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Powdery Mildew Disease – Identification, Control, Prevention

Powdery Mildew Disease – Identification, Control, Prevention

Powdery mildew is a notoriously common problem that affects a wide range of plants. Depending on the time of the year and the seasonal conditions, it can occur in most areas.

The good news is that it is normally not fatal but it is certainly not good for your plants and will have a negative impact on the growth as well as flowers and fruit. It also looks unsightly. 

We will look at what the disease is and what it does, how to identify it, how to treat plants infected with powdery mildew, and how to prevent powdery mildew. 

 


 

Why worry about powdery mildew? 

This fungal infection is likely to be the most common threat to your plants. While it is not a killer powdery mildew is not something you want on your plants. It will do serious damage to a wide range of plants. It spreads rapidly if not treated effectively. Although more common in certain climates or seasons, it occurs on many plants and can be found all over the world. Every gardener needs to be aware of the threat of this fungal infection, how to prevent it, and how to treat it. 

 

What is powdery mildew?

Powdery mildew is a fungal disease. There are multiple species and each one prefers different types of plants. There are hundreds of species and all are in the Erysiphales order. The most commonly seen powdery mildew is the Sphaerotheca fuliginea (Podosphaera xanthii). 

The fungal disease attacks flowers, including roses, fruit, vegetables, and other plants. 

Once plants are infected it can spread rapidly. It will put the plant under strain and slow down growth. Powdery mildew also has a negative impact on the flowers, fruit, or vegetables. Yields will reduce and the taste of most fruit and vegetables will be negatively affected. 

The fungi “feed” on the nutrients from the plant. This is possibly why powdery mildew does not kill the plants but is what causes the damage. 

Apart from the negative health impact on the plant, it looks unpleasant. Everyone wants their plants to look happy and healthy. Powdery mildew is quite noticeable and ruins the appearance of the plants, be they decorative or vegetables and fruit. 

Many people associate fungal infections and mildew in particular with wet cool climates but powdery mildew prefers warm dry climates (roughly  60-80°F / 15-27°C) although it does thrive in humid environments.

Shade plants are more susceptible to the fungal disease than those in full sun. 

The fungi tend to be host-specific. One strain of the infection will prefer a specific plant or type of plant. 

 

How to identify powdery mildew 

Powdery Mildew on th Upperside of the Leaves

The common name of the disease gives it away. Fine white to whitish-gray spots that look like powder appears on the plant. The spots are made up of spores and fungal threads known as mycelia.

Many people compare the appearance to powder or flour that has been sprinkled on the leaves of the plant. The comparison is logical when one looks at a plant infected with powdery mildew. 

They can be seen on the leaves, the stem, and the fruit in some cases. It is more common on the upper-side of the leaves but also occurs on the underside. It is also more likely to be present on new growth. 

It is a fuzzy growth that is quite easy to notice even in the early stages. The spots are small and circular.

Over time they grow bigger until they cover the entire leaf and eventually the stem as well as the flowers or fruit. On closer inspection, the growth might have a spiderweb-type appearance. 

Left untreated the spores will increase on the plant covering large parts of the leaves. They will often spread to other plants. Over time, the leaves begin to yellow and they become dry.

The shape of the leaf often changes as the powdery mildew takes over. They twist, curl, or fall off. Whatever the results, the plant does not look great. 

It is not to be confused with downy mildew. While the appearance is similar, downy mildew is more prevalent on the undersides of the leaves whereas powdery mildew is more noticeable on the top of the leaf.

The environment in which they flourish also differs. Downy mildew is more prevalent in wet cool environments while powdery mildew flourishes in dry hot conditions. 

 

How does powdery mildew spread? 

The light spores of the powdery mildew can easily be carried by the wind. They can also be transferred from one area or plant to another by humans and animals, although this is rare. 

These same spores can also lie dormant in the soil for years so if the area has been infected in the past, it could reappear once conditions for the disease are favorable.

