Occurrence of the F129L mutation in Alternaria solani populations in Germany in response to QoI application, and its effect on sensitivity

Abstract

Early blight caused by Alternaria 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 (QoIs) are commonly used to manage early blight. However, loss of sensitivity to QoIs has previously been reported for A. solani in the United States. Two hundred and three A. solani field isolates collected from 81 locations in Germany between 2005 and 2011 were screened for the presence of the F129L mutation in the cytochrome b gene; of these, 74 contained the F129L 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 F129L 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 F129L 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 F129L isolates in the German A. solani population over the years 2009-2011. It is assumed that the application of QoIs has selected for the occurrence of F129L mutations, which may contribute to loss of fungicide efficacy.

Genetic diversity and occurrence of the F129L substitutions among isolates of Alternaria solani in south-eastern Sweden

“…The shift to reduced sensitivity also tended to be more pronounced in isolates with the F129L substitution collected in 2014 compared to isolates from 2011, indicating that application of QoI fungicides has resulted in reduced sensitivity of the isolates over time. These results are in accordance with the previous study [19], which also reported that isolates carrying this substitution had reduced in vitro sensitivity to QoI fungicides, especially against azoxystrobin.…”

“…In Europe, A. solani isolates containing the F129L substitution were first found in Germany. Isolates possessing the substitution also displayed a shift in sensitivity to different strobilurins in in vitro spore germination assays [19]. None of the Swedish GI isolates carried the F129L substitution, while the F129L substitution was observed in many of the GII isolates.…”

“…Natural mutations may occur more frequently in asexually reproducing isolates than in sexually reproducing ones [35]. Several studies have reported mutations in A. solani populations in the USA, especially in mitochondrial DNA [15, 36] and in Germany [19]. The high usage of fungicides most likely explains why the genetically diverse populations have the same mutation.…”

“…Recently, it has been discovered that A. solani in Europe carries two types of mitochondrial DNA. Populations carrying these two DNA types are referred to as genotype 1 (GI) and genotype 2 (GII) [19]. The latter resembles the American population of A. solani and can only be distinguished by PCR with special primers [15].…”

“…The latter resembles the American population of A. solani and can only be distinguished by PCR with special primers [15]. Analysis of Alternaria populations from Germany revealed the presence of the F129L substitution and the frequency of this substitution increased over the years [19]. Isolates that carried the F129L substitution had reduced in vitro sensitivity to Qol fungicides.…”