This is the first report of variability in sensitivity of Phaeosphaeria nodorum to the fungicide azoxystrobin, and also reports on sensitivity to propiconazole, prothioconazole and cyprodinil. An in vitro sensitivity test of 42 isolates from five Swedish winter wheat fields, collected in 2003 -2005, was performed on malt extract agar amended with six concentrations of each fungicide. Four isolates collected during the early 1990s, before azoxystrobin had been commercially used in agriculture, were used as references. Fragments of DNA from 231 isolates, including the reference isolates, were sequenced for the genes of cytochrome b and CYP51 in search for amino acid substitutions known to cause loss of sensitivity to strobilurins and triazoles, respectively. The majority of the P. nodorum isolates possessed the amino acid substitution G143A, associated with loss of sensitivity in fungi to strobilurins, except in one field where only half of the isolates had the substitution. The EC 50 values varied between 0·66 mg L -1 to estimations far above 1000 mg L . The P. nodorum population is still sensitive to propiconazole, prothioconazole and cyprodinil, even though some isolates varied in sensitivity to triazoles. Part of the CYP51 gene, a possible target for triazole sensitivity, was sequenced but no nonsynonymous substitutions were found.

Mating type ratios and SSR marker analysis were used to study the genetic structure of Phaeosphaeria nodorum , the causal agent of glume blotch in wheat. The study was based on leaf collections in five fields located in different regions in Sweden. In total 302 isolates of P. nodorum were obtained from 203 sampling sites (including eight ascospore isolates). Three strong indications of sexual recombination were found: (i) the two mating types were present at a 1:1 ratio; (ii) the genetic structure was diverse, with many unique genotypes, and 69 of the 93 genotypes were only found once; and (iii) random association of alleles indicated that genetic recombination was frequent. However, asexual reproduction could not be excluded since identical genotypes were found within the fields. The fungal population had experienced a demographic bottleneck, as indicated by a low ratio of number of alleles to microsatellite size range ( M -value) of 0·5.

The fungal communities on wheat leaves showing symptoms similar to stagonospora nodorum blotch were analysed using terminal restriction fragment length polymorphism (T-RFLP). Collection of diseased leaves was carried out in eleven winter wheat fields located in three regions of Sweden during mid-July in [2003][2004][2005]. Fourteen different fungal species were found on the leaves out of which thirteen were identified to the species level and one to the genus level. The majority of the samples had between one and four species present of which at least one was a pathogen. Among the analysed leaves three major leaf pathogens were found: Phaeosphaeria nodorum was common during 2003 and 2004, Mycosphaerella graminicola dominated during 2005. Pyrenophora tritici-repentis was present in all fields, but sometimes in just a few samples. Phaeosphaeria nodorum and P. tritici-repentis often co-occurred on the same leaf. In addition, seven species of yeast and three saprophytes frequently occurred on the leaves every year. The variation in fungal community was largest between the different years while the region of Uppland diverged from the other two regions in species composition. No significant differences in fungal communities were found within a single field, indicating a uniform community at the lowest spatial level.