Cannot be applied if you've used an oil spray in the last month or if the temperatures will exceed 80°F 
  • Certain plant crops are sulfur-shy, and you should not treat them with sulfur. Be sure to research whether or not your plant species or variety is suitable for this type of treatment. 
  • Sulfur Burners: Elemental sulfur vaporization is an option but is highly sensitive. Do not use it at a high concentration. It is not recommended to be used during the plants' flowering cycle and can potentially be hazardous to humans if not managed.

Generally, sulfur does not present a severe, dangerous toxicity level. However, it can irritate the eyes and skin and should not be inhaled.

Copper

The beauty of copper is that it is both a fungicide and bactericidal. That means that it can attack fungus and kill bacteria. The negative, they can also damage the host plant if not applied properly. 

  • Use "Bordeaux" mixtures that combine copper sulfate with lime (calsium hydroxide). This combination makes the application safer for plants by neutralizing the harmful acidic copper sulfate. 
  • Can be purchased as a dust or liquid. 
  • Is able to persist through spring rains and maintain adherence to plants.
  • If applying on young plants, make sure to use a diluted formulation because young foliate is sensitive to copper.
  • Always refer to the label for recommendations according to your plant's age and the type of pest infection you are treating.
  • Generally recommended usage is only before the flowering stage.

Oils

Though often used to manage insects, certain oils can effectively manage powdery mildew. Oils operate as both a fungicide and an insecticide, a double win. 

  • Certain plants are sensitive to oils, make sure you research to make sure the oil you choose is safe to use on your plant.
  • Never apply an oil too soon after a sulfur spray, or it may injure plants. Consult fungicide labels for special precautions and timing recommendations for spray intervals.
  • Do not apply oils when the temperature is over 90°F or below 40°F. 
  • Ideally, use oil when relative humidity is less than 65% so that it can evaporate quickly.

Horticultural Oils are distilled mineral oils free from toxic impurities. Once plant-friendly, combining them with an emulsifying agent helps them mix with water for effective spraying.

Neem Oil works as a preventative tool for repelling powdery mildew spores at a 70% concentration. It is a time-tested anti-fungal used for thousands of years. There are plenty of videos available to walk you through how to apply this oil yourself. 

Jojoba Oil & Canola Oil are both options with proven methods for powdery mildew prevention. However, do not use this method when temperatures exceed 95°F. In studies, these oils have also demonstrated no adverse effects on the tastes of grape wines produced from the treated plants. 

Bicarbonates: Baking Soda

The standard household bicarbonate is that of the sodium nature, baking soda. Many online recipes for fungicide mention mixing baking soda with water and dish soap. 

Sometimes this concoction is referred to as the "Cornell Formula." While some may find it initially effective, in the garden, it can cause several unintended side effects. Plus, it's origin is a myth

Plant pathologist Dr. Ken Horst of Cornell University set the record straight, or tried, with a press release dispelling the myth. His statement clarified that many of these recipes on the internet have limited benefits and may even be phytotoxic. 

Baking soda sometimes can leave behind toxic sodium, which is not suitable for plants, depending on the type of plant of course. The times Dr. Horst observed efficacy could be attributed to the oil used to deliver it. He also does not recommend dishwashing soap or other detergents because of the antibacterial chemicals that may kill off healthy soil bacteria and alter nutrient absorption.

However, many people have had great success with baking soda. The way it works is by changing the pH on the surface of the leaf creating an environment that is hostile to Powdery Mildew, and other fungi too. That being said, it does not kill PM spores, so you still have to be mindful and apply frequent preventative applications.

Bicarbonates: Potassium 

If you can believe it, better and safer bicarbonates exist. Potassium bicarbonate is one of the most common and proven bicarbonate methods. This bicarb also provides potassium to plants, helping them obtain the necessary nutrients to support growth. Plus, it's already safely used in food processing and proven to be safe to consume.

It is important to note that regular and frequent sprayings are necessary to maintain the preventative effects of this method. 

Proven, Unproven, and Anecdotal Fungicide Solutions to Get Rid of Powdery Mildew

  • Milk - many growers swear by this method.
  • Mouthwash - ethanol-based mouthwash might kill powdery mildew.
  • Vinegar - too much vinegar may damage your crops.
  • Garlic - garlic oil may be useful, as it is both an oil and contains sulfur.
  • Compost Tea - organic option, time-consuming.
  • Bacteria - many products containing bacillus subtilis and bacillus pumilis are found effective.
  • Hydrogen Peroxide - can kill molds. 
  • High Alkaline pH Water - many mildews cannot grow in high pH environments.

