Let’s face it… The word “mildew” means different things to people. To the gardener, “powdery mildew” is a fungi on vegetables, especially on cucurbits.
To the homeowner, it may be any discoloration or growth found in the bathroom.
To growers “powdery mildew fungi” means infected plants and three very different fungus plant diseases:
- Black mildew disease – a soot-like coating often found on leaves of slow-growing tropical plants
- Downy mildew disease (white spots on plants) – a delicate, white, frosty coating forming on the undersides of leaves
- Powdery mildew disease – makes infected leaves and stems (powdery spots) look as though they’ve been sprinkled with flour
The last one – powdery mildew – is perhaps the most common and destructive mildew of all in greenhouses during winter and spring months. A detailed discussion of this mildew, therefore, is appropriate at this growing season.
Powdery mildews are obligate parasites – that is, they grow only on living tissues and mainly on the surface of those tissues. Although they are primarily leaf parasites, they may grow upon stems, flower parts or fruits.
On some infected plants the fungal disease causes relatively little apparent injury; others are highly destructive.
A few different species, such as crape myrtle mildew, are highly host-specific as to food preferences and hence attack but a single kind of host plant.
Some powdery mildew species (like Golovinomyces cichoracearum formerly Erysiphe cichoracearum) attack a wide variety of plants.
Others, such as phlox mildew, can attack more than 280 different kinds of ornamentals, including these plants susceptible to powdery mildew:
- Phlox drummondii (the annual phlox)
- Begonia (rex)
- Calendula plants
Difference Of Powdery Mildew Fungus
Powdery mildew fungi differ in one important respect from most other fungi. Their spore germination do not require water to take place.
High humidity on the leaf surface is sufficient for fungal growth in which the mildew thrives. Such a situation exists frequently when plants are grown without good air circulation or when cold nights are followed by warm days.
Powdery mildew on roses has been the most widely studied mildew.
Most of our information on the behavior of mildews and their control is based on this particular one, which goes under the botanical name Sphaerotheca pannosa, variety rosae.
At one time old-fashioned syringing was used to control spider mites on roses in greenhouses, before miticides became widely popular for control.
Syringing with water reduces the severity of mildew. But, it cannot be recommended for mildew control, as it brings on other more destructive diseases, for example, black spot.
How To Treat Powdery Mildew On Squash
Powdery mildew affects a variety of vegetable plants like those in the squash, cucumber and pumpkin family.
Fortunately, it can be easily identified and is treatable. Look for white powdery spots on stems and leaves of infected plants.
NOTE: Most of the white powdery growth is the asexual spores which are the primary means of dispersal of the fungi. Young succulent growth is a favorite target of this disease.
Below are several organic ways for preventing powdery mildew on squash.
1. Milk – Diluted cow milk is a common safe and effective organic way to get rid of powdery mildew. Create a powdery mildew spray of 40% milk and 60% water. It was found to be as effective as chemical fungicides in managing powdery. Treat powdery mildew with milk every week making sure to alternate between other methods. [source]
2. Garlic – Garlic extract is an effective home remedy. Not only this powdery mildew killer is organic and easy to make, but it is also safe for plants and family.
To make this home powdery mildew treatment, blend two bulbs of garlic and add a quart of water and few drops of liquid soap. Once the mixture has blended well, strain and refrigerate.
To prevent germination of asexual spores, dilute the concentrate with 1:10 parts water before spraying the squash.
3. Water – Powdery mildew, unlike other types of mildew, thrives well in dry conditions with high humidity. It sounds strange but by watering your plants can help wash the powdery mildew spores away.
4. Baking Soda (sodium bicarbonate) – can be a better option in treating powdery mildew on plants. Here’s the recipe:
- 1 Tablespoon of baking soda
- 1 Teaspoon of horticultural oil
- 1 Teaspoon insecticidal or liquid soap (not detergent) – or try neem oil insecticide
- 1 gallon of water
Spray on plants every one to two weeks.
Before spraying any plants, test the diluted mixture on several plants to ensure they do not have any issues. Potassium bicarbonate additives also help control powdery mildew problems.
Details on Baking Soda For Plants
5. Mouthwash – Because of its ability to kill germs, mouthwash can be used to destroy mildew spores
Mouthwash is very powerful and therefore, it’s important to test a small area on squash leaves to make sure plants do not suffer any damage.
Dilute mouthwash with water in the ratio of 1 part mouthwash to 3 parts water
Use a generic, ethanol based mouthwash. Jeff Gillman, Ph.D and Associate Professor at the University of Minnesota, Department of Horticulture had very effective control.
His tests used one part mouthwash to three parts water. Be careful when mixing and applying mouthwash as new foliage can be damaged. [source]
6. Vinegar – Vinegar with its acetic acid, when sprayed on powdery mildew, will change the fungus pH thereby killing them effectively. Mix one gallon of water with four tablespoons of vinegar. Spray after every three days until the mildew has been totally wiped out.
