Agriculture Reference
In-Depth Information
visible; the critical nutrient value, which stands for the S concentration above which
the plant is suffi ciently supplied with S for achieving the maximum potential yield or
yield reduced by 5%, 10% and 20%; and the toxicological value, which indicates the
S concentration above which toxicity symptoms can be observed (Haneklaus
et al.
, 2006a).
But there is no one exclusive critical nutrient value for each crop, since it depends on the
growth conditions, the developmental stage of the plant at sampling, the collected plant
part, the determined S species, the targeted yield and mathematical approach for calculat-
ing it. Thus, numerous widely differing thresholds for the S nutritional status of the same
crop exist. Based on a profound and robust statistical procedure, critical S concentrations
have been calculated for cereals, oilseed rape and sugar beet (Haneklaus
et al.
, 1998,
2006a, 2006b). These threshold values are in good agreement with median values that
have been calculated from compiled and categorised available, individual S nutritional
data of studies with varying experimental conditions (Haneklaus
et al.
, 2006a). This
database should be used for evaluating the S nutritional status and adjusting S fertiliser
rates where reliable site-specifi c values for individual crops are not available.
11.2.5.2
Potency and spectrum of SIR against fungal pathogens
Sulphur fertilisation reduced the disease index for various host/pathogen relationships by
5-50% and 17-35% in greenhouse and fi eld experiments, respectively (Haneklaus
et al.
,
2006c). This fi nding refl ects the as-yet untapped potential of SIR, and only a breakthrough
in promoting SIR steadily in production fi elds will facilitate an integration into current
plant protection schemes.
So far, SIR has been demonstrated against biotrophic and necrotrophic pathogens
belonging to the Ascomycetes and Basidiomcyetes, as well as Oomycetes. Results
obtained for members of the Deuteromycotina have been not conclusive.
In greenhouse experiments, S nutrition signifi cantly infl uenced the disease index for dif-
ferent host/pathogen combinations (Wang
et al.
, 2003). The disease index decreased 8-20
days after fungal inoculation by 5% for infections of oilseed rape with stem rot (
Sclerotinia
sclerotiorum
), by 21% for infection of corn with southern blight (
Bipolaris maydis
) and by
44% for infection of winter wheat with sharp eyespot (
Rhizoctonia cerealis
)
(Wang
et al.
,
2003). For infections of cotton with
Fusarium oxysporum
and
Verticillium dahliae,
results
proved to be inconsistent (Wang
et al.
, 2003). In other experiments, the S nutritional status
of oilseed rape was related to resistance against black leg (
Leptosphaeria maculans
),
grey mould blight (
Botrytis cinerea
), and
Phytophthora brassicae
(Dubuis
et al.
, 2005).
In fi eld trials with oilseed rape and potatoes, S fertilisation signifi cantly reduced the
disease index for light leaf spot (
Pyrenopeziza brassicae
) and black scurf (
Rhizoctonia
solani
) (Schnug
et al.
, 1995; Klikocka
et al.
, 2005). Infection rate and disease severity of
potato tubers with
R. solani
were reduced, for example, by 41% and 29%, respectively
(Klikocka
et al.
, 2005). In the case of oilseed rape, SIR ensured that the yield potential
could be more or less fully maintained (Schnug
et al.
, 1995).
11.3
Perspectives in research
A major shortcoming of SIR is that the spectrum and potency are only known for some
host/pathogen relationships. Further systematic studies from lab to fi eld level are required
to quantify interactions between plant, pathogen and environment.
