The growth and spread of fungal pathogens are influenced by the duration of wetness periods. For this reason, leaf wetness duration (LWD) is a key input to many disease-warning systems (decision aids that help growers to efficiently time disease management practices). On-site monitoring of LWD is unacceptable to many apple growers because it is inconvenient, laborious, and often unreliable. An alternative source of LWD data is commercially available, site-specific LWD estimation (e.g. SkyBit, Inc.). These remote estimates do not consider effects of crop canopy microenvironment, however, which may result in erroneous within-canopy LWD estimates. There is a need to calibrate site-specific estimates of LWD to microenvironments in an apple-tree canopy with acceptable accuracy. Hourly averages of LWD at 12 canopy positions were taken in three mature, semi-dwarf apple trees (3.7 to 4.2 m tall, north-south row orientation, cv. Golden Delicious) in an Iowa orchard from mid-July to September 2000 and in four trees from late May to mid-September 2001. Painted electronic wetness sensors (Model 237, Campbell Scientific, Inc.) were mounted at a 45-degree angle from horizontal and facing north at four lateral, east-west positions within the canopy at each of three heights (3.7 m, 2.4 m, 1.2 m.). Four identical sensors were also erected 1 m above mowed turfgrass on an unobstructed site 100 m north of the apple orchard. LWD for nightly periods from noon to 11 am was analyzed using a SAS Mixed Models procedure. During nights with no precipitation, LWD varied within a canopy from west to east (P=0.00075) and LWD was 197% longer in upper eastern than lower western portions of canopies. LWD over the turfgrass area did not differ significantly from the upper canopy of the apple trees. On-site data from both apple canopies and turfgrass was used to evaluate the accuracy of SkyBit estimates of LWD for microenvironments within the apple canopy.