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Rethinking the Role of Leaf Wetness Duration in Plant Disease Management
Tracy RowlandsonUniversity of Guelph
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Outline
• Importance of leaf wetness• Evolution of leaf wetness sensors• So what’s wrong with them?• Other options• Where to go next: a suggestion
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Bregaglio et al. (2011).
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Importance of Leaf Wetness
• Knowledge of leaf wetness duration (LWD) is the basis for any disease management programs
• Sources of leaf wetness:– Intercepted rainfall– Intercepted irrigation– Dew formation: 2 sources of water vapour– Guttation
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Evolution of Leaf Wetness Sensors
• Static (e.g. Duvdevani Dew Gauge)– Wooden block placed outdoors at sunset– Dew formation examined in the morning and
compared to series of photographs– Very poor correlation with LWD
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Evolution of Leaf Wetness Sensors
• Static (e.g. Duvdevani Dew Gauge)– Wooden block placed outdoors at sunset– Dew formation examined in the morning and
compared to series of photographs– Very poor correlation with LWD
• Mechanical (e.g. DeWit leaf wetness recorder)– Measures changes in the length of string, human
hair, or membrane or change in weight of sensor– Typically recorded LWD on a rotating chart
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Evolution of Leaf Wetness Sensors
• Electronic (e.g. resistance sensors)– Interlaced, gold-plated copper wires– When water bridges the wires, the circuit is
complete and resistance drops– Can measure LWD from rainfall, irrigation, and
dew formation
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So What’s Wrong With Them?
• Is the problem with the sensors?• No, the problem is how we use them– What does the sensor represent?– Treatment of sensors– Placement of sensors– Calibration of sensors
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So What’s Wrong With Them?
• There are so many different sensors– Commercially available and those made in-house– Many are sold unpainted– Painting techniques differ between users
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Decagon Environdata Spectrum Netsens
OnSet Campbell Sci. Rainwise Global Water
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Non-painted PaintedSen
sor
Res
po
nse
(K
Oh
m)
Adapted from Lau et al. (2000)
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So What’s Wrong With Them?
• Sensor placement– Position in canopy can have a large impact on the
measured LWD
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• Dew amount and duration varies vertically within some crop canopies
Jacobs and Nieveen, 1995
Maize
Barley
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Leaf Wetness Duration Hours
Batzer et al., 2008
Leaf wetness duration in an apple canopy
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So What’s Wrong With Them?
• Calibration is rarely done– Important especially for multi-seasonal use– Need to determine wetness threshold– Do multiple sensors agree?
• Maintenance is critical– Birds love them– Debris on surface will change wetness threshold
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Other Options
• Models: physical, empirical, or combination• Physical models tend to be more complex and
require more inputs• Physical models valid everywhere– Models are either one source (model dewfall only)
or two source (model both dewfall and distillation)– Consider the canopy to be a ‘big leaf’ or separate
the canopy into layers
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Other Options
• Models: physical, empirical, or combination• Physical models tend to be more complex and
require more inputs• Physical models valid everywhere• Empirical models require fewer inputs• Valid only for the region for which they were
developed• e.g. RH ≥ 90%
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Ames, IA 83%; Elora, ON 85%; Florence, Italy 92%; Piracicaba, Brazil 90%
Sentelhas et al. (2008)
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TomCast ModelLeaf Wetness Duration (h) required to
produce DSVs ofMean T
(°C)0 1 2 3 4
13-17 0-6 7-15 16-20 21+ -
18-20 0-3 4-8 9-15 16-22 23+
21-25 0-2 3-5 6-12 13-20 21+
26-29 0-3 4-8 9-15 16-22 23+
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Other Options
• How well does RH do in a disease model?– Data from LWD sensor from 2010 compared to RH– RH threshold of 83% found to compare the best
• For TomCast model LWD sensor indicated 3 sprays (mid-July to end of August); RH indicated 3 sprays
• RH model indicated spray timing one day later than sensor
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Suggestion
• Take advantage of the standard measurements of RH already available
• Develop networks based on currently existing stations (i.e. fill in where needed)
• Standards are available for installation of RH sensors
• Calibration conducted by manufacturer• There will still be obstacles to overcome
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Weather Stations in North America