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A sensor for the direct measurement of leaf wetness: Construction techniques and testing under controlled conditions
Agricultural and Forest Meteorology, 43 (1988) 241-249
241
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
A S E N S O R F O R T H E D I R E C T M E A S U R E M E N T OF L E A F WETNESS: CONSTRUCTION TECHNIQUES AND TESTING UNDER CONTROLLED CONDITIONS* ALBERT WEISS
Center for Agricultural Meteorology and Climatology, Institute o[ Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68583-0728 (U.S.A.) DONAL L. LUKENS
Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO 80526-2098 (U.S.A.) JAMES R. STEADMAN
Department of Plant Pathology, University of Nebraska, Lincoln, NE 68583-0722 (U.S.A.) (Received September 19, 1987; revision accepted February 20, 1987)
ABSTRACT Weiss, A., Lukens, D.L. and Steadman, J.R., 1988. A sensor for the direct measurement of leaf wetness: construction techniques and testing under controlled conditions. Agric. For. Meteorol., 43: 241-249. Using a standard design technique of independent, adjacent grid elements, details are given for the construction and testing of a leaf wetness sensor that mounts directly on a leaf. Flexibility in sensor design allows for different configurations to be constructed for different shaped leaves. Guidelines for measurement procedures are also outlined. The sensors were tested by comparing duration of dew for two different cultivars of dry edible beans (Phaseolus vulgaris L.) under controlled conditions. A description of the chamber used to control the duration of dew is also given.
INTRODUCTION
Leaf wetness is a necessary condition for the germination of fungal pathogen spores and often for infection by fungal or bacterial pathogens on leaf surfaces (Royle and Butler, 1986). Duration of leaf wetness is also important in problems relating to acid rain and dry deposition on vegetative surfaces, as witnessed by the recent Small Business Innovation Research Program solicitation of the U.S. Department of Commerce for the development of a "surface wetness s e n s o r " , ( A n o n y m o u s , 1 9 8 6 ) . Most recent sensors that measure leaf wetness are in the form of a grid network of independent, but adjacent metallic elements. When this type of sensor *Published as Paper No. 8434, Journal Series, Nebraska Agricultural Research Division.
0168-1923/88/$03.50
© 1988 Elsevier Science Publishers B.V.
242
is connected to the proper electronic circuit, wetness is indicated when moisture is present between two adjacent elements. One form of this type of sensor is a grid etched on a circuit board and then painted with latex paint, followed by a t r e a t m e n t to make the latex paint more sensitive to moisture (Gillespie and Kidd, 1978). A cotton cloth on a grid of fine wires has also been successfully used to measure leaf wetness (Weiss and Lukens, 1981 ). H~ickel (1980) developed different sensor configurations to measure wetness on different plant parts. The electronic requirements to obtain an accurate output signal from this type of sensor are not very demanding. The object of this paper is to describe the construction techniques and some experimental results for a leaf wetness sensor that can be mounted directly on a leaf surface. SENSOR DESIGN
An alternating voltage (to avoid polarizing the water) of low current (to avoid any heating effects) is applied to both -leads of the sensor. The sensor acts as a variable resistor (a dry leaf surface is equivalent to an open circuit) and, in series with a capacitor, forms an RC circuit. The time constant of this type of circuit, which determines the oscillating frequency, is proportional to the amount of water on the grid surface. This sensor can be used with data loggers that are capable of AC conductivity measurements or a simple circuit can be constructed to change the oscillations from the RC network to a voltage which will indicate leaf wetness. The overall size of the sensor is approximately 7 × 20 mm, while the area in direct contact with the leaf is approximately 7 × 7 mm (Fig. 1). The space between the independent wires making up the grid is approximately 1 mm; this value of spacing was determined empirically. The grid wires are held together and in contact with the leaf by silicone adhesive which allows the sensor to conform to the leaf surface. The size of the sensor (length, width, or grid spacing) can be changed to meet specific needs, i.e., the measurement of leaf wetness on leaves with narrow blades. CONSTRUCTION TECHNIQUES
The following materials are used in the construction of this sensor: 0.076mm diameter bare soft stainless steel wire, 0.255-mm diameter insulated copper lead wire, an electronic grade silicone adhesive (GE R T V 162 )*, and polyethylene film (e.g., Saran Wrap ). Soft stainless steel wire was selected because it is easy to work with, easily soldered, and noncorrosive. The tools needed to construct this sensor are: a jig of the desired sensor dimensions (Fig. 2), a *Mention of a manufacturer's n a m e or trade n a m e does not imply e n d o r s e m e n t of a product over those offered by o t h e r manufacturers b u t is merely provided for convenience of the reader.
