Estimation of leaf wetness duration using empirical models in northwestern Costa Rica K.S. Kim a, * , S.E. Taylor b , M.L. Gleason a , R. Villalobos c , L.F. Arauz d a Department of Plant Pathology, Iowa State University, Ames, IA 50011, USA b Department of Agronomy, Iowa State University, Ames, IA 50011, USA c Instituto Meteorolo ´gico Nacional, San Jose ´, Costa Rica d Escuela de Agronomı ´a, Universidad de Costa Rica, San Jose ´, Costa Rica Received 17 August 2004; accepted 29 November 2004 Abstract Leaf wetness duration (LWD) estimation models developed in the midwestern U.S. were assessed for portability to northwestern Costa Rica during wet and dry seasons. The CART/SLD/Wind (CART) model overestimated LWD by 5.4 h at five sites in northwestern Costa Rica during the 1999 wet season. The Fuzzy (FL) model, in contrast, estimated LWD at the same sites within 3 h. During the 2000–2001 and 2002–2003 dry seasons, both models underestimated LWD at most sites, probably due to the prevalence of less humid conditions than in the temperate climate of the midwestern U.S. However, accuracy of the FL model was substantially improved when a correction factor was utilized during the 2002–2003 dry season, indicating that this model could be adjusted to estimate LWD in a semi-arid climate with acceptable accuracy. The results suggested that the adjusted FL model could estimate LWD accurately in other tropical regions with climate regimes similar to northwestern Costa Rica. # 2005 Elsevier B.V. All rights reserved. Keywords: Spatial portability; Fuzzy logic; Dew 1. Introduction Disease-warning systems are management decision aids that help growers apply pesticides more efficiently and economically than traditional calen- dar-based spray timing methods (Gleason, 2000). Implementation of disease-warning systems often results in substantial reduction of spray frequency (Llorente et al., 2000; Madden et al., 2000), which not only reduces the burden of pesticide sprays on the environment but can also delay the development of fungicide and bactericide resistance. Many disease- warning systems gauge the risk of crop infection by monitoring the duration of daily periods when free water is present on foliage and the air temperature during wet periods (Huber and Gillespie, 1992; Pitblado, 1988; Poysa et al., 1993; Latin and Evans, 1996). These systems generally require hourly assessment of leaf wetness duration (LWD). It is difficult for most growers to obtain LWD data, however, because few weather stations measure LWD. www.elsevier.com/locate/agrformet Agricultural and Forest Meteorology 129 (2005) 53–67 * Corresponding author. E-mail address: luxkwang@yahoo.com (K.S. Kim). 0168-1923/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2004.11.009