Simple correlation analyses with moisture-related indices can be
confounded by the fact that low soybean yields can result from both
dry and wet conditions. For example soybean yields in MT in 2006
were reduced not due to drought, but due to the increase in the incidence
and severity of Asian rust and other problems with pests due to
excessive rain and elevated in-canopy wetness duration. The cost of
soybean production increased by 600% in 2006 due to intensive fungicide
spraying to control rust (Soares, 2007). The following year, a
host-free period was introduced in MT to help control future rust outbreaks.
Furthermore, the shift toward shorter season cultivars reduces
the overlap between the soybean growing season and prominent periods
of rust outbreak, lessening damage to yield. Such management
changes can lead to a different moisture-yield behavior in subsequent
years, further disrupting correlationswith satellite-derivedmoisture indicators
such as ESI and TRMM’. In this case additional informationwill
be required to accurately interpret impacts resulting from moisture
anomalies, for example from pest or disease models.