Spring air temperature accounts for the bimodal temporal distribution of Septoria tritici epidemics in the winter wheat stands of Luxembourg

Septoria tritici is the causal agent of leaf blotch in wheat and among the most damaging fungal cereal
pathogens in the humid regions of central Europe. The percentage of the leaf area colonized by S. tritici
was recorded weekly between April and July every season between 2004 and 2010. A total of 11 cultivars
with moderate susceptibility [ratings of 4e6 on a 1 (resistant) to 9 (susceptible) scale] were included. The
disease level was assessed on the upper three leaf layers at 2 locations between 2004 and 2006 and at 3
locations between 2007 and 2010. The period between sowing and the point of time, when 50% of the
leaf area was necrotized due to colonization by S. tritici (T50) was estimated for each year, site, cultivar
and leaf layer by non-linear regression. T50 values followed a bimodal distribution with one maximum
at 245 days after sowing (DAS; early epidemics) and one maximum at 270 DAS (late epidemics). Early
epidemics were preceded by almost constant daily average temperatures of 13.2 0.8 C between 181
and 210 DAS. Late epidemics were preceded by an approximately linear increase in temperature from
8.7 0.9 to 12.1 0.9 C during the same period of time. Based on these differences, it seems possible to
predict whether an early or a late epidemic can be expected at least 35 days before the epidemic
outbreak. Temperature sums calculated with a base temperature of 6.6 C starting at sowing and ending
when T50 was reached were not significantly different between early and late epidemics (P ¼ 0.73) and
averaged 1721 49 days. Fungicide applications, which resulted into a delay of the epidemic development
similar to the difference between early and late epidemics, resulted in a yield increase between
11.7 and 12.6%.Septoria tritici is the causal agent of leaf blotch in wheat and among the most damaging fungal cereal
pathogens in the humid regions of central Europe. The percentage of the leaf area colonized by S. tritici
was recorded weekly between April and July every season between 2004 and 2010. A total of 11 cultivars
with moderate susceptibility [ratings of 4e6 on a 1 (resistant) to 9 (susceptible) scale] were included. The
disease level was assessed on the upper three leaf layers at 2 locations between 2004 and 2006 and at 3
locations between 2007 and 2010. The period between sowing and the point of time, when 50% of the
leaf area was necrotized due to colonization by S. tritici (T50) was estimated for each year, site, cultivar
and leaf layer by non-linear regression. T50 values followed a bimodal distribution with one maximum
at 245 days after sowing (DAS; early epidemics) and one maximum at 270 DAS (late epidemics). Early
epidemics were preceded by almost constant daily average temperatures of 13.2 0.8 C between 181
and 210 DAS. Late epidemics were preceded by an approximately linear increase in temperature from
8.7 0.9 to 12.1 0.9 C during the same period of time. Based on these differences, it seems possible to
predict whether an early or a late epidemic can be expected at least 35 days before the epidemic
outbreak. Temperature sums calculated with a base temperature of 6.6 C starting at sowing and ending
when T50 was reached were not significantly different between early and late epidemics (P ¼ 0.73) and
averaged 1721 49 days. Fungicide applications, which resulted into a delay of the epidemic development
similar to the difference between early and late epidemics, resulted in a yield increase between
11.7 and 12.6%.