Powdery Mildew on Begonias: White Spots, Causes, and Treatment

By Richard Flowers, ACCNP, Green Thumb Nursery Ventura

If your begonia has developed a grayish or white powder on its leaves, stems, or flowers, or you are noticing white spots that look like a dusting of flour, you are almost certainly looking at powdery mildew. It is one of the most common problems I see on these popular shade-loving plants, and it frustrates a lot of gardeners because it shows up on an otherwise beautiful specimen. The good news is that it is both treatable and, with the right growing conditions, largely preventable.

What the White Spots or Powder on Your Begonia Are

That grayish or white powdery growth covering the leaves, stems, and flowers is powdery mildew, a fungal disease that sits on the surface of the plant tissue. The telltale test is simple: if the white coating rubs off between your fingers, it is powdery mildew. It typically appears first as powdery spots or blotches on the older or upper surfaces of the leaves. Over time the tissue underneath the powdery growth may turn yellow or brown, and affected leaves sometimes drop from the plant. You can read more about how these symptoms present in the UC IPM Pest Notes on powdery mildew of ornamentals.

A few look-alikes are worth ruling out before you reach for a treatment. Downy mildew shows up as fuzzy growth on the leaf undersides with yellow patches on the top surface, so it sits below the leaf rather than dusting the top. Edema produces firm, blistery bumps on the undersides and comes from overwatering, not a fungus, and it does not rub off. Mealybugs look like small white cottony tufts tucked into the leaf axils, discrete clusters rather than a flat, even dusting across the surface. If you would like to compare healthy foliage and flowers with what you are seeing, you can browse begonias in our plant library.

Will Powdery Mildew Kill My Begonia?

Usually not. Powdery mildew can only survive in living plant tissue, so it rarely kills an older, established begonia, though it can certainly disfigure the leaves and flowers if left unchecked. Younger plants are more susceptible and worth watching closely. The Missouri Botanical Garden notes that powdery mildew is rarely fatal, which matches what I have seen over the years. Treat it early, keep up good sanitation, and most begonias come through just fine.

What Causes Powdery Mildew on Begonias

Powdery mildew thrives in cool, humid conditions with poor air circulation, which is exactly why begonias in our area are so prone to it. The disease is more problematic in cool coastal zones, while warmer inland areas tend to have less of an issue. It is most prevalent during late summer and fall, and it takes hold when plants are crowded together, grown in too much shade, or watered overhead so the leaves stay wet. One important point: unlike most fungi, powdery mildew does not need standing water on the leaf to infect your plant, only high humidity, which is noted in both the UC IPM guidance above and the Clemson HGIC fact sheet on powdery mildew. Begonias in dry soil are also more susceptible, as are those planted in damp, deeply shaded spots.

How to Prevent Powdery Mildew on Begonias

The best control for powdery mildew is to limit or prevent it in the first place by giving begonias the growing conditions they want. These perennials prefer a fast-draining soil, an open area with filtered light, consistent but light feeding, and enough water to keep them moist but not soggy. Taking each of the following requirements seriously is the key to limiting, and sometimes avoiding, mildew altogether.

• Light: Begonias need good, bright filtered light (not dim light), ideally with an eastern exposure that gives them cooler morning sun. Dark-leafed bedding begonias are the exception and can tolerate more sun. If light is deficient, your chances of getting mildew go up considerably.
• Watering: Water the soil, not the leaves, because wet foliage encourages the fungus. Keep the soil moist but never soggy, and water consistently and thoroughly. In hot, dry summers, grow begonias in containers set in pebble-lined saucers kept filled with water to just below pot level. This pebble-tray technique eliminates overhead watering and works well any time of year.
• Air circulation: Begonias love good airflow, so do not crowd them. Space pots farther apart, and indoors run a fan near the plants or open a window. On taller or bushier types, prune dense stems back to above a healthy set of leaves to increase air movement inside the plant.
• Fertilizing: Feed lightly and consistently so the plant grows vigorously and is better able to resist the fungus. Do not apply nitrogen fertilizer in late summer, since that creates the lush, succulent tissue this pathogen loves. I recommend a half-strength solution of a liquid fertilizer with an NPK around 8-12-4 every 2 to 4 weeks for good flowering and healthy specimens. Clemson likewise advises against excess nitrogen for this reason.

