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What’s that powdery white stuff that’s suddenly appeared on your plants? Although it may look benign, it’s actually a fungal disease called “powdery mildew.”

What is it?

Powdery mildew is a fungal infection that affects plants; it most often appears on the leaves of the plant, but can also affect stems, flowers and fruit. There are several different types of powdery mildew, but they all come from the fungal order Erysiphales. All powdery mildew infections produce similar symptoms.

What does it look like?

Powdery mildew first appears as small white or gray “powdery” patches or spots that look like talcum powder, usually on lower leaves and stems. It can spread quickly as fungal spores multiply, covering the entire plant.

It will change color as it progresses, from white or gray to yellow-brown, and then black.

How does it start and spread?

Powdery mildew fungus first takes hold on a plant’s surface (usually the leaves), where water content is high.

It can happen because of water stress, when water supply for plants is inadequate for a period of time.

It begins on the surface of the plant (usually the leaves) and then creates microscopic dispersal spores that go airborne.

Note: Powdery mildew does not need particularly high humidity conditions to start or spread, as with some other fungal infections, although it does need high relative humidity. The spores have a high moisture content, which means they can infect more easily under drier conditions than most fungi can.

What are its symptoms?

Pinhead-sized spores first appear as white spots that look like talcum powder. These white spots multiply, turn a yellow-brown color, and finally turn black.

Your plants may look healthy when the infection first occurs, but can ultimately become discolored as the infection progresses. Because it hinders photosynthesis, powdery mildew will ultimately stunt growth and can even kill the plant.

Can you treat powdery mildew?

There are treatments for powdery mildew, although previously infected plants may not be suitable for harvest. More on treating and preventing it, next time.


Published on Feb 11 2015

Last Updated on Sep 23 2021