243
Fig. 1. Leaf wetness sensor on leaf with silicone adhesive. T h e active sensing area is approximately 7 m m X 7 mm.
@
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SCREW
IJ
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J Fig. 2. T h e jig used to form a n d connect grid wires to the lead wires. T h e dashed line indicates t h e continuous p a t h of the stainless steel wire from screw 1 to screw 2 around the rectangular posts.
244
soldering iron, electronic solder, liquid soldering flux, flux removing solvent, a single edge razor blade, four small aluminum blocks, and a magnifying glass. The construction and use of a sensor is described in the following steps: (i) Cut the bare stainless steel wire into a 0.25-m length. Attach one end of the wire between the two washers of screw 1, note the straight edge on the washers which allows for soldering wires near the screw, and weave the wire around the rectangular posts on the jig. Attach the free end of the wire between the washers, also with a straight edge, of screw 2. Using a magnifying glass, inspect the position of the wires on the rectangular posts to ensure that they make uniform contact at the base of these posts. (ii) Strip 19 mm of insulation from the center of a 0.3-m length of lead wire and 13 mm from each end. Apply liquid flux to the bare center of the lead wire and to the stainless steel wire in contact with the smaller sides of the rectangular posts and "tin" these wires. (iii) Position the lead wire in the jig using screws 3 and 4 so that the bare center of the lead wire is in contact with the stainless steel wire. Apply liquid flux again and solder the stainless steel wire to the lead wire (Fig. 2). Repeat this procedure for the wires on the opposite side of the jig. (iv) While still in the jig, use the single edge razor blade to cut the network of wires in half with the aid of the offset guide. Remove the two pieces of wire from the jig and cut each pair of grid wires (with diagonal pliers) in half again, but this time at the center of the lead wire (Fig. 3). Now there should be four
7ram
1second cut
gr,dw,re
---f'rst °at 13.4mm
T]
lead wire )
--
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Fig. 3. Sequence of cutting the connected grid elements to.form four lead wires each with four grid wires. Fig. 4. Wire configuration for assembling leaf wetness sensor.
245 p i e c e s o f lead wire, each with four grid wires. Remove excess ends of lead wire and straighten individual grid wires so they are perpendicular to the lead wire. Rinse the wires in a mild flux removing solvent. (v) A sensor, consisting of a grid network of independent, adjacent wires can now be assembled (Fig. 4). Two lead wire sections are positioned on an aluminum block covered with polyethylene film. A pattern of lead and grid wire separation is outlined on the block so that there is a separation of 13.4 mm between lead wires and 1 mm between grid wires. A 7 mm wide, polyethylene-covered aluminum block is centered between the lead wires. Another pair of blocks holds the lead wires in place. The silicone R T V is applied to both sides of the centering block to encapsulate the lead and grid wires. A small piece of polyethylene is placed over each silicone section in order to obtain a fiat surface. After the silicone sets, a thin layer of silicone is applied to the bottom side of the sensor. W h e n the silicone on the b o t t o m side sets, the rough edges of the silicone can be trimmed with a single edge razor blade. Electronic grade silicone is recommended because it is easy to use and sets with a smooth surface. Other types of silicone may set with a rough surface which will allow water to stay in depressions along the interface of the silicone and the grid wires. Leaf wetness will be indicated by this artifact of construction even though the leaf is dry. Electronic grade silicone does not appear to affect leaf physiological processes. No necrotic areas were found around sensors which had been on leaves for several weeks. (vi) Each sensor is tested before it is used. A new sensor is connected to the circuit that will be used to make the actual measurement. The sensor's offset voltage in air is noted. The sensor is then placed in a small beaker of distilled water for about ten minutes. The sensor is removed from the water and allowed to dry; another voltage reading is made which should be similar to the first reading. If this last reading greatly differs from the initial reading, the sensor is not functioning properly. It should be allowed