Precipitation and dew in a soybean canopy: Spatial variations in leaf wetness and implications for Phakopsora pachyrhizi infection

Abstract

With the occurrence of soybean rust ( Phakopsora pachyrhizi) now a seasonal problem in the United States, increased research is needed to ascertain the potential transmission of this disease among soybean ( 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 trifoliolates for several hours in order to infect the plant. These moist conditions can be achieved through any form of wetness. This wetness was measured in a soybean canopy at West Lafayette, IN, during the summer of 2007 using resistance-grid wetness sensors located at three heights representative of the top, mid-level, and lowest trifoliate of the canopy. Temperature, relative humidity, wind speed, and precipitation measurements were used to determine the cause of wetness. Over the course of the experiment, wetness events were classified as dew, rain, or neither. The dew events were then compared to rain events with an emphasis on differences in vertical and horizontal duration of wetness. Dew events were found to be the dominant wetness source in the upper canopy. Rain events were the most dominant cause of wetness in the lower canopy. Upper canopy wetness events had the longest duration. Horizontal variation in wetness and wetness duration was large.