Powdery Mildew is a global threat, with hundreds of fungal species that can cause it and the ability to infect more than 10,000 plant species.
According to Master Gardener Jim Cooper of Washington State University Extension, it has been estimated that when total loss of crop yield and plant growth is taken into account, powdery mildew causes the greatest loss of any disease. of plants in farmers’ fields. like home gardens.
Infected leaves appear as white or gray spots in the early stages of an infection. They can quickly enlarge to completely cover infected parts of the plant with a nasty pest.
Although these fungi do not usually kill their hosts, they do weaken them. However, there is hope!
In addition to weather patterns and symptoms to watch out for, if you’re if you’re interested to know how to treat Powdery Mildew, read on. There are many options that I will cover in this guide, as follows:
The Basics Of Powdery Mildew
The fungi that cause powdery mildew are unusual in that they require their hosts to be alive, due to their intimate relationship with plants.
This is believed to be the reason they do not kill their host. They need the plant’s nutrients to survive.
And unlike most types of fungi, they cause more severe cases of illness in hot, dry weather.
A mild case may go away on its own. But without gardener intervention and a little extra care, a serious infection can spell the end for your prized plants.
About Powdery Mildew?
Powdery mildew is the name given to the disease caused by hundreds of species of fungi of the order Erysiphales.
This name comes from the white to grayish talc-like powder you’ll notice on the leaves of infected plants. This powder is a combination of spores and mycelia (fungal threads).
Like many fungi, these pathogens reproduce both sexually and asexually. These stages look quite different from each other, to the extent that there are different scientific names for each of the stages.
Spores of the asexual stage spread the disease, while the sexual stage produces fruiting bodies that are important for overwintering. These fruiting bodies used to be called cleistothecia, but are now designated chasmothecia.
Some common types include Podosphaera which infects roses and Golovinomyces which attacks cucurbits. Powdery mildew is common wherever cucurbits are grown, so vertical gardening is often recommended to improve airflow.
How These Fungi Attack Their Host
While the fungal threads of almost all species grow on the surface of leaves, these fungi send out specialized structures that live within the plant cells to siphon their nutrients. These root-like structures are called haustoria.
Because of this specialized relationship, most powdery mildews are specific for their hosts. These fungi won’t attack any plant in the area like many other pathogens that assault a wide variety of species (Anthracnose and white mold come to mind).
In other words, just because the field of barley next door is exhibiting signs of powdery mildew doesn’t mean your roses will succumb to it as well. Cucumbers and zucchini, on the other hand, may both be infected by the same species of fungi.
Conditions For Infection
Unlike most fungal pathogens, all species that cause powdery mildew can infect their host in the absence of free water.
In fact, free water can kill the spores of most types of fungi that cause powdery mildew and inhibit mycelial growth.
However, the water in the air (moisture) is necessary for the spores to germinate. These fungi cause the most serious infections when humidity is high at night and low during the day.
Spore formation is favored by high humidity, while spore dispersal is accelerated by low humidity.
Moderate temperatures in the 70-80°F range favor infection. Therefore, powdery mildew is usually a problem in spring and autumn, when there are large temperature differences between day and night in many regions.
Although the spores are often spread by the wind, insects such as woolly aphids can spread the infection by feeding on plants.
Infection is common in crowded plantings that have poor air circulation and in shady, moist areas.
While healthy plants can often protect themselves from infection or recover on their own, stressed plants that are grown in the wrong places or that are cared for improperly or incorrectly can more easily fall victim to this fungal disease.
Most infections first show white or grayish spots on the tops of the leaves and on the new tender shoots. Some gardeners say that this looks like a layer of flour.
The lower leaves are the most commonly affected. However, as the spots increase in size, the mold can spread throughout the plant.
Flowers, young fruit, shoots, and young stems can be infected, and fungus can overwinter inside shoots in warmer climates.
As the infection progresses, the fungi produce fruiting structures the size of a pinhead. They start out white before turning yellowish brown and then black.
Leaves can twist and distort before wilting and dying.
Leaf loss can lead to sunburn on plant fruits. This is a particular problem with cucurbits, and infected plants tend to produce fruit that is prone to sunburn, tastes bad, and does not store well.
Some types of succulents show different symptoms when infected with powdery mildew. Infections on sedum can manifest as crusty brown spots, for example, while kalanchoe can develop corky brown spots.
Powdery mildew on tomato plants often starts with pale yellow spots on the leaves that turn into the typical white, powdery growth.
Types of Plants Affected
With the exception of gymnosperms, such as conifers, most of your plants are at risk of powdery mildew infection if conditions are right.
With the exception of gymnosperms, such as conifers, most of your plants are at risk of powdery mildew infection if conditions are right. A short list of susceptible plants reads like who’s who in the plant world.