to dry overnight and then be repotted. If the sensor still does not behave properly, it should be discarded. (vii) The sensor is mounted on a leaf by applying a small amount of silicone to the b o t t o m of the sensor and then placing the sensor in the desired location. Care should be exercised to avoid letting the sensor slide on the leaf. It may be necessary to hold the sensor in place until the silicone sets. This can be accomplished by placing the mounted sensor between two thin pieces of rigid plastic which can then be held together by two clothes pins. (viii) The end of the lead wires are attached to cables going to the recording device (data logger, computer, etc.) by microclips. Follow the manufacturer's instructions for making this AC conductivity measurement, i.e., connect resistor networks and excitation voltages to the proper channels. (ix) In using this sensor, interest usually lies with the duration, rather than the intensity, of leaf wetness. If the voltage output from a sensor is greater than zero after taking into account the offset voltage determined in step (vi),
246
then the leaf is wet. If the recording device has programmable capabilities, then the period of leaf wetness per unit period (e.g., minutes per hour) can be determined as a function of the recording device's scan initiation rate (how many times per unit period the sensors are read). Otherwise, a computer program can be written to analyze the raw data for periods of leaf wetness per unit time based on the above procedures. DEW CHAMBER
This sensor was designed for a series of controlled environment experiments of leaf wetness duration on different types of leaf surfaces and at the interface of a senescent blossom and the leaf surface. This latter environment is where the cosmopolitan pathogen Sclerotinia sclerotiorurn (Lib.) de Bary initiates infection via ascospore germination and blossom colonization. In field trials of dry edible bean (Phaseolus vulgaris L. ) cultivars, the navy bean Ex Rico 23 was not as susceptible to S. sclerotiorum as was Pinto UI 114; however, stem inoculation tests showed no difference in susceptibility (unpublished data). Lyons ( 1985 ) suggested that blossom infection may be involved in Ex Rico's reaction. Since moisture duration plays a major role in S. sclerotiorum germination and infection, but accurate measurements of this variable are lacking, a series of experiments was begun. The object of the first experiment was to determine if there was a significant difference in the duration of leaf wetness between the two cultivars. A chamber system to control the duration of dew on leaf surfaces was designed and constructed (Fig. 5) incorporating the radiative exchange mechanism of a chamber used to study freezing injury on leaves (Marcellos, 1981 ). A cold, blackened, copper plate acted as a heat sink which allowed leaves to cool below the dew point temperature of the air, thus causing dew to form on leaf surfaces. A simple t e m p e r a t u r e / d e w point control system supplied water vapor to the lower part of the chamber and worked in conjuction with the blackened copper plate to determine the intensity and duration of dew. The chamber was divided into two sections by the layers of polyethylene (Fig. 5 ). As previously noted, the upper section of the chamber (0.4 × 0.4 X 0.2 m) was devoted to the heat sink. The lower section of the chamber (0.4 × 0.4 × 0.9 m) was for plants or, using the removable floor, for leaf samples. In a closed system (consisting of the PVC pipe, desiccant, a heater and fan), warm dry air was circulated between the polyethylene, keeping condensation from forming on the polyethylene. Condensation-free polyethylene, which was transparent to short and long wave radiation, allows for radiative cooling of the leaf a n d / o r plant surfaces to occur. Thus, it was possible to keep the blackened copper plate at - 15 °C and the lower part of the chamber at 20 ° C. The blackened copper plate was kept at a low temperature by pumping a coolant (such as automobile antifreeze) through copper coils soldered to the
247
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TEMPERATURE/
TEMPERATURE
DEW POINT CONTROL
PUMP
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CONTROLLED
PUMP
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Fig. 5. Schematic representation of the main elements of the dew duration control chamber: ( 1 ) insulated wall, (2) copper coils, (3) blackened copper plate, (4) PVC pipe, (5) polyethylene film, (6) manifold and (7) removable floor.
