Agriculture Reference
In-Depth Information
11.1
The Situation for Site-Specifi c Fungicide Applications
It is a well known fact that fungal crop infections in most cases start with a few
small and very discrete
patches
. At the beginning of a fungal epidemic, these few
tiny infected loci within a fi eld are hard to detect. Their visual patterns depend very
much on the respective fungi, on the infection stage, on crop development as well as
on varieties and on the weather. Therefore, traditionally crop epidemiologists have
been
detectives of patterns
with microscopes as manual tools.
The early detection of an infection is crucial, because it allows timely counterac-
tions, hence to eliminate further spreading of the disease and to prevent serious
damage to the crop. It is also a prerequisite for site-specifi c application. Because at
the far end of a fungal epidemic and much damage, a more uniform spatial distribu-
tion of the disease will exist.
If the challenge of an early and site-specifi c detection of infected patches is met,
there probably are no other farming operations where treating the whole fi eld uni-
formly can be so far off the actual needs. However, these challenges also mean that any
successful site-specifi c application of fungicides must comply with extremely high
spatial and temporal resolutions. So to come up to the prerequisites for site-specifi c
spraying against fungi is not easy, yet the prospects might justify the efforts.
As a consequence of this situation, two different approaches for site-specifi c
application of fungicides have evolved. The fi rst approach is based on a
full-area
preventive concept
. Since for practical applications the detection of the initial tiny
infected loci is not yet solved, still the whole area is treated in a precautionary man-
ner. The timeliness for these largely prophylactic applications is oriented at local
epidemic forecasts of extension services. However, there might still be a site-specifi c
control of the application rate. This control might be based on more general crop
properties such as biomasses or leaf-area-indices.
The second approach aims at
discrete spot spraying
of the few just infected loci
and if possible not beyond these. This approach - if well conceived and executed -
would allow a radical reduction of fungicide use. However, whereas the full area
preventive concept with site-specifi c application is state of the art, the discrete spot
spraying of fungicides still is in an experimental stage. And especially the latter con-
cept needs sprayers that allow for separate section- or even better separate nozzle
control. Modern sprayers that use
direct injection
of the pesticides into the water
close to the nozzles instead of premixed batches provide the technical prerequisites
for such a resolution in the application (Vondricka and Schulze Lammers
2009
).
11.2
The Full-Area Preventive Concept Based on Biomass
It is general experience that dense, lush crops are more susceptible to fungal
infections than thin and less developed canopies. On a site-specifi c basis, the bio-
mass densities or leaf-area-indices of crops vary as the soils do. With the usual
