Discsusion on leaf wetness cliamte data
The following is an email discussion which took place on agromet-L. It was compiled by Jeffery White.
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Subject: SUM: leaf wetness
To: agromet-l@mailserv.fao.org
Cc: "EDMEADES, GREGORY" <G.EDMEADES@CGNET.COM>,
"Banziger,Marianne" <m.banziger@CGNET.COM>,
"Harrington,Larry" <lharrington@cimmyt.mx>,
"Corbett, John" <J.CORBETT@CGNET.COM>,
"JEFFERS, DANIEL" <D.JEFFERS@CGNET.COM>
X-Mailer: Internet Mail Service (5.5.1960.3)
Summary on: Regional characterization of leaf wetness duration as an indicator of disease incidence
(In most cases, I've presented the original message +/- minor editing,
but I've split the message among topics where appropriate.)
-------------------------------------
1. The original query:
Our maize pathologist would like us to generate maps of an indicator
related to leaf wetness for maize growing regions of sub-Saharan Africa.
This might be probability of a having a wet leaf surface for X number of
hours per day and would be estimated on a monthly basis. Leaf wetness
is usually a good proxy for incidence of certain foliar diseases.
Our base maps for Africa are interpolated long-term mean monthly climate
data (Tmax, Tmin, precipitation and solar radiation), which were
generated by John Corbett (Texas A&M) using Hutchinson's ANUSPLIN
package.
Has anyone established simple relations between leaf wetness and basic
climate data that would serve for regional characterization? Please
realize that this is very different from the usual que wetness assessment …
I did quite a bit of work on wetness measurement and modeling, and I have
a perspective on the general question of wetness assessment. Your project is
extremely interesting for me
because my previous studies were done at field or canopy scale.
Therefore it would give me an opportunity to be involved in a leaf wetness study at
the temporal and spatial scales you are interested in. Also, I quite like
your probabilistic approach applied to this question. Maize is interesting
for me because I have not worked with this plant on the question of leaf
wetness :in France fungal diseases are well controlled on this crop.
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3. Comments on measurement/instrumentation:
Summary:
The standard for leaf wetness measurement continues to be mock leaves
made with printed circuit grids (e.g., Gillespie & Kidd, 1978, Can J
Plant Sci 58:179-187). These are available from several sources (e.g.,
http://www.ats.com.au/monitor/lw01.htm,
http://www.geneq.com/catalog/en/wetsens.htm) and can include
conditioning circuits packaged on the sensor board. Costs seem to be
$40 to $50 US. Spectrum Technologies
(http://www.specmeters.com/p22.htm) sells a leaf wetness/temp logger
with programmable intervals from 1 to 120 min (US $ 300). Another
option might be to link a sensor to a Hobo data logger
(http://www.onsetcomp.com). Their 4 channel logger costs about $100.
From: Michael Savage[SMTP:savage@agron.unp.ac.za]
Leaf wetness is difficult to predict so measurements may be essential.
Campbell Scientific Africa sell leaf wetness sensors that can connect to
CR10X dataloggers. Their E-mail is jvisagie@mbv.co.za
The fraction of time that these sensors are wet each day can be
calculated in real-time and these loggers also allow for a variety of
relative humidity and temperature sensors to be logged continuously.
Julian Ward at Cedara (Department of Agriculture) has used this system
on maize.
Currently there is work being conducted on a better (and more standard)
leaf wetness sensor.
From: Albert Weiss[SMTP:agme011@unlvm.unl.edu]
Most leaf wetness measurements use a sensor that acts as a variable
resistor. Infinite resistance when dry and changing resistance as the
sensor gets wet. The sensor can take several forms. For this type of
measurement, the data logger has to have the capability to measure ac
conductivity. Campbell Scientific makes such battery powered data
loggers.
…
From: John Sadler[SMTP:sadler@florence.ars.usda.gov]
In 1996, I gave a talk at the Ag & For Met conf (22nd) joint with 12th
conf on biomet and aerobio. Just before my talk, Adrie FG Jacobs and Joost P
Nieveen spoke on "Formation of dew and the drying process within crop
canopies" paper J1.2:2-5.
My presentation was:
Sadler, E. J. Considerations for determining leaf wetness using
infrared
thermometry in humid regions. Proc. 12th Conf. On Biometeorology and
Aerobiology. J1.3:6-7. American Meteorology Society, Boston, MA. 1996.
From:
peter.dzikowski@agric.gov.ab.ca[SMTP:peter.dzikowski@agric.gov.ab.ca]
Dr. Terry Gillespie at U of Guelph (Ontario, Canada) has done a fair bit
of work on leaf wetness, mostly at daily resolution. He has lots of
experience with leaf wetness sensors and may be able to advise on a low
cost option.
One version I have used, with some success, is a leaf wetness sensor
hooked up to a run time meter, which is fairly inexpensive, robust, easy to use
& runs off 12 volt batteries. All you need is the right sensor,
electronics, exposure and someone to read & record the data daily. The tricky part
is getting the sensor right & getting the electronic circuitry tuned right
to turn on & off when nearby "wetness" conditions start & end. The
electronics should be fairly easy & not too expensive to build & quite
robust (solid state). I expect it can provide data that has better than
+/- one half to one hour resolution.
From: Dr. Terry Gillespie[SMTP:tgillesp@lrs.uoguelph.ca] (Agmet
Group, University of Guelph.
I notice that Peter Dzikowski has replied to your question about leaf
wetness estimators and sensors. I'm not sure you want to get into a
network of such sensors, since it
would take some time to accumulate a "climatology". Peter has indicated
that the least expensive route his probably to get some simple
electronics built so that the sensor turns on a cheap lab timer while
its wet. But the typical flat-plate sensors work best if they are
painted, and getting this done properly is a bother. There is a nice compact unit
sold commercially by Spectrum Technologies in the US (about $300) that
has a sensor and micro-logger combined in a 6x4x4cm package (including the battery). You dump the
data to a computer, either in the field or take the unit back to the lab
(maybe too expensive and impractical for your locations?)
Best regards and my sincere thanks to responders,
Jeff White
CIMMYT