Best Solutions for How to Get Rid of Powdery Mildew During Flowering

Generally speaking, PM growth during flowering is a huge problem. Nobody wants their buds to become infected with mold or mildew, no matter what you are growing. 

At this stage, it is also essential to maintain plant vitality and bud or flower purity. So choosing one of the newer technologies is the safest. There's no need to apply any fungicide that may alter the health, appearance, or taste of your plant's harvest. 

While there are many products that are time tested and age-old, many are simply "preventative". They required constant application, every few days or every week. The best thing to choose is something like Gard'nClean which will eradicate the fungus and spores completely, so it will only come back if it blows in via contaminated air. 

How to Remove Powdery Mildew From Buds and Flowering Plants

We've determined these are the best options to employ for how to get rid of powdery mildew during flowering:

  1. Pure Chlorine Dioxide (Gard’nClean
  2. UV Light Technology 
  3. Certain oils, such as Neem Oil or Jojoba Oil 
  4. Potassium Bicarbonate 

What to Avoid: No Sulfur Burner During Flowering and More

You should not use the following during flowering as they can impact the bud quality and, on edibles, the taste or safety:

  1. Sulfur (not recommended during flowering) 
  2. Copper (not recommended during flowering) ✕
  3. Synthetic fungicides ✕
  4. Harsh Oils ✕

Should You Consider An Ozone Generator For Grow Room?

One method we didn't include on our list is using an ozone generator. It is certainly effective at knocking out harmful pathogens. However, it can also be harmful to humans. Finding the balance between effective and safe is a fine line to walk. 

Here is a handy break down of the good, the bad, and the useful if you would like to learn more about using these. One takeaway we'd like to point out is that when UV rays and O₃ molecules mingle it can be dangerous. If we have to hold our breath as we leave the room, it might be a little too risky.

Do You Know How to Identify Powdery Mildew? 

Powdery mildew makes its home on the surface of plant tissue. It can be challenging to spot, and once you do, it is already rapidly spreading. So keeping an eye out for signs of PM is crucial for crop health.

Locations to survey: Vegetative and reproductive plant surfaces

  • Often spotted first on the upper-side of lower, younger leaves. 
  • Then the mildew can spread to the stem and weaken the stem tissue. 
  • The mildew can appear on any plant tissue located above ground.
  • Infected leaves often curl and drop.

Colors to watch for: White Spots On Fan Leaves or Gray/White Coating

  • Early signs include blistering or small raised bumps. 
  • The first stages will appear as grayish or brown-white spots on young plant tissues.
  • Later, a fine, powdery growth will take over and appear as a full layer of white or gray dust that you can easily wipe off with your finger.
  • Flower buds may never open and be white with mildew.
  • Older infected areas turn brown and appear shriveled.

It's important to observe your plants as they will quickly start to develop symptoms once powdery mold spores have landed on the plant. If you don't catch the initial signs, subsequent symptoms include stunting, discolored leaves, and distorted plant growth. If your plant bears fruit, you may have fruit that is scarred or distorted.

According to research performed on hops, some powdery mildew can remain invisible as they build up a network on the plant. So, it's essential to be actively taking preventative measures.

How Long is Powdery Mildew Spore Lifespan?

Airborne spores will only survive a few days without a host to leech nutrients from. That's why it spreads so rapidly, it's evolved to quickly latch onto a host and multiply.

It is an obligate biotrophe, meaning it is necessary for it to get nutrients from another organism. If it doesn't, it won't survive. 

Now You Know How to How to Get Rid of Powdery Mildew During Flowering and More

Powdery mildew is definitely a nuisance. It is not just common, but it can rapidly take over your garden or greenhouse. It's especially problematic during the plant flowering stage.

The first thing to know for how to get rid of powdery mildew during flowering directly leads to prevention strategies. Make sure you follow cultural practices like proper ventilation and climate control. Since PM thrives with high humidity and restricted airflow, make sure your tactics include plant spacing, watering, and pruning. 

If you do find your plants infected with powdery mildew during flowering, the best solutions are the safest. Never use strong chemicals that may negatively impact the final harvest of the crop. Using environmentally-safe and easy to apply solutions that actually will wipe out the fungus is the best solution. Our recommendation is to stick with an affordable and effective option like Gard'nClean's chlorine dioxide treatment that not only prevents powdery mildew but actually gets rid of it. 

 

Sources:

Science.gov

Sciencedirect.com

Chemicalsafetyfacts.org

Researchgate.net

Pacific Northwest Pest Management Handbook - pnwhandbooks.org

Agriculture and Natural Resources, University of California - ipm.ucanr.edu

 

 

 

 


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