What parameters are used in forecasting grape powdery mildew?

The correct answer focuses on the key environmental factors that influence the development and spread of grape powdery mildew. Specifically, leaf wetness duration is a critical factor because this fungal disease requires a certain amount of moisture on the leaf surface to germinate and infect the plant. The duration of leaf wetness is directly related to the presence of free water on the leaves, which is a vital component for spore germination.

Daily maximum temperature and average hourly temperature also play significant roles in forecasting. Powdery mildew thrives under warm temperatures, typically between 68°F and 86°F (20°C to 30°C). Monitoring temperature helps determine whether conditions might be conducive to outbreaks of the disease, as extremely high or low temperatures can adversely affect the lifecycle of the pathogen.

In summary, the combination of leaf wetness duration, along with daily maximum and average hourly temperatures, provides critical insights into the potential for grape powdery mildew development, enabling growers to implement timely management strategies.