back of the plate. The temperature-controlled refrigerated bath can take different forms, from a complete system ordered from a catalog to a self-built system, depending on the resources available. We used an air conditioner, a thermistor-controlled relay setable to different temperatures, a closed 8-liter insulated container filled with antifreeze, a stirrer and an automobile fuel pump. The t e m p e r a t u r e / d e w point control system can also assume different degrees of sophistication. Ideally, and also as a function of the experiment to be carried out in the chamber, it should be possible to control the air and dew point temperature of the air entering the chamber through the mainfold. In testing these sensors on different leaf surfaces, our interest was in the duration of moisture on each leaf subjected to the same conditions. If our interest had been in the effect of different leaf temperatures a n d / o r intensity of moisture on the development of a fungal pathogen, then there would be more demands on the t e m p e r a t u r e / d e w point conditioning system. We simply bubbled air at a fixed flow rate through warm water (slightly higher than the ambient air temperature) and then into the chamber. This procedure worked well for this application. After a desired period of wetness, drying of the leaves was accom-
248
plished by passing air through a desiccant rather than through the bubbler. With a more sophisticated t e m p e r a t u r e / d e w point control system, drying of the leaves could be accomplished by different combinations of changing air a n d / o r dew point temperatures. SENSOR TESTING
Leaves of Ex Rico and Pinto 114 were placed inside the chamber with two sensors attached to each leaf. The petioles of the leaves were cut under distilled water and kept in vials of distilled water for the duration of the experiment. The sensor leads were attached to an electronic circuit that yielded a voltage proportioned to the degree of wetness measured by the sensor. This voltage output was recorded on a Campbell Scientific, Inc. CR5 data logger for later analysis. The leaves were subjected to five hours of wetness before dry down procedures were initiated. Each leaf was used for two days before new leaves were used in the chamber. At this time, in order to avoid introducing a possible bias of different sensor variation in the measurement of duration of leaf wetness, the sensors were switched to measure wetness on a different leaf. The sensor that measured wetness on the Ex Rico cultivar now measured wetness on the Pinto 114 cultivar and vice versa. W h e n there was too large a build-up of silicone on the sensors, the old sensors were discarded and replaced by new ones. This series of measurements was replicated for 19 sets of leaves, but due to physical or biological problems only 29 individual comparisons between the Ex Rico and Pinto 114 were used in analyzing the data. The average duration (hours and minutes) of leaf wetness for the Pinto 114 and Ex Rico cultivars was 8:53 and 8:30, respectively. A t-test comparing the paired observations was performed and the null hypothesis was accepted, there was no significant difference between the mean values of duration of leaf wetness for the two cultivars. The duration of moisture at the leaf/blossom interface will be tested in a subsequent study. CONCLUSION
Construction techniques for a sensor to directly measure leaf wetness have been described. This sensor was successfully tested by comparing duration of dew on two different cultivars of dry edible beans in a chamber designed to control the duration of dew. Differences in susceptibility between the cultivars of dry edible beans was not related to leaf wetness duration per se, but may be due to the duration of wetness at the leaf/blossom interface. This will be investigated in another series of experiments.
249 ACKNOWLEDGEMENTS
The jig was designed in conjunction with and built by Mr. J. Schafer (Precision Machine Company, 2933 North 36th Street, Lincoln, NE 68504). Ms. Terri Lewandowski assisted in the data collection. Drs. K.G. Hubbard and G.E. Meyer provided valuable comments during the preparation of this paper.
REFERENCES Anonymous, 1986. Small Business Innovation Research for FY 1987. Department of Commerce Program Solicitation. DOC 87-1. Gillespie, T.S. and Kidd, G.E., 1978. Sensing duration of leaf moisture retention using electrical impedance grids. Can. J. Plant Sci., 58: 179-187. H~ickel, H., 1980. New developments of an electrical method for direct measurement of the wetness-duration on plants. Agric. Meteorol., 22: 113-119. Lyons, M.E., 1985. Recurrent selection for resistance to Sclerotinia sclerotiorum in Phaseolus spp. and the expression of resistance in blossom tissue. M.S. thesis, Cornell University, Ithaca, NY, 109 pp. Marcellos, H., 1981. A plant freezing chamber with radiative and convective energy exchange. J. Agric. Eng. Res., 26: 403-408. Royle, D.J. and Butler, D.R., 1986. Epidemiological significance of liquid water in crop canopies and its role in disease forecasting, pp. 139-156. In: P.G. Ayres and L. Boddy (Eds.), Water, Fungi and Plants, Cambridge University Press, 413 pp. Weiss, A. and Lukens, D.L., 1981. Electronic circuit for detecting leaf wetness and comparison of two sensors. Plant Dis., 65: 41-43.