As soon as the soil is tilled or disturbed or new growth changes the soil structure, the spores can become active and infect plants.  

Other infected plants or weeds in the area could also transfer the disease to healthy plants. This can happen over relatively long distances so while your area might be safe, spores could travel from neighboring properties. 

No moisture is necessary for infection, unlike most other fungal infections. 

High temperatures 90°F / 32°C plus tend to slow down the spread as does rain. Having said that, the spores of powdery mildew can survive harsh winters. Although they will become dormant they can flare up again when conditions are more favorable. 

Powdery Mildew on Roses

 

What does powdery mildew do to your plants?

The fungus sends out threads into the plant as it develops. These fungal threads (haustoria) absorb vital nutrients from the infected plant. While they generally appear on the leaves they can also be present on the stem and later the fruit, buds, and other areas. 

They are host-specific. In other words, certain fungi prey on specific plants. Again, there are hundreds of varieties so they attack a wide range of plants. 

In some respects, this could be good news. The powdery mildew on your tomatoes, as an example, is unlikely to attack your roses. That is not to say you can be complacent as the fungus spreads rapidly and can quickly attack a range of plants. 

The other bad news is that, although not fatal, powdery mildew will weaken the plant. This not only affects growth and flowering or yields, but also leave the plant more susceptible to other disease, insects, and problems.

A healthy plant is tough and can often handle most problems. One weakened by a fungal infection will not be able to put up much of a fight. 

Powdery Mildew on a Bunch of Grapes

 

How to treat powdery mildew 

Powdery mildew can affect more 10 000 species of plants so it is important to always keep a careful eye out for it and treat it as quickly as possible.

Despite not killing the plant, in most cases, it is a menace that you do not want on your plants. It spreads rapidly so the sooner you can treat it the better. 

As with most diseases and pests, they tend to prey on plants that are weak or in poor health. Most strong plants are more resilient. We will get to how to prevent powdery mildew but let’s first look at what to do if your plants are already infected. 

There are several ways to go about treating this fungal disease. One option is to treat with commercial products which are generally chemical fungicides.

You can get commercial products that are based on organic ingredients. The other option is using natural products you can mix yourself, some might call them home remedies. 

Both chemical and organic products can be effective options depending on the severity of the infection, their environment, and the type of plant. 

We will look at all options. You might need to experiment a bit in order to determine which is best for your situation. Often a combination is recommended. 

 

Home solutions

 

Baking soda 

A safe, inexpensive, and effective home solution is a solution of sodium bicarbonate (baking soda). It has many amazing uses in the garden and preventing and treating powdery mildew is just one of them. 

To make this, simply mix a teaspoon of baking soda in water (1 quart/ 950 ml). Using a spray bottle, drench the plant in the solution. It is only effective where it makes direct contact. 

You can improve on this mix by adding a horticultural oil. Simply add just over two tablespoons of the oil to the mix above. 

As with many of these treatments, they need to be reapplied frequently. Once or twice a week is the minimum. 

Remember that excess bicarbonate buildup in the soil will slow the absorption of iron and reduces magnesium and calcium. Always monitor the process and results closely. 

This mixture is often more effective if one adds a small amount of liquid soap to the blend. 

 

Milk

While there are no conclusive scientific studies to prove the effectiveness of milk, many people are using it to treat powder mildew. It is used mostly on roses, cucumbers, and squash. It is believed that it acts as an antiseptic. It is mixed in a ratio of one part milk to three parts water and sprayed onto the plant. 

One of the first reports of this treatment was reported in 1999 by Wagner Bettiol, a Brazilian scientist.  Other tests were conducted by  Peter Crisp, a researcher from Australia. The results, in both cases, were positive. 

We are by no means saying that this solution prevents or treats powdery mildew but there are many people that swear by it. 

 

Mouthwash

Another unusual and controversial treatment is mouthwash. It is mixed in the same ratio as milk, one part mouthwash to 3 parts water, and needs to be sprayed regularly.

