Assessing the Belgian potato Alternaria population for sensitivity to fungicides with diverse modes of action

Abstract

Early blight and brown spot, caused by respectively Alternaria solani and Alternaria alternata, can lead to severe yield losses in potato-growing areas. To date, fungicide application is the most effective measure to control the disease. However, in recent years, a reduced sensitivity towards several active ingredients has been reported. To shed light on this issue, Alternaria isolates were collected from different potato fields in Belgium during two growing seasons. Subsequently, the sensitivity of these isolates was assessed using four widely used fungicides with different modes of action. Demethylation inhibitors, quinone outside inhibitors, a dithiocarbamate and a carboxylic acid amide were included in this study. Although all fungicides reduced spore germination and vegetative growth of Alternaria species to some extent, the interspecies sensitivity was very variable. In general, A. solani was more suppressed by the fungicides compared to A. alternata. The effectiveness of the dithiocarbamate mancozeb was high, whereas the quinone outside inhibitor azoxystrobin showed a limited activity, especially towards A. alternata. Therefore, a subset of the A. alternata and A. solani isolates was tested for the presence of, respectively, the G143A substitution and the F129L substitution in the cytochrome b. The frequency of A. alternata isolates bearing the resistant G143A allele (approximately 65%) was comparable in both sampling years, although sensitivity of isolates decreased during the growing season. This finding points to a shift of the population towards resistant isolates. Both the European genotype I and American genotype II were present in the A. solani population, with genotype I being the most prevalent. None of the genotype I isolates carried the F129L substitution, whereas in 83% of the genotype II isolates this substitution was present. Our results demonstrate for the first time that the Belgian Alternaria population on potato comprises a considerable broad spectrum of isolates with different sensitivity to fungicides

Long term field sampling in Sweden reveals a shift in occurrence of cytochrome b genotype and amino acid substitution F129L in Alternaria solani, together with a high incidence of the G143A substitution in Alternaria alternata

“…When considering our results, showing that the shift from non-mutated GII to GII F129 L occurred the year before the shift from GI to GII, regardless of mutation, the genotype change is most probably due to selection caused by strobilurin applications. We found only five samples with GI F129 L. A higher occurrence of F129 L in GII compared to GI has also been observed in Belgium (Landschoot et al 2017a). It is possible that the intron in the gene encoding the GI genotype has a diminishing effect on the ability to mutate or that GII isolates have a better fitness compared to GI.…”

“…Three different nucleotide substitutions where the wild type codon TTC has changed to CTC, TTA or TTG, leading to the same amino acid substitution, have been found (Pasche et al 2005). These substitutions result in a reduced field efficacy of strobilurins in A. solani, which has been confirmed in in-vitro sensitivity tests (Rosenzweig et al 2008a;Pasche et al 2005;Landschoot et al 2017a). In Sweden, field observations of reductions in field efficacy of strobilurins have been made since 2014.…”

“…The correlation between loss of sensitivity to azoxystrobin and presence of F129 L is strong. Isolates carrying the substitution have often an EC 50 -value that is far above the threshold level for resistance (Pasche et al 2005;Landschoot et al 2017a;Odilbekov et al 2016). Observations of reduced field efficacy in south-eastern Sweden has been observed in a field trial outside Kristianstad in 2014, where two applications of Amistar (azoxystrobin, Syngenta) resulted in the same disease level as the untreated control (Liljeroth 2016).…”

“…The substitution is referred to as F129 L since the amino acid phenylalanine is changed to leucine at codon position 129 in the gene encoding cytochrome b. Many of the analysed GII samples from Germany, Belgium and Sweden possessed the F129 L substitution, here referred to as GII F129 L (Leiminger et al 2014;Odilbekov et al 2016;Landschoot et al 2017a). Three different nucleotide substitutions where the wild type codon TTC has changed to CTC, TTA or TTG, leading to the same amino acid substitution, have been found (Pasche et al 2005).…”

Molecular identification of Alternaria spp. in potato crop

“…Devido à importância da doença como fator limitante à cultura da batata, diferentes métodos de controle integrado são usados, como por exemplo, espaçamento adequado, utilização de batatas-sementes sadias, cultivares menos suscetíveis, retirada e destruição de plantas doentes, sendo estas da cultura ou voluntárias, eliminação de restos de cultura e controle químico (ZANOTTA 2019;NOSSLLALA;2016;BOSCO et al, 2010;TAYLOR et al, 2003). Destes, os fungicidas desempenham um papel decisivo no controle da pinta preta (TÖFOLI et al, 2016) Eficácia, modo de ação, risco de resistência, efeitos colaterais, aspectos econômicos e sociais e legislação são fatores que devem ser tecnicamente considerados em programas de manejo que visem a sustentabilidade da cadeia produtiva da batata.…”