241
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
A S E N S O R F O R T H E D I R E C T M E A S U R E M E N T OF L E A F WETNESS: CONSTRUCTION TECHNIQUES AND TESTING UNDER CONTROLLED CONDITIONS* ALBERT WEISS
Center for Agricultural Meteorology and Climatology, Institute o[ Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68583-0728 (U.S.A.) DONAL L. LUKENS
Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO 80526-2098 (U.S.A.) JAMES R. STEADMAN
Department of Plant Pathology, University of Nebraska, Lincoln, NE 68583-0722 (U.S.A.) (Received September 19, 1987; revision accepted February 20, 1987)
ABSTRACT Weiss, A., Lukens, D.L. and Steadman, J.R., 1988. A sensor for the direct measurement of leaf wetness: construction techniques and testing under controlled conditions. Agric. For. Meteorol., 43: 241-249. Using a standard design technique of independent, adjacent grid elements, details are given for the construction and testing of a leaf wetness sensor that mounts directly on a leaf. Flexibility in sensor design allows for different configurations to be constructed for different shaped leaves. Guidelines for measurement procedures are also outlined. The sensors were tested by comparing duration of dew for two different cultivars of dry edible beans (Phaseolus vulgaris L.) under controlled conditions. A description of the chamber used to control the duration of dew is also given.
INTRODUCTION
Leaf wetness is a necessary condition for the germination of fungal pathogen spores and often for infection by fungal or bacterial pathogens on leaf surfaces (Royle and Butler, 1986). Duration of leaf wetness is also important in problems relating to acid rain and dry deposition on vegetative surfaces, as witnessed by the recent Small Business Innovation Research Program solicitation of the U.S. Department of Commerce for the development of a "surface wetness s e n s o r " , ( A n o n y m o u s , 1 9 8 6 ) . Most recent sensors that measure leaf wetness are in the form of a grid network of independent, but adjacent metallic elements. When this type of sensor *Published as Paper No. 8434, Journal Series, Nebraska Agricultural Research Division.
0168-1923/88/$03.50
© 1988 Elsevier Science Publishers B.V.
242
is connected to the proper electronic circuit, wetness is indicated when moisture is present between two adjacent elements. One form of this type of sensor is a grid etched on a circuit board and then painted with latex paint, followed by a t r e a t m e n t to make the latex paint more sensitive to moisture (Gillespie and Kidd, 1978). A cotton cloth on a grid of fine wires has also been successfully used to measure leaf wetness (Weiss and Lukens, 1981 ). H~ickel (1980) developed different sensor configurations to measure wetness on different plant parts. The electronic requirements to obtain an accurate output signal from this type of sensor are not very demanding. The object of this paper is to describe the construction techniques and some experimental results for a leaf wetness sensor that can be mounted directly on a leaf surface. SENSOR DESIGN
An alternating voltage (to avoid polarizing the water) of low current (to avoid any heating effects) is applied to both -leads of the sensor. The sensor acts as a variable resistor (a dry leaf surface is equivalent to an open circuit) and, in series with a capacitor, forms an RC circuit. The time constant of this type of circuit, which determines the oscillating frequency, is proportional to the amount of water on the grid surface. This sensor can be used with data loggers that are capable of AC conductivity measurements or a simple circuit can be constructed to change the oscillations from the RC network to a voltage which will indicate leaf wetness. The overall size of the sensor is approximately 7 × 20 mm, while the area in direct contact with the leaf is approximately 7 × 7 mm (Fig. 1). The space between the independent wires making up the grid is approximately 1 mm; this value of spacing was determined empirically. The grid wires are held together and in contact with the leaf by silicone adhesive which allows the sensor to conform to the leaf surface. The size of the sensor (length, width, or grid spacing) can be changed to meet specific needs, i.e., the measurement of leaf wetness on leaves with narrow blades. CONSTRUCTION TECHNIQUES
The following materials are used in the construction of this sensor: 0.076mm diameter bare soft stainless steel wire, 0.255-mm diameter insulated copper lead wire, an electronic grade silicone adhesive (GE R T V 162 )*, and polyethylene film (e.g., Saran Wrap ). Soft stainless steel wire was selected because it is easy to work with, easily soldered, and noncorrosive. The tools needed to construct this sensor are: a jig of the desired sensor dimensions (Fig. 2), a *Mention of a manufacturer's n a m e or trade n a m e does not imply e n d o r s e m e n t of a product over those offered by o t h e r manufacturers b u t is merely provided for convenience of the reader.
