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A 5-by-5-to-an-inch graphing paper can be downloaded from
SDSU
Extension Fact Sheet 905 and copied onto a transparency. Any
graphing paper will work but counting will be more tedious in graphing
papers with smaller sized cells or grids. This procedure of
estimating percent defoliation is not new and is the basis of electronic
leaf area meters. We already have compared this graphical procedure
with a $5,000 electronic leaf area meter and we found it to be an
accurate, fast, and low-cost means of estimating percent defoliation on
soybeans.
Step 1.
Obtain representative soybean leaflets from the field. More
samples will result to more accurate estimate of the defoliation level on
the field. The leaflet with the intermediate damage in a trifoliate
may be picked to minimize bias. Place the leaflets underneath a
transparency with 5-by-5-to-an-inch cells or grids.
Step 2.
Count the total number of cells occupied by the entire leaflet (as if the
leaflet were undamaged). Missing margins of badly damaged leaflets
may be traced or estimated. Count only cells that are at least 50%
covered by the leaflet. Click on the thumbnail for examples.
Step 3.
Count the number of cells occupied by the missing leaf tissues.
Count only cells that are at least 50% "empty." Click on
thumbnail for examples.
Step 4.
Calculate percent defoliation as the number of "missing" cells
multiplied by 100, then divided by the total number of cells occupied by
the leaflet.
References:
Catangui,
M. A. 2000.
Grasshopper and bean leaf beetle economic thresholds in soybeans.
Fact Sheet 905, South Dakota State University, Brookings.
LI-COR, Inc. 1988. LI-3000A portable leaf area meter,
LI-3050A transparent belt conveyor
instruction manual. Lincoln, Nebraska.
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