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Powdery Mildew

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What is Powdery Mildew?

Powdery mildew on plants is a fungus that can infect a wide range of plants. It's easy to notice since it looks like light grey or white powdery spots on infected leaves, but it can also be found below, on stems, flowers, fruit, or vegetables. Powdery mildew has a wide variety of species, each of which targets a particular type of plant. Cucurbits (squash, pumpkins, cucumbers, melons), nightshades (tomatoes, eggplants, peppers), roses, and legumes are all regularly affected in the garden (beans, peas).


A layer of mildew made up of many spores grows on the top of the leaves when the fungus begins to take over one of your plants. The wind then carries these spores to neighboring plants. Powdery mildew can stifle your plant's growth and, if the infection is serious enough, impair fruit yield and quality.


Causes of Powdery Mildew

Powdery mildew flourishes in hot, dry conditions, but it also requires a high level of humidity, such as the hot days and cool nights of late spring and early summer. Powdery mildew thrives in settings where there isn't enough sunlight and there isn't enough air circulation.


Although rarely lethal, it can cause major damage to your plants by depriving them of water and nutrients if left untreated. Plants can become weak, bloom less, and develop slower as a result of most illnesses, which cause mild harm such as leaves turning yellow or getting withering or deformed.


Powdery mildew spores are usually carried into your garden by the wind, but if you've had previous outbreaks ( Infected before ), new outbreaks could be caused by dormant spores in old vegetative material or surrounding weeds.


Powdery mildew, unlike many other fungal diseases, flourishes in warm, dry conditions (60-80°F / 15-27°C), albeit it does require a high relative humidity (i.e., humidity around the plant) to spread. It does not spread as quickly in cooler, rainier locations, and it is also delayed by temperatures above 90°F (32°C). Plants in gloomy places are also more affected than those under full sunlight.


Symptoms of Powdery Mildew

  • Powdery mildew-infected plants or powdery mildew disease appear to have been sprinkled with flour.

  • It normally appears as round, powdery white patches on leaves, stems, and occasionally fruit.

  • Powdery mildew normally grows on the tops of the leaves, although it can also grow on the undersides.

  • Damage to young foliage is the most common. The leaves become yellow and shrivel up.

  • Some leaves may twist, shatter, or become deformed as a result of the fungus.

  • Powdery mildew's white patches will spread to cover the majority of the leaves or affected areas.

  • In addition, the leaves, buds, and developing tips will be deformed. Late in the growth season, these symptoms frequently occur.


Powdery Mildew Prevention

  • Powdery mildew (powdery mildew causal organism) is best controlled through proactive prevention, as it is with many pests and diseases.

  • Powdery mildew-resistant plants are ideal for your landscape. Many mildew-resistant cucurbit cultivars (melons, cucumbers, squash, and so on) have been produced and are available from major seed companies.

  • Powdery mildew on plants thrives in shaded regions, so locate plant in the sunniest spots possible.

  • To assist reduce relative humidity, trim overloaded areas to encourage air circulation around your plants.

  • Maintain enough distance between plants and keep them away from walls and fences to promote proper air circulation and lower relative humidity.

  • Watering from above can aid in the removal of spores from leaves. Wet foliage, on the other hand, can often lead to the development of other common diseases, so don't rely on it as a preventative measure.


Powdery Mildew Disease -How to Get Rid of It?

Powdery mildew management is tough to eradicate the illness once plants have become extensively infested, therefore focus on preventing it from spreading to other plants. Remove all sick foliage, stems, and fruit from the plant and dispose of it in the garbage or burn it. Any affected plant should not be composted because the illness can still be transferred by the wind and survive in the composted materials. 


Allowing pruning shears to contact healthy leaves after removing sick areas is not a good idea. To begin, use rubbing alcohol to sterilize your pruners.


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Powdery Mildew Control - Preventative Measures Made at Home

If plants develop powdery mildew there are a variety of ecologically friendly ways to get rid of the condition, few of them including:

  • Sulfur, lime-sulfur, neem oil, and potassium bicarbonate are all effective organic fungicides for treating powdery mildew. These are most helpful when administered before an infection or when symptoms of the disease first appear.

  • Many gardeners have found baking soda to be beneficial in curing powdery mildew. 1 quart of water + 1 teaspoon baking soda Because the solution will only destroy fungus that it comes into touch with, spray the plants extensively.

  • Another effective home cure is milk spray. Spray on roses after diluting the milk with water (1:10) at the first indication of infection or as a prophylactic precaution.


Powdery Mildew Fungicide

Powdery mildew infections are usually minor, which is excellent news. Plants that are otherwise healthy will typically recover on their own after weather changes and with adequate care to increase airflow.


Often, treatment isn't required. Treatment methods that do not need the use of conventional fungicides are available for serious infections or a widespread problem throughout the garden. Fungi can acquire resistance to fungicides if they are used frequently. Both agriculture and human health are affected by this. But, many fungicides are there, which are very effective for controlling powdery mildew. But we should apply the fungicide that leaves little trace, and lasts for a longer period. Triadimefon is one such example. It can be sprayed with 1000–1200 WP of 15% wettable powder, 1 time every 10 days, and 2–3 times every 2–3 days. However, check with your local nursery for permitted fungicides in your area.


Powdery Mildew Treatment

Powdery mildew appears as dusty splotches of white or gray powder on the leaves and stems of diseased plants, as the name implies. Powdery mildew is often identified by splotches on the surface of leaves, but the disease usually begins on the undersides of leaves and spreads to stems, flower buds, and even fruit.


