Occurrence of the F 129 L mutation in A lternaria solani populations in G ermany in response to Q o I application, and its effect on sensitivity

Early blight caused by A lternaria solani is a highly destructive disease of potatoes. Control of early blight mainly relies on the use of preventive fungicide treatments. Because of their high efficacy, azoxystrobin and other quinone outside inhibitors ( Q o I s) are commonly used to manage early blight. However, loss of sensitivity to Q o I s has previously been reported for A . solani in the U nited S tates. Two hundred and three A . solani field isolates collected from 81 locations in G ermany between 2005 and 2011 were screened for the presence of the F 129 L mutation in the cytochrome b gene; of these, 74 contained the F 129 L mutation. Sequence analysis revealed the occurrence of two structurally different cytb genes, which differed in the presence (genotype I) or absence (genotype II) of an intron, with genotype I being the most prevalent (63% of isolates). The F 129 L mutation was detected only in genotype II isolates, where it occurred in 97%. Sensitivity to azoxystrobin and pyraclostrobin was determined in conidial germination assays. All isolates possessing the F 129 L mutation had reduced sensitivity to azoxystrobin and, to a lesser extent, to pyraclostrobin. Early blight disease severity on plants treated with azoxystrobin was significantly higher for A . solani isolates with reduced fungicide sensitivity in the conidial germination assay compared with sensitive isolates. Data suggest an accumulation of F 129 L isolates in the G erman A . solani population over the years 2009–2011. It is assumed that the application of Q o I s has selected for the occurrence of F 129 L mutations, which may contribute to loss of fungicide efficacy.