Barbedo SpringerPlus 2013, 2:660 Page 3 of 12
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arguing that their system can be used to monitor plants
in greenhouse s dur ing the night, but more research is
nee ded for its use during the day, when lighting conditions
vary more intensely.
Quantification
The methods presented in this section aim to quantify the
severity of a given disease. Such a severity may be inferred
either by the area of the leaves that are affected by the
disease, or by how deeply rooted is the affection, which
can be estimated by means of color and texture features.
Most quantification algorithms include a segmentation
step to isolate the symptoms, f rom which features can be
extracted and properly processed in order to provide an
estimate for the severity of the disease.
It is worth noting that the problem of determining the
severity of a disease by analyzing and measuring its symp-
toms is dif ficult even if performed manually by one or
more specialists, which have to pair the diagnosis guide-
lines with the symptoms as accurately as possible. As
a result, the manual measurements will always contain
some degree of subjectivity, which in turn means that ref-
erences used to validate the automatic methods are not
exactly “ground truth ”. It is important to take this into
consideration when assessing the performance of those
methods.
The methods presented in the following are grouped
according to the main strategies they employ to estimate
the se verity of the diseas es.
Thresholding
One of the first methods to us e digital image processing
was proposed by Lindow and Webb (1983). The images
were captured using an analog video camera, under a red
light illumination to highlight the necrotic areas. Those
images were later digitized and stored in a computer. The
tests were per formed using leaves from tomatoes, bracken
fern, sycamore and California buckeye. The identification
of the necrotic regions is done by a simple thresholding.
The algorithm then apply a correction factor to compen-
sate for pixel variations in the healthy parts of the leaves,
so at least s ome of the pixels from healthy regions that
were misclassified a s part of the diseased areas can be
reassigned to the correct set.
Price et al. (1993) compared visual and digital image-
processing methods in quantifying the severity of coffee
leaf rust. They tested two different imaging systems. In the
first one, the image s were captured by a black and white
charge coupled device (CCD) camera , and in the second
one, the images were captured with a color CCD cam-
era. In both cases, the segmentation was performed by a
simple thresholding. According to the authors, the image
processing-based systems had better performance than
visual evaluations, e specially for ca ses with more severe
symptoms. They also observed that the color imaging had
greater potential in discriminating betwe en rusted and
non-rusted foliage.
The method proposed by Tucker and Chakraborty
(1997) aims to quantify and identify diseas es in sunflower
and oat leaves. The first step of the algorithm is a seg-
mentation whose threshold varies according to the disease
being considered (blight or rust). The resulting pixels are
connected into clusters representing the diseas ed regions.
Depending on the characteristics of the lesions, they are
classified into the appropriate category (type a or b in case
of blight and by size in case of rust). The authors reported
goo d results, but observed some errors due to inappropri-
ate illumination during the capture of the images.
Martin and Rybicki (1998) proposed a method to quan-
tify the sy mptoms caused by the maize streak virus. The
thresholding scheme adopted by the authors was based on
the strateg y des cribed by Lindow and Webb (1983) and
briefly explained in the previous paragraph. The authors
compared the results obtained by visual assessment, by
using a commercial software package and by employing
a custom system implemented by themselves. They con-
cluded that the commercial and custom software packages
had approximately the same performance, and that both
computer-based methods achieved better accuracy a nd
precision than the visual approach.
The method proposed by Skaloudova et al. (2006) mea-
sures the damage caused in leaves by spider mites. The
algorithm is based on a two-stage thresholding. The first
stage discriminates the leaf from the background, and the
second stage separates damaged regions from healthy sur-
face. The final estimate is given by the ratio betwe en the
number of pixels in damage regions divided by the total
number of pixels of the leaf. The authors compared the
results with two other methods based on the leaf dam-
age index and chlorophyll fluorescence. The y concluded
that their method and the leaf damage index provided
superior results when compared with the chlorophyll
fluore scence.
In their work, Weizheng et al. (2008) presented a strat-
eg y to quantify lesions in soybe an leaves. The algorithm is
basically composed by a two-step thresholding. The first
threshold aims to separate leaf from background. After
that, the image containing only the leaf is converted to the
HSI color space, and the Sobel operator is applied to iden-
tify the lesion edges. A second threshold is applied to the
resulting Sobel gradient image. Finally, small objects in the
binary image are disc arded and holes enclo sed by white
pixels are filled. The resulting objec ts reveal the diseased
regions.
Camargo and Smith (2009a) proposed a method to
identify reg ions of leaves containing lesions caused by
disea ses. The tests were performed using leaves from a
variety of plants, like bananas , maize, alfalfa, cotton and