A methodological approach was conducted to investigate how fast isolates of a “one-race-population” adapt to an ingredient with fungicidal activity. A major objective was to maintain a continuous selection pressure on the fungal populations. Different concentrations of strobilurin azoxystrobin (Amistar®) were applied on wheat and barley cotyledons to ensure selection pressure. The fungi reproduced on fungicide-treated leaves in order to display a “worst-case-scenario”. Wheat powdery mildew (Blumeria graminis f. sp. tritici) adapted rapidly to higher concentrations of the fungicide. Already the fifth generation of the fungus was able to grow and reproduce on leaves containing high concentrations of azoxystrobin (0.5x of the recommended dosage in the field). Two replicates led to the same results. Although rapidity of the adaptation was remarkable, the adapted isolates apparently lost their fitness. In contrast, Blumeria graminis f. sp. hordei was significantly more susceptible to azoxystrobin. The dosage of the fungicide was reduced by the factor of 10 to 40. The lowest concentration of Amistar® efficient against wheat powdery mildew was equal to the highest tolerable concentration applicable for barley powdery mildew. Even when the fungus could adapt to these low concentrations, a change to resistance is not likely, because in the field, 0.1x of the recommended dosage will normally not be used. The third fungus investigated, glume blotch of wheat (Septoria nodorum) was tested ad planta and in vitro. Ad planta, the fourth and fifth generation of the fungus seemed to be less susceptible against the active ingredient azoxystrobin, but in vitro no such effects were observed. The methods used in these studies seem to offer an easy and fast tool to estimate the risk of adaptation and to compare sensitivity levels of different fungal species. The experiments showed significant differences in fungicide susceptibility between the two powdery mildews. We presume that disruptive as well as adaptive resistance mechanisms occur in both pathogens and that the two resistance mechanisms are correlated. Fungi possessing high affinity to both are more assertive than other fungi. A disadvantage is that these artificial circumstances do not exactly reflect the situation in the field because sexual reproduction is not considered. Nevertheless, the risk of a continuous selection pressure caused by a single-site- inhibiting fungicide was obvious in these experiments.
