Rethinking the Role of Leaf Wetness Duration in Plant Disease Management

Tracy RowlandsonUniversity of Guelph

Outline

• Importance of leaf wetness• Evolution of leaf wetness sensors• So what’s wrong with them?• Other options• Where to go next: a suggestion

Bregaglio et al. (2011).

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

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

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

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

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

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

Decagon Environdata Spectrum Netsens

OnSet Campbell Sci. Rainwise Global Water

Non-painted PaintedSen

sor

Res

po

nse

(K

Oh

m)

Adapted from Lau et al. (2000)

So What’s Wrong With Them?

• Sensor placement– Position in canopy can have a large impact on the

measured LWD

• Dew amount and duration varies vertically within some crop canopies

Jacobs and Nieveen, 1995

Maize

Barley

Leaf Wetness Duration Hours

Batzer et al., 2008

Leaf wetness duration in an apple canopy

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

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

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%

Ames, IA 83%; Elora, ON 85%; Florence, Italy 92%; Piracicaba, Brazil 90%

Sentelhas et al. (2008)

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+

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

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

Weather Stations in North America