Phlox is especially subject to powdery mildew problems.
This fungal infection is attributed to several different genera of fungus.
No matter which type attacks your phlox, the result is a powdery gray dusting of mildew on the plant‘s leaves.
This condition usually begins on the plant’s lower leaves and then moves upward.
Powdery mildew is a problem for many reasons.
First, it simply makes your plant look ugly.
Second, it causes plants to become weak and may even kill formerly healthy plants.
Not only does the mildew block sunlight, but it also attaches itself to leaf tissues and absorbs nutrients from the leaves.
Lack of ability to perform photosynthesis and loss of nutrients result in very sick or dead plants.
What Causes Powdery Mildew?
The mildew likes moderate temperatures and high humidity. It also likes intermittent rain showers.
The fungus grows best when the nights are cool and the days are warm.
Low humidity or an extended rainy season will inhibit the development of the disease, as will consistently high or consistently low temperatures.
Consistent temperatures higher than 86° degrees Fahrenheit (30° C) will kill the mold spores.
This is especially true if humidity levels are low.
Windy weather spreads the infestation by releasing spores and distributing them.
This might be a negative, but it’s also important to remember wind helps keep leaves dry and may thwart spore germination.
Like all fungi, powdery mildew does not like the sun.
Therefore, plants in shady places are far more likely to develop this disease.
Very lush plants having been given an abundance of nitrogen are far more likely to develop powdery mildew.
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How Can You Tell Your Plant Has Powdery Mildew?
At first, you’ll notice a light dusting of white powder on the lower leaves.
This will gradually move upward, and if left unchecked, will coat the entire plant.
At late stages of infection, affected leaves turn yellow, become brown and crispy and then fall off.
In advanced stages of infestation, you’ll see a dusting of pepper-like structures on badly infected leaves.
These structures contain mold spores and are resistant to extremes in weather, thus allowing the fungus to survive year-after-year.
What Can You Do About Powdery Mildew?
Unfortunately, the fungus is everywhere, all the time and there’s no getting rid of it.
As with other types of fungus, do your best to prevent allowing it to get a foothold.
Here are some preventative measures to take to keep powdery mildew away from your Phlox:
- Provide good air circulation. Be sure your plants are not overcrowded, and prune back dead foliage and flowers as needed to improve circulation and reduce the opportunity for any kind of fungus to set up shop.
- When your plants become overcrowded, be sure to dig up and divide them to make more space and improve air circulation.
- Avoid overhead watering.
Always water at ground level to keep the leaves dry.
When leaves are wet for a short period, powdery mildew is encouraged.
On the other hand, a forceful spray of water will knock the spores off the leaves.
- Water in the morning so your plants become thoroughly dry before nightfall.
- Avoid fertilizing excessively.
You should especially avoid giving your plants too much nitrogen as this is quite attractive to the fungi.
- Clear away dead plant matter at the end of the growing season so fungi will not have a place to harbor over the winter months.
- Apply fungicidal spray regularly to protect your plants’ leaves against the disease.
It’s important to understand spraying fungicide will not cure affected leaves, but preventative spraying can keep the fungus from being able to take hold on healthy leaves.
What Can You Do To Manage Powdery Mildew?
Examine your plants carefully throughout the growing season for signs of powdery mildew.
Early detection and action can help prevent the spread of this disease.
The moment you notice powdery mildew infestation take steps to eliminate it by pruning affected areas of the plant and using a fungicidal spray.
Maintain active preventative measures throughout the growing season.
Are There Organic Solutions?
You may want to create a natural baking soda spray with water.
Combine 1 1/2 tablespoons of baking soda with a gallon of water and apply the spray weekly.
Horticultural oil, or Neem oil may also be sprayed. More on Controlling Powdery Mildew with Neem Oil.
Combine three tablespoons of the oil with a gallon of water and spray every two weeks.
The baking soda solution can combine with the horticultural oil spray solution.
Apply this double-duty spray every two weeks.
Plant Mildew Resistant Types Of Phlox
There are quite a few Phlox cultivars resistant to powdery mildew.
These include:
- Orange Perfection
- Marshall’s Delight
- Prime Minister
- Blue Stocking
- Violet Queen
- Shortwood
- Catherine
- Starfire
- David
It’s important to understand some types of Phlox are resistant to powdery mildew in one part of the United States and not in another.
Check with your local agricultural extension to find out which will perform best in your area.