One of the biggest problems that I had faced in my bonsai journey was a disease called powdery mildew- a white, powdery fungus mainly caused by high humidity. So, if the leaves of your bonsai get covered with a white substance? It’s probably an indication that your bonsai might also be affected by powdery mildew. Don’t worry the fungus is easy to identify and treat and even easier to prevent. But if you ignore it for longer it can fatally harm the bonsai’s looks. So, let’s learn how to identify, treat, and prevent this disease. 

What is powdery mildew and how to identify it (symptoms of powdery mildew):

Powdery mildew is one of the most common fungi that affects a wide variety of bonsai trees. You can identify it easily, by looking at the leaves of your plant, they will appear as light grey or white powdery flecks-like spots, especially on the upper side of the leaves, but can also be found underneath the leaves, or on stems of the bonsai. The spots outspread and will ultimately cover most of the parts of the bonsai.

Powdery mildew loves warm, dry climates; but, it also requires relatively high humidity — like the warm days and chilly nights in late spring to early summer. Lack of sunlight and poor air circulation also contribute to states that promote powdery mildew.

Sometimes, a little bit of humidity in the air is sufficient for powdery mildew to spread. It’s become more aggressive when the temperature warms up. Powdery mildew doesn’t practically kill the plant, but it reduces photosynthesis. The powdery layer blocks photosynthesis. 

Mildew spores are circulated by the wind in warm, dry climates, but don’t circulate well when the atmosphere is rainy and cold. Spores can endure cold winters in leaf piles and on bonsai, so it’s necessary to discard and not compost any plant debris that fell from infected plants to control our spreading or allow it to resurface the next spring.

How to identify Powdery mildew:

  • White layer of powdery coating will be detectable at the upper surface of the leaves. In severe cases it can spread to the lower surface of the leaves as well. Under favourable states, the patches broaden and integrate to cover the entire tree.
  • New growths, especially the leaves, water sprouts, and green shoots are usually most harshly affected.
  • Leaf edges curve or twist upwards like a cupped shape.
  • Purple to reddish spots may also form on leaves.
  • Small, rounded, black fungal structures (cleistothecia) may also be present on the underneath of the leaves.
  • Buds and fruit of bonsai are usually spared the white mildew, but infected plants have low yields and imperfect fruits in terms of shape and quality.
  • When heavily infected, leaves of the bonsai may turn yellow and fall prematurely during the spring.
  • In some cases around the end of the summer or before autumn, you will notice the growth of orange or black spores within the white layer. 
  • Easily getting wipes out after cleaning. 

Treatment 

Chemical treatment:

Powdery mildew effects on the surface level of the tree and can be removed easily with a cloth. But it can reappear, so normal fungicide will be enough to remove them permanently. Apply fungicides at one to two weeks intervals to provide constant protection throughout the growing season (spring and fall). Follow the guidelines on the fungicide label for use on detailed plant species, types, rates to be used, timing of applications, and waiting periods. Here are a few fungicides that work best on powdery mildew treatment are;

Sulphur:

Sulphur remains the go-to preference of bonsai growers for limiting powdery mildew spreading. Sulphur has minimal to no chance of disease-causing fungi developing resistance to it. Sulphur burns the disease-spreading spores by disrupting respiration within their cells. It does this either by residual activity or, at higher temperatures, by volatilization. One of the best sulphur enriched fungicides that I have been using for years is Bonide (BND141).

Neem oil:

Plants with fuzzy leaves can’t handle the burning effects of sulphur. For that, you should opt for horticultural oil. If mild to normal amounts of powdery mildew are there, horticultural oils and any kind of plant-based oils such as neem oil can be used to remove the spores completely.

Triforine:

Triforine is marked for the management of black spots, powdery mildew, and rust on bonsai and is one of the fungicides preferred by bonsai growers. On certain ornamentals, it is marked for managing powdery mildew. Triforine breaks down rapidly in the climate.

Potassium bicarbonate:

Mix 1 tablespoon potassium bicarbonate and half a teaspoon of mild liquid soap in 1 gallon of water. Sprinkle liberally on all infected parts of the tree. This remedy may work better than other methods as a remedy for living infections. You can go for Earth’s Ally Disease Control for Plants, it’s affordable and works like a charm. 

Homemade Treatment:

Baking soda solution: 

Mix 1 tablespoon baking soda and ½ teaspoon mild liquid soap in 1 gallon of water. Cover the entire tree, getting top and bottom leaf surfaces and any infected places. This method may work nicely as a preventive action, although it does have some impact on existing powdery mildew as well.

Milk: 

Combine 1 part milk with 2 parts water(1:2 ratio) and use it on the affected area. While the science behind this method isn’t fully explained, it seems to work fairly well. It is acknowledged that naturally occurring blends in the milk not only fight the disease but also promote the tree’s immune system.

Prevention of powdery mildew:

Powdery mildew thrives on our contact with small mistakes, which is why prevention is important. Maintain the following methods to keep the bonsai healthy for a long run;

Keep the foliage dry:

To keep leaves dry and prevent powdery mildew formation, water your bonsai tree near the soil surface instead of overhead watering. Make sure there is good spacing between branches to allow for air circulation and avoid overcrowding plants to lessen moisture buildup.

Remove or trim out infected leaves:

You can prevent further spreading of mildew by removing the leaves that are infected. It would be better if you isolate the bonsai from others as well. If the tree has dense foliage, try thinning out a bit. 

Promote good air circulation:

For indoor bonsai, you should choose a place for your bonsai where it will have good air circulation. Open the windows in the morning for a few hours. If you have no way to provide good air circulation, you can use a fan as well. 

Properly place your plant:

Overcrowding can cause mildew formation as well. Give them space to breathe freely. 

Scouting:

Scout for disease on a routine schedule to determine outbreaks before they evolve and widespread.

Tips for powdery mildew:

  • Do not fertilize infected bonsais unless it is suggested by a soil test to fix a nutrient deficit. Fertilizer will force the tree to grow young shoots which are highly liable to powdery mildew.
  • Prune the tree during winter months to boost light penetration and enhance air circulation throughout the canopy.
  • Place bonsai in proper sunlight according to their requirements.
  • Disinfect pruners or shears after usage on infected trees. 
  • Do not do overhead watering early in the morning or in the evening. 

Conclusion

Powdery mildew is likely one of the most typical, easily identified, and widely spread infections. This disease is mostly of ornamental concern since it usually results in deformed and unattractive trees rather than plant death. Signs of the condition are easily recognized by the white, powdery spread of the fungus on the tree. This post discussed the symptoms, treatments, factors for disease development, and strategies to prevent these common diseases. Hope you can take control whenever you notice the disease.