Which of the following describes the appearance of powdery mildew?

The appearance of powdery mildew is characterized by a white, powdery coating on the surfaces of leaves and stems. This distinctive coating is caused by fungal growth and can spread rapidly under certain environmental conditions, such as high humidity and moderate temperatures. The presence of this powdery substance can be easily recognized, making it a common issue among gardeners and agricultural professionals.

The other options describe symptoms associated with different plant problems. For example, brown spots on leaves often indicate leaf blight or fungal infections that cause necrotic damage. Yellowing leaves may suggest nutrient deficiencies, water stress, or other diseases. Black spots on stems are typically associated with different fungal infections or pests, but not with powdery mildew specifically. Understanding these distinctions is important for effective plant management and diagnosing issues in agricultural practices.