Dew

On clear, calm nights, surfaces such as leaves radiate heat to the sky and cool faster than the surrounding air. If a surface cools to the dew-point temperature of the adjacent air, water vapour condenses onto it as dew. Cloud cover, wind, and dry air all tend to reduce dew formation — clouds slow radiative cooling, wind mixes warmer air down to the surface, and dry air has a lower dew point that is harder to reach.

The most agriculturally important effect of dew is the prolonged leaf wetness it produces. Many fungal and bacterial pathogens require a film of water on the leaf surface for their spores to germinate and infect, and the number of hours that leaves stay wet is a key variable in the development of diseases such as downy mildew, apple scab, and various leaf spots and blights. Powdery mildews are a notable exception: they infect under high humidity and are inhibited rather than favoured by a film of free water. Dew that persists into the morning extends this wetness period.

• Overnight dew can supply the leaf wetness some pathogens need to infect
• The duration of leaf wetness is used in many disease-forecasting models
• Dense canopies and poor air movement can prolong wetness and raise disease risk
• Managing crop architecture and airflow can help leaves dry faster

Beyond disease, dew provides a small direct input of moisture that can be a minor benefit in dry environments and can slightly ease water stress in some situations. Dew and the leaf wetness it creates also interact with the timing and effectiveness of some field operations, such as spraying and harvesting, which are often adjusted around morning wetness. Its role, however, is generally as a modifier rather than a major water source.

This entry explains dew as a concept and its main agricultural effects qualitatively. How much dew forms, how long leaves stay wet, and how that translates into disease risk depend on local climate, canopy, and the specific pathogens present, and are not quantified here.