Agriculture Reference
In-Depth Information
50
40
30
20
10
0
cercospora leaf spot
(
Cercospora beticola
)
disease severity
0 %
10 %
20 %
50 %
80 %
50
40
30
20
10
0
50
40
30
20
10
0
powdery mildew
(
Erysiphe betae
)
sugar beet rust
(
Uromyces betae
)
400
500
600
700
800
900
1000
wavelength in nm
blue
green
red
near-infrared
Fig. 11.3
Refl ectance spectra of sugar beet leaves infected with various fungi (From Mahlein
et al.
2010
, altered)
wavelengths and thus to simplify the technology and - if properly selected - still
can provide a high precision.
Attempts with
standard refl ectance indices
have been made for sensing fungal
infections in wheat. The leaves were artifi cially infected. Among many standard
indices, the best
differentiations
between infected and healthy leaves for yellow
rust (
Puccinia striiformis
) were provided by either
the Photochemical Refl ectance Index =
RR
RR
531
−
+
570
•
(Huang et al.
2007
) or
531
570
1
550
1
700
•
the Anthocyanin Refl ectance Index =
−
(Devadas et al.
2009
).
R
R
It should be noted that all standard refl ectance indices originally were developed
for defi ned purposes. The photochemical refl ectance index was created for assess-
ing the effi ciency of radiation-use in photosynthesis (Penuelas et al.
1995
). And the
anthocyanin refl ectance index was developed for estimating the anthocyanin accu-
mulation in plants (Gitelson et al.
2001
). The fact that these indices depend on the
development of rust fungi does not imply that they are the best possible choice for
any refl ectance indices that are conceivable. Because none of the standard refl ec-
tance indices was developed with the aim of sensing fungi.
