Precipitation and Dew in Soybean Canopies: An “In-Depth” Look at the Differences in Wetness with Canopy Height

With the occurrence of soybean rust (Phakopsora pachyrhizi) becoming imminent in the United States, increased research is needed to ascertain the potential transmission of this disease among soybeans (Glycine max L.) in major growing regions throughout the nation. The fungal disease spores are deposited on leaves in the lower region of the canopy through rain events or wind transport from nearby plants during the growing season and must then have moist conditions on the lower trifoliates for several hours in order to infect the plant. These moist conditions can be achieved through any form of wetness, i.e. drizzle, mist, fog, rain, or dew.

Wetness was measured in a soybean canopy at West Lafayette, IN, during the summer of 2005 using resistance-grid wetness sensors located at three heights representative of the top, mid-level, and lowest trifoliate of the canopy. For additional data, a temperature and relative humidity probe and anemometer were placed at approximately 8 cm above the canopy height. Measurements were taken at three-minute intervals for 47 days. Wetness sensors were checked for accuracy before and after the experiment, while in-field data logger checks confirmed accuracy of the other instruments.

Over the course of the field experiment, 24 events of interest were studied; seven dew events and seventeen late afternoon or evening rains. The dew events were then compared against the rain events to examine differences in the temporal formation and dry-down periods. Dew events themselves were compared and analyzed for duration length, differences in formation and dry-down periods, and other quantifiable factors. Two days of human observation after heavy dewfall events were also conducted and compared to dry-down readings by the wetness sensors. An average length of time for wetness events was established, and attempts were made at describing the dry-down process of early morning wetness transfer throughout the canopy.