Agriculture Reference
In-Depth Information
It might be helpful to remember that in-season nitrogen sensing also started with
standard refl ectance indices that too originally also were created for other purposes
(Heege and Reusch
1996
). It turned out that the prediction of these standard indices
could be improved by special indices, which solely were determined for nitrogen
developed by systematically checking all theoretically possible ratios of narrow
refl ectance bands from the visible range plus the adjacent near-infrared range
(Reusch
2003
,
2005
; Müller et al.
2008
).
Hence for sensing of fungi it probably is reasonable as well to systematically
check mathematical combinations of discrete narrow wavelengths along a sensible
full spectrum for their sensitivities in this respect. The efforts needed for this
systematic searching
will be immense, since many different fungi and various
crops should be considered. A start in this direction has been made by Mahlein et al.
(
2013
) for fungal diseases of sugar beets.
Instead of sensing by indices from narrow wavelengths, sometimes
image sensing
for the detection of fungi is proposed. These images rely on broad wavelength bands,
e
.
g
. the red, green and blue (RGB) bands of the spectrum. An obvious advantage of
imaging is that the locations of the infected loci or discrete spots directly can be seen
on the records. A disadvantage of images is that the broad wavelength bands tend to
hide early effects of fungal infections. The detection is possible not until when symp-
toms appear as obvious lesions. Yet with suitable spectral indices from narrow wave-
lengths, the sensing can start already slightly before the fi rst sporulation (Moshou
et al.
2011
). And a still earlier detection might be possible when fl uorescence pro-
vides the signals. This will be dealt with in the next sections.
11.4
The Discrete-Spot Sensing Concept Based
on Fluorescence
11.4.1
Indirect Measuring with In-Situ Sensor System
Fluorescence is the emission of radiation that follows an absorption of light energy.
The emitted radiation has longer wavelengths than the absorbed light. Fluorescence
requires an adequate substance called
fl uorophore
. Normally, the sensing of fungi
by means of fl uorescence is an indirect method. This means that the fungi are non
fl uorescent or only show a very weak fl uorescence according to their low concentra-
tion in the leaf when compared with fl uorescent plant pigments. Only some biotro-
phic fungi like powdery mildew and yellow rust show a blue fl uorescence if excited
with UV light (Zhang and Dickensen
2001
).
The normally called “plant fl uorescence” originates from substances insides the
leaves that are natural fl uorophores. This is different from the fl uorescence detection
tools used in serological or molecular methods where the samples must be prepared
to show fl uorescence.