How To Get Rid Of Powdery Mildew on Rose Bushes
The powdery meldew cure!
The first powdery mildew symptoms appear on young leaves. The leaves hold their color but begin to crinkle. Small patches of mold develop with spore-bearing fungal filaments.
These spread to the stems and other parts of the rose including the buds. As the fungal disease spreads it anchors itself to the foliage. From there, the fungus will draw on the nutrients and moisture within the leaves. [source]
NOTE: Enzymes structures produced by powdery mildew help break up the plant cells by forming haustoria in its host. “Haustoria are specialized organs formed inside living plant cells, which absorb nutrients and anchor the fungus.” [source]
This destructive disease can kill the plant within a short duration. Take appropriate measures to prevent powdery mildew or put out the disease in its early stages.
The drawing below shows how powdery mildew attacks a rose leaf.
- A – exterior “conidia”
- B – “mycelium” on the leaf surface
- C – “haustoria” of the mildew growing within the epidermal cells of the leaf
- D – the interior cells of the leaf
Inspection – Inspecting your plants for any signs associated with the disease. The signs are likely to be found on new leaves although it can occur on older parts as well. Carefully inspect the foliage and the blooms.
Prune – Like most fungal diseases, powdery mildew tends to affect crowded plants, which receive little sunlight and reduced air flow around and through them.
Pruning areas affected, this reduces the treatment area and allows more air movement If the infection is not very serious, you can simply pluck off the affected buds and leaves.
Water Spray – Just like the above water spray on squash, the same can be used on roses. Spraying with water washes off the spores before they have time to embed.
Apply Fungicide Spray – You can apply a copper fungicide spray once a week until all the signs of the mildew are gone. Apply early in the morning or late evening to avoid leave burn.
Powdery Mildew Baking Soda Spray Recipe from Rose Magazine:
- 1 tbsp vegetable oil
- 1 gallon unchlorinated water
- 1 tbsp apple cider vinegar
- 1 tsp Listerine (yes, the famous mouthwash, not mint flavor, just regular)
- 1 tbsp liquid soap
- 1 ½ tbsp baking soda
- Pump sprayer (large)
Before the mildew has affected your roses or to avoid re-infection you can use these tips
- Plant roses in locations that receive plenty of sunlight
- Improve air circulation around bushes by pruning
- Leave enough space between plants to facilitate good airflow
- Avoid overwatering creating conducive environments for mildew development. Consider drip irrigation for regular watering.
General Recommendations For Controlling Powdery Mildew
A high humidity condition allows and causes powdery mildew to spread and thrive on plant parts including:
- Plant surfaces
- Plant tissues
- and even plant debris
The usual preventative measure recommendations are to avoid excessive high humidity, drafts and sudden changes in temperature to prevent outbreaks of mildew.
However, exhaustive state university studies on the effect of temperature and humidity on powdery mildew of roses concluded that this disease…
“cannot be effectively controlled under greenhouse conditions through regulation of temperature and humidity, but that it may he held somewhat in check by keeping both temperature and relative humidity as low as possible for the cultivation of roses and by avoiding drafts and extreme changes in temperature. Since these conditions are difficult to meet, except possibly in winter, the application of fungicides seems to remain an indispensable measure in the control of the disease.”
How to Kill Powdery Mildew With Chemical Controls
Over the years many fungicide applications have been tested to control or kill powdery mildew.
Some have proved to be effective eradicants even after the mildew is quite abundant.
NOTE: According to the Journal of Plant Pathology Vol. 92, No. 3 (November 2010), pp. 775-779 – Powdery mildew on raspberry is genetically different from strawberry powdery mildew. This is why using and applying a chemical labeled for the disease is important. [source]
One reason for this is that the powdery mildews do not penetrate deeply into leaf tissue as do some diseases such as black spot.
Sulfur has long been a favorite way of combating mildew.
Bayer Advanced Disease Control for Roses contains the active ingredient Tebuconazole, providing three-way action: killing existing fungi while forming a protective barrier on the outside of the plant and is absorbed into the plant to keep on protecting regardless of weather conditions. More Here…
The best “natural control” is to grow mildew-resistant varieties or mildew-tolerant varieties of vegetables, crape myrtle, and roses.
However, here are a couple recommended natural controls (aka homemade organic fungicides) for powdery mildew homeowners can try courtesy Organic Gardening:
- To try this at home, mix 1 part milk with 9 parts water and spray the stems and tops of leaves with the solution. Reapply after rain.
- Spraying leaves with baking soda (1 teaspoon in 1-quart water) raises the pH, creating an inhospitable environment for powdery mildew.
Correct Timing Essential
As with disease control on outdoor plants, the correct timing of spray applications is most essential. Mildew is more easily controlled if the sprays are applied when the disease first appears. It is far more difficult to control on plants already heavily infected.
Although, control can be a challenge if your plants experience problems with powdery mildew look to improve air circulation and reduced humidity does seem to be a good start in reducing outbreaks.