What causes powdery mildew on plants?

Powdery mildew is caused by a type of fungus, which is the reason why this choice is correct. This fungal disease is characterized by white or grayish powdery spots that commonly appear on the upper surfaces of leaves, stems, and buds of affected plants. The fungi thrive in warm, dry environments, and typically spread when humidity levels are moderate, making it a prevalent issue in many gardens and landscapes.

Understanding the etiology of powdery mildew is important for effective management and prevention. Measures can include improving air circulation around plants, ensuring proper spacing, and applying fungicides when necessary. Since the other options refer to diseases caused by bacteria, viruses, or insects, they do not account for the fungal nature of powdery mildew, which is critical to diagnosing and managing this common plant disease effectively.