Ornamental plants that can stand up to this attack include: asters, bee balm, begonias, dahlias, delphiniums, Hibiscus, lilacs, moms, peonies, roses, succulents, Verbena, Zinnias.
While fruits and vegetables include: apples, Beans, beets, carrots, Cantaloupe, cucumbers, Eggplant, Grapes, Onions, parsnips, pears, Green peas, Peppers, Pumpkin, Strawberries, Tomatoes, Watermelons
Even your herbs aren’t safe: cilantro and rosemary can get infected, as can your fruit trees, including apples and avocados.
This disease can also occur indoors, on houseplants such as African violets and poinsettias.
It’s also worth noting that some plants are known to be highly susceptible to powdery mildew under the right conditions, while others, such as annual phlox and Belladonna delphiniums, have a natural resistance.
Keep an eye out for resistant cultivars to plant in the garden and avoid others known to succumb to this disease more easily, such as Sedum ‘Matrona’.
How to Treat Powdery Mildew
The good news is that powdery mildew infections are usually mild. Otherwise, healthy plants will often recover on their own after changes in weather and with proper care to improve airflow.
Treatment is often unnecessary.
For severe infections or a widespread problem throughout the garden, there are treatment options that avoid the use of conventional fungicides.
Frequent use of fungicides can lead to the development of resistance in fungi. This has implications for both agriculture and human health.
>>> See more: cauliflower pests and diseases
Surely many of you have had a rosemary plant that succumbed to this ailment. Why could that be?
It is often a clear case of killing your plants too lovingly and attacking the problem with the wrong solution, usually too much or too little water.
Although the fungus itself needs a living host to live, and although its goal may not be to kill your beloved rosemary, catch 22 here is that stressed plants are more prone to infection.
Without proper care, you’re not doing your plants a favor and paving the way for disease, as well as other problems like pest infestations, or the inability to produce dazzling flowers or high-yielding crops.
This is where cultural controls come into play.
A key element of organic gardening and integrated pest management (IPM), cultural controls are those elements of farming and gardening that deal with modifications to the growing environment.
These may include making changes to modify the nutritional content or pH of the soil, watering more or less, and making sure the plants get the right amount of sunlight.
Giving plants the conditions they need to thrive is key to keeping them healthy, happy, and hopefully disease-free.
Adequate spacing is another important factor to consider, and if you’ve been ignoring the USDA hardiness zones and recommended planting times for your region thus far, take heed: While you may be able to grow a tropical plant all fall in a cold climate, or is “only” two zones away from what is listed on the plant label, those recommendations are there for a reason.
Since damp, crowded foliage is more susceptible to infection, prune foliage as needed to encourage proper airflow.
The sun is the enemy of fungal diseases, generally speaking, so be sure to give your plants the level of sun exposure they need to thrive, whether that’s shade, partial sun, or full sun.
Overhead watering is sometimes recommended as a way to prevent this disease, but others advise against the practice, as it will increase humidity inside the plant. Such increased moisture could encourage infection or increase the severity of an existing infection.
Follow our advice: always make sure to water plants at ground level and avoid wetting the foliage. It’s usually best to water early in the day (rather than overnight), as the plants will have time to dry out before nightfall.
Lush, newly formed tissue is more susceptible to powdery mildew. So if you have a problem with this disease in your garden, avoid fertilizing with nitrogen in late summer and always be careful not to over-fertilize your plants with this nutrient.
If your plants are infected, prune off the diseased tissue and destroy it. Don’t put it in your compost pile, or the infection could spread to other parts of your garden.
Dormant spores can remain on plant matter, and nearby weeds can also serve as hosts. Be sure to keep your garden beds and borders clean and free of weeds.
According to Dr. Gary Moorman of Penn State Extension, some growers who grow their crops in greenhouses inject water into the leaves of susceptible plants during the day when humidity is low. This has been shown to work well in inhibiting powdery mildew infections as the spores will not germinate in the presence of water.
But this practice can greatly increase the chances of plants contracting other types of diseases, such as fungal leaf infections. We do not recommend it.
Spraying milk on plant areas showing symptoms will help control powdery mildew, especially if applied early in an infection.
This technique is used by many organic growers around the world and has been shown to be effective in studies conducted for over 60 years on tomatoes, grape vines, apples, pumpkins and other types of winter squash, cucumbers, squash and other Plants types.
The results of one of the studies mentioned above on pumpkins and acorn squashes indicated that this readily available product could provide control similar to that achieved using conventional chemical fungicides, while compost tea applications were not effective. Milk treatment was more effective in the early stages of infection.
Common advice is to dilute the milk 1:10 with water and spray it on the plants at the first sign of infection.
Powdered or liquid milk can be used. However, based on the research available to date, higher concentrations of milk may be more effective.
Try a 50/50 mix of liquid milk with water for moderate to severe infections, or even plain milk if you don’t have a large area to cover.