Mouthwash is designed to kill germs safely and appears to be effective on the spores of powdery mildew. It could have a negative effect on new growth so be careful with this method and keep a close eye on the plant. 

 

Garlic

Another common home remedy for many plant issues is garlic and it can be effective against powdery mildew. To create this mix, blend two or three garlic bulbs in a quart (950 ml) of water, and add a small amount (a few drops) of liquid soap.

Strain the solution to leave only the liquid and cool in a refrigerator. Use one part of this concentrate to 10 parts of water and spray the plant. If the fungus has already started to spread, increase the concentration. 

 

Apple cider vinegar

This has been used to protect against and treat powdery mildew for many years. Mix in a ratio of 2 tablespoons of the vinegar to a quart (950 ml) water and spray the plant thoroughly. Spray frequently and observe the plant to ensure it is working.  

 

Compost tea

Another popular antifungal is compost tea. It is relatively easy to make but you can buy commercial kits that will help with the process. The tea takes a bit of time to create but goes a long way. It is completely safe, effective against powdery mildew, and offers other benefits to the plants. 

 

Oils

These are used in many commercial organic products but can also be made at home. Oil such a canola oil works well but soap needs to be added to emulsify the mixture. Use about 3 tablespoons of oil, a quarter-teaspoon of soap (liquid dish-washing soap is fine) to a quart (950 ml) of water. 

Neem oil can be used in the same way and also protects the plant from other pests and diseases. Other oils used by some growers and gardeners include rosemary oil or mint oil. 

 

Commercial Organic Fungicides to treat powdery mildew 

Here, you will pay a bit more but there are many great products on the market that are safe and effective.

It is always best to use organic solutions where possible as they are effective against unwanted problems and diseases without harming beneficial organisms, insects, birds, and other aspects of the delicate balance in your garden ecosystem. 

The better products have been tried and tested and are safe to use. This is particularly important for use on fruits and vegetables. 

Preventative (protectant) fungicides are usually used (as the names suggests) to prevent the appearance of fungal infection. If you catch the powdery mildew early enough and it has not multiplied prolifically, these products should help. 

Ingredients to look out for include neem oil, lime-sulfur, sulfur, and potassium bicarbonate. 

If the infection has multiplied and advanced you will need an eradicant fungicide.

 

Bacteria solutions

Both Bacillus pumilis and Bacillus subtilis are bacteria that prevent and treat powdery mildew. They also protect against other diseases. 

Popular brands include Sonata (mainly used for roses),  Rhapsody (popular with commercial growers) and Serenade which is the main option found at garden centers for home use. 

 

Antitranspirants

These sprays are designed to reduce plant water loss (transpiration). They are often used on cut  flowers and Christmas trees as well as shrubs that have been transplanted.

They stop the plant from drying out but can also be effective against powdery mildew. They are also beneficial for plants with blackspot and downy mildew. 

The two most common brands on the market are Wilt Pruf and Vapor Guard. While these sprays are natural and safe they can reduce essential photosynthesis. Only use them when conditions are sunny and apply when new growth appears.

 

Air purification

For plants grown indoors, there are commercial air purification systems that reduce the risk and spread of powdery mildew. This is especially important when grow lights are used. Fans are often used with the lights to disperse the heat but this also helps to spread airborne pathogens. 

Air purification systems “clean” the air and destroy powdery mildew spores as well as many other harmful pathogens. 

 

Chemical fungicides (protectant or contact fungicides)

Sometimes a chemical solution is best. It can also be used in conjunction with other solutions we have discussed. 

The most common chemical treatments for powdery mildew contain propiconazole, triadimefon, myclobutanil, hexaconazole, or penconazole. Sometimes these are used in combination. 

Your local garden center should have a range of products specifically designed to combat fungal infections and most should be effective on powdery mildew. 

Most general-purpose fungal sprays should be effective but as we will discuss, it is important to not always use the same ones. 

 

Strobilurin Fungicides

These are systemic fungicides, part of the FRAC group 11 fungicides. They are effective against a range of pathogens and fungi. There is a range of options available. They work by preventing respiration in fungi. 

