Which factor primarily determines the growth rate of powdery mildew?

• A. Soil moisture
• B. Humidity
• C. Temperature (optimal around 25C)
• D. Light exposure

Answer: (C)

The main factor shaping how fast powdery mildew grows is temperature. The pathogen’s metabolism and infection processes peak at a warm, moderate range, with an optimum around 25°C. At this temperature, spore germination, attachment to the leaf, and rapid colony expansion happen most efficiently, so the disease progresses quickly.

Humidity and leaf wetness influence infection to some degree, but they are not the primary driver. Powdery mildew forms a powdery mycelium on the leaf surface and can spread under relatively dry conditions, and excessive leaf moisture can even hinder spore dispersal in some cases. Light exposure and soil moisture have even less direct impact because this is a leaf-surface disease, not one rooted in the soil or heavily dependent on light. So, the temperature around 25°C best explains why growth rate is fastest.