Powdery mildew is one of the world’s most common plant diseases of both ornamental and food crops. It is very widespread, but luckily, easily recognizable and it can be treated. Powdery mildew doesn’t usually kill plants, but it sucks the nutrients and water out and weakens and damages their growth and development.
Powdery mildew is caused by a closely related group of fungi that thrive when warm days are followed by cool, humid nights. Air currents and wind carry the fungal spores which land on leaf surfaces and germinate. Some types of white powdery mildew are inhibited by moisture on plant leaves while others thrive on wet leaf surfaces.
What is powdery mildew?
There are hundreds of species of powdery mildew. But they are host specific and can’t survive without a particular type of host plant.
S. Newman and Laura Pottorff of the Colorado State University Extension explain that the powdery mildew species, Uncinula Necator, which causes powdery mildew on linden and grapes won’t attack lilacs. Microsphaea Alni, which attacks lilacs, catalpa, elms, and oak trees, won’t attack turfgrass.
The species of fungi that attack African violets is Oidium. Golovinomyces spadiceous (previously known as Erysiphe chicoracearum) attacks zinnias.
The plant specialists at the Utah State University (USU) Extension name two species of fungi that cause white powdery mildew on cucurbits – cucumber, melon, pumpkin, and squash. They are Podosphaera xanthii and Golovinomyces spadiceus (Erysiphe chicoracearum), with P. xanthii being more common in Utah.
A fact sheet developed at the USU Extension lists some of the powdery mildew species that affect common fruits and vegetables.
Erysiphe cruciferarum affects broccoli, cabbages, cauliflower, kale, and Brussels sprouts.
Erysiphe polygoni affects chard, beets, and beans.
Erysiphe pisi affects peas.
Erysiphe heraclei affects carrots and celery.
Golovinomyces cichoracearum & Podosphaera xanthii affect cucumbers, melons, and both summer and winter squash.
Leveillula taurica affects onions, peppers, and tomatoes.
How Does Powdery Mildew Spread?
Irrespective of the kind of powdery mildew plants are attacked by, it spreads the same way – in the air. Michelle Grabowski, Extension educator at the University of Minnesota, says powdery mildew spores are easily carried hundreds of miles by the wind.
Once the mildew spores land on a host plant, they germinate quickly and start a new infection. But they do better in some conditions than others.
For example, powdery mildew fungal disease thrives in warm, dry climates. When the weather is dry during the day, air currents enable the spread of powdery mildew spores. And it thrives in the moderate temperature range between 70 and 80℉.
The University of Maryland Extension warns that when conditions are right, it takes less than a week for the fungal infection to set in. And there are commonly continual cycles within each growing season.
To make matters worse, the fungi overwinter on the plants and quickly initiate new infections in spring.
As Grabowski says, some types of powdery mildew fungi survive on green leaves throughout winter. This makes them a threat to many greenhouse plants.
Other types create round, hard, dark structures that protect the powdery mildew spores during harsh winter weather. When the weather clears, the spores are released and spread by wind, splashing raindrops, or even by insects.
How to Identify Powdery Mildew Damage
Despite the many different species of powdery mildew that spread fungal disease, they all produce very similar symptoms on plants. Generally, the mildew spores create patches or spots of white-gray growth that are similar in appearance to talcum powder.
The fungi, which are essentially parasites, infest the plant and block out the light needed for photosynthesis and ultimate survival. Even though the infections are not usually lethal, in addition to the powdery blotches it causes, it also causes leaves to yellow and brown.
Optimum conditions for its growth are warm days followed by cool, humid evenings and nights. Also, young plants growing in heavy shade are usually the most seriously affected by this fungal disease.
Overwintering fungus takes the form of tiny spherical fruiting structures, the size of a pinhead, that start off white and then turn yellow-brown, and sometimes black.
Even though they damage their host plants, powdery mildews never invade plant tissue. They grow on the surface of the plant, feeding by sending up root-like structures into the top epidermal cells of the plant.
How to treat powdery mildew?
If disease buildup is severe, it may be necessary to use a proprietary fungicide. But as Jim Cooper, Master Gardener at Washington State University (WSU) Extension says, to avoid adverse environmental impact, it’s best to avoid these as much as possible.
Instead, there are various sulfur, carbonates, and petroleum-derived spray oils that effectively treat powdery mildew. If you do use a chemical fungicide, use one that is approved for home use.
Organic Treatments for Powdery Mildew
One of the most popular treatments is neem oil, but you need to catch the infection in its early stages for it to be effective.
Sulfur is a classic fungicide that kills mildew spores on contact. You can also use it preventatively because it keeps the spores from returning to areas you treat.
There are many sulfur products, including those that are organic. There are also copper fungicides and various bio-fungicides that are organic.
But, whether you use a fungicide or a compound that is approved for organic gardening, timing and proper application are vital, warns Cooper.
How do you prevent powdery mildew?
The best way to prevent powdery mildew is to plant species that are resistant to or at least tolerant of the disease. But additionally, you can take precautions that will minimize the risk of white powdery mildew.
These are some precautions that are recommended by many plant experts including Laura Pottorff and Newman, and Jim Cooper.
Don’t plant in low, shady, humid areas of your garden. Avoid overhead watering that tends to increase relative humidity.
Prune plants selectively to prevent and overcome overcrowding. This will help to increase air circulation and reduce relative humidity and ultimately reduce infection.
Avoid feeding with nitrogen fertilizers in late summer. This will help to limit the production of succulent plant tissue which is susceptible to infection.
When you spot powdery mildew fungus disease, remove infected parts of the plant as you can. If annuals and vegetables are infected, it’s usually best to dig them out in the fall to prevent the fungus from overwintering.
A final warning: Don’t compost infected plant debris. It’s very unlikely that temperatures of rotting organic matter will be high enough to kill the fungus. Rather burn it.
Powdery mildew is a common, very widespread disease that affects a wide range of plants, including fruits and vegetables. The vast number of species can be overwhelming, but, in reality, all produce almost identical effects.
It makes sense to know what powdery mildew is and how it spreads. As our garden tips reveal, the good news is that there are preventative measures you can take to minimize the risks of powdery mildew plant diseases.