Some experts say that raw or organic milk is best, and that the fats in whole or 2% milk can serve as a deterrent to treatment, while others say it doesn’t matter which kind you use.
Add the liquid to a spray bottle and be sure to cover all affected areas of your plants, including stems and the undersides of leaves. Continue with applications twice a week until you see results and be sure to reapply if it has rained.
You shouldn’t worry about milk spots on plant leaves (at least less unpleasant than mildew, if you see any), although the spray can develop a sour smell in the hot sun. Better than having sick plants on your hands, of course!
Milk can also help prevent infections if you spray it weekly on your plants.
Sodium And Potassium Bicarbonate
Researchers have been testing the effectiveness of sodium bicarbonate (baking soda) as a fungicide since the early 1930s and found that it is not effective as a fungicide on plants, although it can inhibit mold growth in the laboratory.
According to Dr. Linda Chalker-Scott, associate professor of plant pathology and extension specialist at Washington State University, it can be helpful in keeping harvested fruits and vegetables from spoiling.
But we cannot recommend it as a treatment for powdery mildew.
MilStop, Biological Fungicide
However, there are some fungicides that combine the closely related compound, potassium bicarbonate, with additional components.
Both formulations are listed by the Organic Materials Review Institute (OMRI) as approved for organic growers and safe for home use. Read product labels for application instructions.
Sulfur is a classic fungicide that is effective in controlling powdery mildew, but the problem is that it must be applied preventively, before symptoms appear. You can spray it on plants known to be susceptible, to prevent infection.
It is considered to be one of the most effective and least expensive products to use against the fungi that cause this disease.
Many sulfur product labels will state that it can be applied immediately at the onset of infection, but results may vary.
Some powdered products include surfactants to make them easier to mix with water and spray on plants.
Bonide Sulfur Plant Fungicide
Sprays should generally be applied weekly and after rain, but this will depend in part on the type of plant you’re treating.
Do not apply sulfur to any plants that have been treated with horticultural oils for at least two weeks, and avoid application if temperatures are expected to exceed 80°F.
Some gardeners also choose to brush or dust sulfur products on the leaves of their plants, but it is important to note that these products can irritate the eyes and throat and are not suitable for use on all types of plants.
Be sure to wear proper protection and read all product labels carefully before use. Its use should be avoided on certain types of apples, grapes, pumpkins and melons.
There are several different types of microbes that are marketed to control this disease and are OMRI approved as organic.
An advantage of using biofungicides is that they leave no toxic residue and fungi are much less likely to become resistant to them.
The active ingredient in Actinovate AG is a well-studied strain of Streptomyces lydicus, while Double Nickel 55 LC and WDG contain the commonly used biocontrol bacteria Bacillus amyloliquefaciens, as do Serifel and Taegro 2.
Aviv and Companion contain different strains of Bacillus subtilis.
All of these microbes are sprayed onto the leaves, usually once a week, but every three days if you choose to use Cease to treat active infections. Again, always read product labels and follow recommended application instructions.
Copper is a common fungicide that can be effective against powdery mildew and can also be used to control some types of bacteria. Many preparations are certified organic.
However, some experts claim that it has limited effectiveness, especially when compared to sulfur.
Bonide Liquid Copper
Arbico Organics sells Bonide Liquid Copper Fungicide, a low-concentration copper octanoate spray. This is a form of copper that binds to a soap.
The ready-to-use formulation connects to your hose and then you spray it on your plants. It is more effective if you apply it preventively, similar to sulfur.
If your plant is infected, apply copper as soon as you notice symptoms and repeat every seven to 10 days. Be sure to spray both the tops and bottoms of the leaves and reapply if it rains.
Rated “slightly toxic” by the EPA, be sure to closely follow product directions, cover exposed skin, and avoid eye contact or inhalation when applying this product.
Copper is toxic to fish, so don’t use it near ponds or streams.
Don’t be afraid if you see dusty growths on your plants. These infections are treatable and it is not even necessary to resort to the use of conventional fungicides.
There are a variety of options available, and many of them are certified organic.
However, as with most treatments, they are the most effective if you get an infection in the early stages.
If you’ve had recurring problems with powdery mildew in your garden, consider planting resistant varieties of your favorite flowers, fruits, and vegetables next year. Cultivar descriptions in catalogs and on seed packets will include this information.
Many types of produce have been bred for resistance to this disease, particularly certain varieties of cucumbers and other cucurbits such as pumpkins, melons, and various types of summer squash, as well as Brussels sprouts, bush beans, and strawberries, just to name a few . few.
Hardy annual and perennial flower cultivars include certain types of bee balm, black-eyed Susans, delphiniums, hardy geraniums, phlox, salvia, verbena, and zinnias.
Have you fought powdery mildew in your garden? If so, share your experiences with Answer The Question in the comments below.