Journal of Plant Pathology - Sipav.org

S1.12 Fungicide sensitivity of Septoria tritici Journal of Plant Pathology (2011), 93 (1, Supplement), S1.7-S1.13
2009). In contrast, triazole sensitivity of S. tritici fluctuated
(Table 1, Fig. 3A), suggesting that the factors affecting
sensitivity development over time were different
between G. zeae and S. tritici. G. zeae causes a monocyclic
disease that is targeted by one spray per season at
maximum, whereas S. tritici elicits a polycyclic disease
targeted by 1 to 3 sprays per season. Hence, effects of
fungicide regimes on sensitivity are likely to become obvious
in a shorter period of time in S. tritici than in G.
zeae. Furthermore, mycotoxin levels associated with
Fusarium head blight pathogens were hardly reduced
by QoIs (Chala et al., 2003), and, therefore, azoles were
preferentially applied against Fusarium species, whereas
QoIs were largely avoided, resulting in a moderate but
permanent selective pressure of DMIs on G. zeae with
hardly an interference by QoIs.
Leroux et al. (2007) reported that some mutations in
the CYP51 gene were associated with changes in sensitivity
of European S. tritici strains towards DMIs. Data
by Brunner et al. (2008) suggest that CYP51 mutations
conferring partial resistance towards azoles probably
arose in Denmark or UK and then spread towards eastern
Europe. Furthermore, no CYP51 mutations conferring
partial resistance towards azoles have been found
in non-European populations so far (Brunner et al.,
2008). Stammler et al. (2008) screened 615 strains sampled
in 2007 throughout Europe for their sensitivity towards
epoxiconazole. They found no consistent relationship
between CYP51 haplotypes and sensitivity. In
our study, less isolates from the same region were tested,
but over a longer period of time, allowing us to study
the effects of temporal factors on fungicide sensitivity,
largely unbiased by spatial factors. A general interpretation
of our results focusing on the effect of the composition
of fungicide sprays on sensitivity over time is given
below.
General interpretation. Changes of fungicide sensitivity
over time were characteristic and very consistent
for each group of fungicides having the same mode of
action (Fig. 3). In 1999, DMIs were used on a routine
basis against S. tritici in northern Germany (Fig. 3B). In
subsequent years, QoIs use increased dramatically (Fig.
3B), such that the selective pressure on S. tritici was not
exerted by DMIs alone. Despite approximately constant
use of DMIs at high frequency (Fig. 3B), EC 50
values
for DMIs slightly decreased during the period of frequent
QoI use until 2004.
First S. tritici isolates resistant towards strobilurins
were found in UK in 2002 (Fraaije et al., 2005) resulting
in a decrease of QoI use for S. tritici control in subsequent
years. Hence, the fraction of selective pressure applied
by DMIs increased again and is reflected by an increase
of EC 50
for azoles until 2008. Interestingly, prothioconazole
followed the sensitivity pattern of azoles, even
though it was not yet introduced to the market in 1999.
Hence, at least a limited degree of cross resistance between
(tri)azoles seems to exist in S. tritici. Given the
continuous use of DMIs at high frequency and earlier reports
on mutations of the DMI target gene CYP51 (Leroux
et al., 2007), resistance phenomena observed in our
study were surprisingly small, raising the question
whether CYP51 is the only DMI target in S. tritici.
Fungicides having a different mode of action such as
folpet or chlorothalonil resulted in a different pattern of
sensitivity development over time. Generally, development
of resistance towards multi site inhibitors is considered
to be more unlikely than development towards single
site inhibitors, because several toxic mechanisms have
to be circumvented by the target organism at the same
time with multi site inhibitors, whereas only one mutation
can confer resistance to single site inhibitors (Deising
et al., 2008). Even though the changes in sensitivity
were non-significant for the multi site inhibitors folpet
and chlorothalonil in our study, the consistency of EC 50
changes over time suggests that S. tritici populations have
the potential to adapt also to multi site inhibitors on a
long term perspective. Furthermore, EC 50
s for multi site
inhibitors slightly increased during the period of QoI use
and decreased again afterwards. This phenomenon might
be interpreted as evidence for a limited degree of cross
resistance between trifloxystrobin and the multi site inhibitors,
raising the question whether the multi site inhibitors
used here may – at least partly – affect mitochondrial
respiration, as trifloxystobin does.
The low number of strains available for 2004 from
the particular region of interest was rather unsatisfying.
However, the main effect (interaction between time and
fungicidal mode of action) of this paper did not depend
on the data from 2004. In fact, the effect of time × fungicide
on log(EC 50
) (same analysis as given in Table 1)
was significant at P