Agriculture Reference
In-Depth Information
(Haneklaus
et al.
, 2006a). The major criticism of critical values for the interpretation of
tissue analysis is the small experimental basis and adequacy of the method of interpretation.
Data about symptomatological S values, critical nutrient values and toxicological values
have been compiled and statistically computed for different crops for a better appraisal of
the S nutritional status of a crop plant (Haneklaus
et al.
, 2006a).
11.1.2
Elemental sulphur - a retrospective
Elemental S (S
0
) is an approved fertiliser according to the German decree on fertilisers
(Anon, 2003). S
0
is, however, better known for its fungicidal effect, which was discovered
by William Forsyth (1802) and has been widely used for this purpose in agricultural
production since the end of the nineteenth century (Hoy, 1987). The application of S
0
is
recommended, among other uses, to 'encourage natural pest-control mechanisms' and
to 'limit the use of pesticides to the minimum effective level' in the guidelines for the
cultivation of medicinal plants (WHO, 2003).
S
0
proved to be most effi cient against rust and powdery mildew (Coleno, 1987; Cook,
1987; Hoy, 1987; Bourbos
et al.
, 2000; Reuveni, 2001), but was also successfully used
against other diseases, such as downy mildew in cereals (Hoy, 1987), common scab of
potato (Vlitos & Hooker, 1951; Mortvedt
et al.
, 1963) and Alternaria black spot of oilseed
rape (Anon, 1988). Recently, it was shown that repeated, foliar-applied S
0
applications
signifi cantly reduced the infection rate by Fusarium head blight after artifi cial inoculation
under fi eld conditions (Haneklaus
et al.
, 2007b). Here, a reduction of the infection rate by
30% under high infection pressure was comparable to the effi ciency of soil tillage opera-
tions and crop rotation in reducing deoxynivalenol content (Beyer
et al.
, 2006). Next to its
fungicidal effect, S
0
is an acaricide and used to combat mites (Hoy, 1987).
The effi cacy of S
0
is either related to the direct toxicity of S
0
or that of its reduction
product hydrogen sulphide (H
2
S) outside the fungal hyphae, or reduction of S
0
to H
2
S
after entering the fungal cell (Heitefuss, 1975; Boerner, 1997). S
0
, which is lipophilic,
may enter directly into the cell wall of the fungi where it disturbs redox reactions in the
metabolism of the pathogen, resulting in the synthesis of cytotoxic levels of H
2
S. A fungi-
cidal action of the oxidation product, sulphur dioxide, is also possible (Boerner, 1997).
The results of an
in situ
experiment suggested a direct inhibitory effect of soil-applied
S
0
on
Rhizoctonia solani
(Haneklaus
et al.
, 2007c), which is corroborated by the experi-
ments of Klikocka
et al.
(2005), where S
0
reduced the infection rate and severity of pota-
toes infected with the same pathogen. Intriguing is the fi nding of Cooper
et al.
(1996) that
S
0
deposits in vascular plant tissue only occur in tomato varieties resistant to
Verticillium
dahliae
. Such S
0
depositions were rapidly induced in Solanaceae; in Brassicaceae S
0
depositions were found constitutively (Cooper & Williams, 2004).
Pezet & Pont (1977) reported that S
0
in fungi is a key factor for self-inhibition or spore
dormancy. Dormant fungal spores have a reduced respiratory capacity, and the addition
of low concentrations of S
0
to fungal spores yielded a similar effect because S acted as
an acceptor of hydrogen, particularly in the terminal respiratory chain (Pezet & Pont,
1977; Beffa, 1993). The supplement of high concentrations of S
0
resulted in an increased
capacity to reduce S, whereby this process was independent of the respiratory activity
and obviously due to the reduction of S
0
by proteic and non-proteic sulphhydryl groups,
which caused the fungicidal effect (Beffa, 1993).