243
Fig. 1. Leaf wetness sensor on leaf with silicone adhesive. T h e active sensing area is approximately 7 m m X 7 mm.
@
OFFSETCUTTINGGUIDE @
o ~Jr ~ii~ ® ~)~ t ~'LJtRlll-JLl"d~ ~ ~. ~ L
SCREW
IJ
RECTANGULAR POST
(~ (~) SCREW SCREW SCREW 4" 4 @ #1 "#2
J Fig. 2. T h e jig used to form a n d connect grid wires to the lead wires. T h e dashed line indicates t h e continuous p a t h of the stainless steel wire from screw 1 to screw 2 around the rectangular posts.
244
soldering iron, electronic solder, liquid soldering flux, flux removing solvent, a single edge razor blade, four small aluminum blocks, and a magnifying glass. The construction and use of a sensor is described in the following steps: (i) Cut the bare stainless steel wire into a 0.25-m length. Attach one end of the wire between the two washers of screw 1, note the straight edge on the washers which allows for soldering wires near the screw, and weave the wire around the rectangular posts on the jig. Attach the free end of the wire between the washers, also with a straight edge, of screw 2. Using a magnifying glass, inspect the position of the wires on the rectangular posts to ensure that they make uniform contact at the base of these posts. (ii) Strip 19 mm of insulation from the center of a 0.3-m length of lead wire and 13 mm from each end. Apply liquid flux to the bare center of the lead wire and to the stainless steel wire in contact with the smaller sides of the rectangular posts and "tin" these wires. (iii) Position the lead wire in the jig using screws 3 and 4 so that the bare center of the lead wire is in contact with the stainless steel wire. Apply liquid flux again and solder the stainless steel wire to the lead wire (Fig. 2). Repeat this procedure for the wires on the opposite side of the jig. (iv) While still in the jig, use the single edge razor blade to cut the network of wires in half with the aid of the offset guide. Remove the two pieces of wire from the jig and cut each pair of grid wires (with diagonal pliers) in half again, but this time at the center of the lead wire (Fig. 3). Now there should be four
7ram
1second cut
gr,dw,re
---f'rst °at 13.4mm
T]
lead wire )
--
A
Fig. 3. Sequence of cutting the connected grid elements to.form four lead wires each with four grid wires. Fig. 4. Wire configuration for assembling leaf wetness sensor.
245 p i e c e s o f lead wire, each with four grid wires. Remove excess ends of lead wire and straighten individual grid wires so they are perpendicular to the lead wire. Rinse the wires in a mild flux removing solvent. (v) A sensor, consisting of a grid network of independent, adjacent wires can now be assembled (Fig. 4). Two lead wire sections are positioned on an aluminum block covered with polyethylene film. A pattern of lead and grid wire separation is outlined on the block so that there is a separation of 13.4 mm between lead wires and 1 mm between grid wires. A 7 mm wide, polyethylene-covered aluminum block is centered between the lead wires. Another pair of blocks holds the lead wires in place. The silicone R T V is applied to both sides of the centering block to encapsulate the lead and grid wires. A small piece of polyethylene is placed over each silicone section in order to obtain a fiat surface. After the silicone sets, a thin layer of silicone is applied to the bottom side of the sensor. W h e n the silicone on the b o t t o m side sets, the rough edges of the silicone can be trimmed with a single edge razor blade. Electronic grade silicone is recommended because it is easy to use and sets with a smooth surface. Other types of silicone may set with a rough surface which will allow water to stay in depressions along the interface of the silicone and the grid wires. Leaf wetness will be indicated by this artifact of construction even though the leaf is dry. Electronic grade silicone does not appear to affect leaf physiological processes. No necrotic areas were found around sensors which had been on leaves for several weeks. (vi) Each sensor is tested before it is used. A new sensor is connected to the circuit that will be used to make the actual measurement. The sensor's offset voltage in air is noted. The sensor is then placed in a small beaker of distilled water for about ten minutes. The sensor is removed from the water and allowed to dry; another voltage reading is made which should be similar to the first reading. If this last reading greatly differs from the initial reading, the sensor is not functioning properly. It should be allowed to dry overnight and then be