How to Treat Powdery Mildew Once You Have It

Once powdery mildew shows up, start with sanitation, because the fungus spreads by wind and by splashing water. If practical, pick off the diseased leaves and bag them for the trash. If a plant is severely affected, it may be best to remove and destroy it to protect the rest. Clear away any fallen leaves on the ground that look mildewed, since removing that debris helps break the life cycle and reduces the spread to healthy begonias. Whenever you prune or clean up infected plants, disinfect your tools afterward; Clorox Disinfecting Wipes make this quick and convenient.

If you still need a fungicide after improving conditions and sanitation, several appropriate options for powdery mildew are widely available and compatible with organic gardening. Whatever you choose, read, follow, and understand the entire product label before applying, and plan on repeat applications roughly every 7 to 14 days. The UC IPM Pest Notes on powdery mildew cover these materials in more detail.

• Horticultural oil or neem oil: Neem is organic and works both to prevent and to knock back existing infections.
• Sulfur: A good preventive, but do not apply it in high heat (roughly above 90 degrees F), as it can damage the plant.
• Potassium bicarbonate: Organic and effective at eradicating active mildew.
• Bacillus subtilis or Bacillus amyloliquefaciens biofungicide: A modern, widely available biological option that is organic-compatible.

Bring Us a Sample at Green Thumb Nursery

If you are not sure whether you are dealing with powdery mildew or one of its look-alikes, bring us a sample and we will help you sort it out. Take a fresh piece of the affected plant, seal it in a plastic bag so nothing spreads on the way over, and visit one of our five Green Thumb Nursery locations. We are always happy to look at it with you and point you toward the right fix for your begonias.

Begonia Powdery Mildew FAQ

Why are my begonia leaves turning white?

A white or grayish powdery film on begonia leaves is almost always powdery mildew, a fungal disease. The telltale sign is that the white coating rubs off between your fingers. It usually starts on older or upper leaves and spreads in humid, shady, poorly ventilated spots.

What causes powdery mildew on begonias?

Powdery mildew is caused by fungi that thrive in cool, humid conditions with poor air circulation. Begonias are most at risk in cool coastal areas, in late summer and fall, when crowded together, when grown in too much shade, and when the leaves get wet from overhead watering. Unlike most fungi, it does not need standing water on the leaf, only high humidity.

What are the white spots or white powder on my begonia leaves?

They are the fungal growth of powdery mildew sitting on the leaf surface. If the white coating wipes off, it is powdery mildew. If you instead see fuzzy growth on the leaf undersides with yellow patches on top, that points to downy mildew, and firm blistery bumps on the undersides usually mean edema (overwatering), not a fungus.

How do I treat powdery mildew on begonias?

Pick off and bag the infected leaves, clear any fallen debris, and disinfect your pruning tools (Clorox Disinfecting Wipes work well). Improve airflow, keep water off the leaves, and ease off nitrogen fertilizer. If it persists, apply an organic fungicide such as horticultural or neem oil, sulfur, potassium bicarbonate, or a Bacillus subtilis biofungicide every 7 to 14 days, always following the product label.

Will powdery mildew kill my begonia?

Usually not. Powdery mildew needs living tissue to survive, so it rarely kills an established begonia, though it can disfigure leaves and flowers. Younger plants are more vulnerable, so treat early and remove badly affected plants to protect nearby begonias.

Sources:

• UC IPM Pest Notes: Powdery Mildew on Ornamentals
• Clemson HGIC: Powdery Mildew on Landscape and Garden Plants
• Missouri Botanical Garden: Powdery Mildew (Outdoors)

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