Some popular brands and the active ingredients include:

  • Flint – trifloxystrobin
  • Cabrio – pyraclostrobin
  • Quadris – azoxystrobin 
  • Quadris Top – azoxystrobin + difenoconazole, 11 + 3
  • Quadris Opti -azoxystrobin + chlorothalonil, 11 + M5
  • Pristine – pyraclostrobin + boscalid, 11 + 7
  • Quilt – azoxystrobin + propiconazole, 11 + 3

 

You will note that some of the products are used in a combination of ingredients. What is important to note is that in many cases the fungi can develop a resistance to the chemicals.

It is necessary to use a range of fungicides to avoid this. Alternate the chemicals used to prevent fungicide resistance. 

 

Other commercial treatments for powdery mildew:

SuffOil-X is another option. It needs to be sprayed regularly for prevention, control, and removal. Do not apply when it is extremely hot and bright as it could burn the plants. 

Monterey Complete Disease Control and Monterey Bi-Carb Fungicide are other options. 

For large applications, you can use MilStop.

 

How to prevent powdery mildew 

Once you have this disease on your plants it can and will spread rapidly. The best measure is always to defend against it and prevent it before it starts. If your plants do become infected, you need to ensure that it is treated and removed quickly so that it does not spread to other plants. 

Remember that spores can remain dormant for a few years so you need to prevent against reinfection down the line. 

 

Plant powdery mildew resistant plants

While many plants are susceptible to the disease, certain varieties have been bred or are naturally more resilient to the disease. This is especially important in climates where the disease is more common.

By using these plants you reduce the risk of infection and the spread of this fungal disease.

Even some of the most susceptible plants, including squash, melons, and cucumbers, have strains that are all but immune to powdery mildew. With a bit of research, you can find the strain that is best suited to your environment. 

If you are experiencing regular problems with powdery mildew it is important to research and find plants that are more resilient to the disease. 

If you find that certain plants or crops are particularly susceptible to powdery mildew it is probably best to remove these plants altogether and plant something else.

There is no point fighting an ongoing battle. It will only put your other plants at greater risk. 

 

Remove infected plants as soon as possible

The spores are light and travel quickly. If powdery mildew is noticed the best bet is to remove all infected plants immediately.

If this is not possible or viable, remove heavily infected leaves, treat, and monitor. 

Do not discard nearby or compost the infected plants as this will only worsen the problem. Bag and discard or burn the plants. 

 

Water well from above

One of the conditions that the disease thrives in is hot and particularly dry conditions. Without overwatering the plants, give them sufficient water so that they are not distressed. Water from above for maximum effect. 

 

Plant in the sun

While this cannot be done with all plants it is possible with most of the more susceptible plants. Powdery mildew prefers shade so it stands to reason that plants in the sun have a better chance of not being infected. 

You need to prune carefully around plants so that they get enough light and air to reduce the risk of powdery mildew. This pruning also reduces humidity which the disease thrives on. 

 

Ensure there is good air circulation

Poor circulation around plants leads to many problems and can contribute to an increase in powdery mildew. As mentioned above prune and clear the area around the plant to improve the airflow. Keep some space between plants so that they are not overcrowded. 

 

Do not over-fertilize 

It is best to use a slow-release type fertilizer to encourage natural growth. Powdery mildew is likely to occur on new growth so rapid growth could make the problem worse. 

 

Mulch

Applying a decent layer of compost (quality organic compost) after preparation is a good practice. This will cover any dormant spores that might be present in the soil and prevent them from getting to the plant. 

If you practice all of these precautions and preventative measures it is unlikely that you will need to resort to any organic or chemical remedies. A healthy protected plant in the correct environment is not likely to be affected by powdery mildew. 

 

Which plants are particularly at risk of powdery mildew?

As there are so many varieties of powdery mildew it affects a wind range of plants. 