Although powdery mildew can affect a wide range of plants, each fungal infection is "host-specific," which means the fungi that infect that plant are unique to that varietal. In other words, the powdery mildew on your lilacs will not spread to other plants in your landscape.


Did You Know?

  • Powdery mildew of rubber - Young rubber leaves, buds, inflorescences, and other immature tissues are affected by powdery mildew disease, which reduces rubber latex output by up to 45 percent. The disease is found across the rubber-growing world, with a high incidence rate in subtropical areas.

  • Spots or patches of white to grayish, talcum-powder-like development characterize powdery mildews. 

  • Powdery mildews thrive in hot, dry environments. Powdery mildew resistance and tolerance have been established in a variety of plants. Infection is more likely in succulent tissue.


Conclusion

When plant foliage is dry, illumination is low, temperatures are moderate, and humidity is high, powdery mildew develops. Powdery mildew thrives in the late spring and early summer, when the evenings are still cool and humid but the days are starting to warm up.


Powdery mildew isn't always lethal to plants; it's more of a cosmetic concern in many circumstances. Powdery mildew, on the other hand, can leach nutrients from the plant if left untreated, eventually causing the leaves to wither and yellow. This can make flowers look unattractive and expose veggies and fruits to sunburn. Powdery mildew can impair a plant's potential to produce and alter the flavor of fruits and vegetables.

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FAQs on Powdery Mildew

1. What is powdery mildew?

Powdery mildew is a common fungal disease that affects a wide variety of plants. It is easily identifiable by its characteristic white or grey powdery spots that appear on the leaves, stems, and sometimes flowers of infected plants. The fungi responsible for this disease belong to a group known as Ascomycetes, and they are obligate parasites, meaning they can only survive on a living host plant.

2. What are the common symptoms of a powdery mildew infection on plants?

The most obvious symptom of powdery mildew is the appearance of blotches or spots of white to greyish powder-like growth. Other symptoms include:

  • The powdery spots typically appear first on the upper surface of the leaves.
  • As the disease progresses, the spots can merge, covering the entire leaf surface.
  • Infected leaves may turn yellow or brown, become distorted, and drop prematurely.
  • In severe cases, it can also affect flowers and fruit, reducing the overall vigour and yield of the plant.

3. What is the main cause of powdery mildew and which fungi are responsible?

Powdery mildew is caused by various species of fungi belonging to the order Erysiphales. Unlike many other plant diseases, these fungi are highly host-specific. For example, the powdery mildew species that infects roses will not infect lilacs or cucumbers. These fungi are obligate biotrophs, meaning they obtain nutrients exclusively from living plant cells, establishing a parasitic relationship. This type of parasitism is a key characteristic that distinguishes many fungi from algae. For more details on this, you can read about the Difference Between Algae and Fungi.

4. Why does powdery mildew thrive in high humidity but on dry leaf surfaces?

This is a key characteristic that often confuses gardeners. Unlike downy mildew, which requires wet leaves to develop, powdery mildew spores can germinate and infect in conditions of high ambient humidity alone. The spores do not need free-standing water on the leaf surface. In fact, rainy conditions can sometimes wash the spores off the leaves and inhibit the spread of the fungus. Therefore, warm days with shady conditions and high humidity create the perfect environment for a powdery mildew outbreak.

5. What are the effective methods for treating and controlling powdery mildew?

Controlling powdery mildew involves a combination of cultural practices and treatments. Effective methods include:

  • Cultural Control: Improve air circulation by pruning and properly spacing plants. Avoid overhead watering and remove and destroy infected plant debris to reduce spore survival.
  • Organic Treatments: Solutions made from neem oil, potassium bicarbonate, or even diluted Hydrogen Peroxide can be effective when applied regularly.
  • Chemical Fungicides: For severe infections, fungicides containing active ingredients like sulfur or myclobutanil can be used. These chemicals are designed to kill fungi, a property indicated by the suffix '-cide' as explained in Biology Root Words Starting with Cide.

6. How can one differentiate between powdery mildew and downy mildew?

While both are fungal diseases, they have distinct differences:

  • Appearance: Powdery mildew appears as white, flour-like dust on the upper surface of leaves and can be wiped off. Downy mildew causes yellowish spots on the top of leaves and a fuzzy, greyish or purplish mould on the underside.
  • Location: Powdery mildew is primarily on the top surface, while downy mildew is on the bottom surface of the leaves.
  • Conditions: Powdery mildew prefers warm, dry weather with high humidity. Downy mildew thrives in cool, wet conditions.

7. How does powdery mildew actually harm a plant's health and yield?

Powdery mildew harms a plant by interfering with its vital functions. The fungal growth, or mycelium, spreads across the leaf surface, physically blocking sunlight and severely hindering photosynthesis. The fungus also penetrates the plant's epidermal cells with specialised structures called haustoria to siphon off nutrients. This dual attack weakens the plant, leading to stunted growth, reduced flower and fruit production, and overall poor health, which can ultimately cause the plant to die.

8. Can powdery mildew on plants cause harm to humans or pets?

Generally, the types of fungi that cause powdery mildew on plants are not pathogenic to humans or pets and are considered non-toxic. However, the abundant spores produced by the fungus can act as an allergen for sensitive individuals, potentially triggering asthma attacks or allergic reactions upon inhalation. While it is not recommended to consume heavily infected produce due to poor quality and taste, accidental ingestion is unlikely to cause illness.