repotted. If the sensor still does not behave properly, it should be discarded. (vii) The sensor is mounted on a leaf by applying a small amount of silicone to the b o t t o m of the sensor and then placing the sensor in the desired location. Care should be exercised to avoid letting the sensor slide on the leaf. It may be necessary to hold the sensor in place until the silicone sets. This can be accomplished by placing the mounted sensor between two thin pieces of rigid plastic which can then be held together by two clothes pins. (viii) The end of the lead wires are attached to cables going to the recording device (data logger, computer, etc.) by microclips. Follow the manufacturer's instructions for making this AC conductivity measurement, i.e., connect resistor networks and excitation voltages to the proper channels. (ix) In using this sensor, interest usually lies with the duration, rather than the intensity, of leaf wetness. If the voltage output from a sensor is greater than zero after taking into account the offset voltage determined in step (vi),
246
then the leaf is wet. If the recording device has programmable capabilities, then the period of leaf wetness per unit period (e.g., minutes per hour) can be determined as a function of the recording device's scan initiation rate (how many times per unit period the sensors are read). Otherwise, a computer program can be written to analyze the raw data for periods of leaf wetness per unit time based on the above procedures. DEW CHAMBER
This sensor was designed for a series of controlled environment experiments of leaf wetness duration on different types of leaf surfaces and at the interface of a senescent blossom and the leaf surface. This latter environment is where the cosmopolitan pathogen Sclerotinia sclerotiorurn (Lib.) de Bary initiates infection via ascospore germination and blossom colonization. In field trials of dry edible bean (Phaseolus vulgaris L. ) cultivars, the navy bean Ex Rico 23 was not as susceptible to S. sclerotiorum as was Pinto UI 114; however, stem inoculation tests showed no difference in susceptibility (unpublished data). Lyons ( 1985 ) suggested that blossom infection may be involved in Ex Rico's reaction. Since moisture duration plays a major role in S. sclerotiorum germination and infection, but accurate measurements of this variable are lacking, a series of experiments was begun. The object of the first experiment was to determine if there was a significant difference in the duration of leaf wetness between the two cultivars. A chamber system to control the duration of dew on leaf surfaces was designed and constructed (Fig. 5) incorporating the radiative exchange mechanism of a chamber used to study freezing injury on leaves (Marcellos, 1981 ). A cold, blackened, copper plate acted as a heat sink which allowed leaves to cool below the dew point temperature of the air, thus causing dew to form on leaf surfaces. A simple t e m p e r a t u r e / d e w point control system supplied water vapor to the lower part of the chamber and worked in conjuction with the blackened copper plate to determine the intensity and duration of dew. The chamber was divided into two sections by the layers of polyethylene (Fig. 5 ). As previously noted, the upper section of the chamber (0.4 × 0.4 X 0.2 m) was devoted to the heat sink. The lower section of the chamber (0.4 × 0.4 × 0.9 m) was for plants or, using the removable floor, for leaf samples. In a closed system (consisting of the PVC pipe, desiccant, a heater and fan), warm dry air was circulated between the polyethylene, keeping condensation from forming on the polyethylene. Condensation-free polyethylene, which was transparent to short and long wave radiation, allows for radiative cooling of the leaf a n d / o r plant surfaces to occur. Thus, it was possible to keep the blackened copper plate at - 15 °C and the lower part of the chamber at 20 ° C. The blackened copper plate was kept at a low temperature by pumping a coolant (such as automobile antifreeze) through copper coils soldered to the
247
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(4)
15)~ - ~
,
121
_
~'~'
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(6) ~ _ i~)
V
~_\\\\\~\\\\\\\\\\\
i
1
_(7)
TEMPERATURE/
TEMPERATURE
DEW POINT CONTROL
PUMP
SYSTEM
CONTROLLED
PUMP
REFRIOERATED BATH
Fig. 5. Schematic representation of the main elements of the dew duration control chamber: ( 1 ) insulated wall, (2) copper coils, (3) blackened copper plate, (4) PVC pipe, (5) polyethylene film, (6) manifold and (7) removable floor.