Flowers at risk include roses, sunflowers, dahlias, zinnias, begonias, and chrysanthemums. Other plants at risk include hydrangea, phlox, gerbera daisy, and verbena. 

Vegetables and fruit that are particularly susceptible include tomatoes, peppers, cucumbers, potatoes, peas, beans, eggplants, melons, squash, pumpkins, grapes, parsley, and lettuce. Many fruit trees are also at risk. 

Marijuana is now a major legal crop in many parts of the world and there are also many enthusiastic home growers. Powdery mildew is a major threat to this plant especially when grown indoors. 

This list is not exhaustive and many other plants are susceptible. It is therefore important to keep a careful eye on all plants. 

 

Frequently asked questions about powdery mildew 

 

When do I need to worry about powdery mildew? 

One should always be on the lookout for pests and disease but when the weather is particularly hot and dry, you want to inspect regularly for signs of powdery mildew. It can occur overnight and spread rapidly. The sooner you identify and treat it, the easier it is to eradicate. 

 

Is it difficult to see and identify powdery mildew?

No, one thing about this fungal infection is that it is easy to spot even in its early stages. The white to gray spots are fairly obvious and can be seen with the naked eye. 

 

Are my plants safe from powdery mildew in a greenhouse?

Unfortunately, they are not. A greenhouse, in fact, is an ideal breeding ground for the disease and extra care should be taken in preventative measures. One should always be on the lookout for any sign of the infection and take fast and decisive action if it occurs. 

 

Does all powdery mildew look the same?

The answer is no. While the vast majority of cases present with white/gray spots that start on the leaves there are other variants of the disease that look different. 

A good example is  Leveillula taurica. This forms as yellow patches and is normally found on tomatoes and artichokes. The patterns and behavior are the same but the color is different. 

 

Can I not just ignore powdery mildew?

Just because it does not kill the plants is no reason to not be concerned. The disease literally sucks the nutrients out of plants and if left untreated it can spread rapidly. 

It makes the plants look unpleasant and unhealthy. In flowering plants you will have fewer flowers and the quality will be poor and possibly misshapen. In fruit and vegetables the yield will be significantly lower and of a poor quality size and the taste will not be great.  

 

Is powdery mildew harmful to humans?

The fungi are not harmful to humans. If you touch it or eat a fruit or vegetable that has powdery mildew it will not do you any harm. The only harm is to your attractive plants and your fruit and vegetable yields. 

 

What is the difference between powdery mildew and downy mildew? 

Both are fungal diseases but there are distinct differences. For instance, downy mildew is more prevalent on the underside of the leaves while powdery mildew is usually more noticeable on top of the leaves. Also, the spores are smaller and more difficult to notice. The color is also different. Downy mildew is darker, red to purple or brown in color. 

Downy mildew preys on similar plants and is also common but is more common in cooler climates or seasons. It also likes high humidity. 

 

Is powdery mildew the same as sooty mold? 

Not at all. While they are both caused by fungi they are completely different. Sooty mold, as the name suggests, is dark in color. The leaves turn black and start to die off. The fungus is prevalent on plants with an insect infestation. They secrete honeydew which is what attracts the sooty mold. 

 

Final thoughts

Powdery mildew is a serious fungal infection that is extremely common all over the world. The good news is that it seldom, if ever, kills the plants.

That is not to say it should not be taken seriously. It is a serious plant disease that needs to be treated quickly and effectively. 

Knowing how to identify it is critical. Prevention, as always, is better than cure. Take the necessary precautions to reduce the risk of infection. Inspect plants regularly to see if any plants have been infected.

If noticed, it should be treated correctly and regularly. Keep a very close eye on plants to ensure the treatment is working and do what is necessary to prevent the spread or future infection of plants. 

When the rights steps are followed for prevention and treatment, powdery mildew is nothing more than an irritation. Left untreated it can be disastrous. Do not think that just because it does not kill plants it is not a major threat. 

Look after your plants, treat them well, and avoid this dreaded fungal disease. 

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