back of the plate. The temperature-controlled refrigerated bath can take different forms, from a complete system ordered from a catalog to a self-built system, depending on the resources available. We used an air conditioner, a thermistor-controlled relay setable to different temperatures, a closed 8-liter insulated container filled with antifreeze, a stirrer and an automobile fuel pump. The t e m p e r a t u r e / d e w point control system can also assume different degrees of sophistication. Ideally, and also as a function of the experiment to be carried out in the chamber, it should be possible to control the air and dew point temperature of the air entering the chamber through the mainfold. In testing these sensors on different leaf surfaces, our interest was in the duration of moisture on each leaf subjected to the same conditions. If our interest had been in the effect of different leaf temperatures a n d / o r intensity of moisture on the development of a fungal pathogen, then there would be more demands on the t e m p e r a t u r e / d e w point conditioning system. We simply bubbled air at a fixed flow rate through warm water (slightly higher than the ambient air temperature) and then into the chamber. This procedure worked well for this application. After a desired period of wetness, drying of the leaves was accom-
248
plished by passing air through a desiccant rather than through the bubbler. With a more sophisticated t e m p e r a t u r e / d e w point control system, drying of the leaves could be accomplished by different combinations of changing air a n d / o r dew point temperatures. SENSOR TESTING
Leaves of Ex Rico and Pinto 114 were placed inside the chamber with two sensors attached to each leaf. The petioles of the leaves were cut under distilled water and kept in vials of distilled water for the duration of the experiment. The sensor leads were attached to an electronic circuit that yielded a voltage proportioned to the degree of wetness measured by the sensor. This voltage output was recorded on a Campbell Scientific, Inc. CR5 data logger for later analysis. The leaves were subjected to five hours of wetness before dry down procedures were initiated. Each leaf was used for two days before new leaves were used in the chamber. At this time, in order to avoid introducing a possible bias of different sensor variation in the measurement of duration of leaf wetness, the sensors were switched to measure wetness on a different leaf. The sensor that measured wetness on the Ex Rico cultivar now measured wetness on the Pinto 114 cultivar and vice versa. W h e n there was too large a build-up of silicone on the sensors, the old sensors were discarded and replaced by new ones. This series of measurements was replicated for 19 sets of leaves, but due to physical or biological problems only 29 individual comparisons between the Ex Rico and Pinto 114 were used in analyzing the data. The average duration (hours and minutes) of leaf wetness for the Pinto 114 and Ex Rico cultivars was 8:53 and 8:30, respectively. A t-test comparing the paired observations was performed and the null hypothesis was accepted, there was no significant difference between the mean values of duration of leaf wetness for the two cultivars. The duration of moisture at the leaf/blossom interface will be tested in a subsequent study. CONCLUSION
Construction techniques for a sensor to directly measure leaf wetness have been described. This sensor was successfully tested by comparing duration of dew on two different cultivars of dry edible beans in a chamber designed to control the duration of dew. Differences in susceptibility between the cultivars of dry edible beans was not related to leaf wetness duration per se, but may be due to the duration of wetness at the leaf/blossom interface. This will be investigated in another series of experiments.
249 ACKNOWLEDGEMENTS
The jig was designed in conjunction with and built by Mr. J. Schafer (Precision Machine Company, 2933 North 36th Street, Lincoln, NE 68504). Ms. Terri Lewandowski assisted in the data collection. Drs. K.G. Hubbard and G.E. Meyer provided valuable comments during the preparation of this paper.
REFERENCES Anonymous, 1986. Small Business Innovation Research for FY 1987. Department of Commerce Program Solicitation. DOC 87-1. Gillespie, T.S. and Kidd, G.E., 1978. Sensing duration of leaf moisture retention using electrical impedance grids. Can. J. Plant Sci., 58: 179-187. H~ickel, H., 1980. New developments of an electrical method for direct measurement of the wetness-duration on plants. Agric. Meteorol., 22: 113-119. Lyons, M.E., 1985. Recurrent selection for resistance to Sclerotinia sclerotiorum in Phaseolus spp. and the expression of resistance in blossom tissue. M.S. thesis, Cornell University, Ithaca, NY, 109 pp. Marcellos, H., 1981. A plant freezing chamber with radiative and convective energy exchange. J. Agric. Eng. Res., 26: 403-408. Royle, D.J. and Butler, D.R., 1986. Epidemiological significance of liquid water in crop canopies and its role in disease forecasting, pp. 139-156. In: P.G. Ayres and L. Boddy (Eds.), Water, Fungi and Plants, Cambridge University Press, 413 pp. Weiss, A. and Lukens, D.L., 1981. Electronic circuit for detecting leaf wetness and comparison of two sensors. Plant Dis., 65: 41-43.