Early blight, caused by the fungus Alternaria solani, is a common foliar disease in potato. Quinone outside inhibitor (QoIs) fungicides have commonly been used against A. solani. To avoid or delay development of fungicide resistance it is recommended to alternate or combine fungicides with different modes of action. Therefore, we compared two different fungicide programs against early blight in field trials and studied within season changes in the pathogen population. An untreated control was compared with treatments using azoxystrobin alone and with a program involving difenoconazole followed by boscalid and pyraclostrobin combined. Isolates of A. solani were collected during the growing season and changes in the population structure was investigated. We also screened for the amino acid substitution in the cytochrome b gene and investigated changes in sensitivity to azoxystrobin. Treatment with azoxystrobin alone did not improve disease control in 2014 when the disease pressure was high. However, lower severity of the disease was observed after combined use of difenoconazole, boscalid and pyraclostrobin. The efficacy of both fungicide treatments were similar during the field trial in 2017. Two mitochondrial genotypes (GI and GII) were found among isolates, where all isolates, except two, were GII. All GII isolates had the F129 L substitution while the two GI isolates were wild type. Population structure analysis and principal component analysis (PCA) of amplified fragment length polymorphisms (AFLP) data revealed within season changes in the A. solani populations in response to fungicide application. Isolates with the F129 L substitution had reduced sensitivity to azoxystrobin in vitro and their sensitivity tended to decrease with time.
Early blight, caused by Alternaria solani, is a common potato disease worldwide. Reduced field efficacy of the fungicide boscalid against this disease has been reported in several countries. Boscalid resistance has been mostly studied with in-vitro and/or greenhouse experiments. Field studies validating this phenomenon are largely missing. Here, for the first time in Scandinavia, we validated boscalid resistance in a Swedish population of A. solani both in the field and in the laboratory. Field trials between 2014 and 2017 in Nymö showed significant efficacy reduction by year. The target regions of the A. solani genes encoding the succinate dehydrogenase subunits (Sdh) B, C and D of samples collected from Nymö, and additional fields in south-eastern and central Sweden, were analysed for substitutions associated with loss of boscalid sensitivity. In 2014, the SdhC-H134R mutation was found at several sites at a low frequency, while, in 2017, the majority of the samples had either the SdhB-H278Y or the SdhC-H134R substitution. No mutations were detected in the gene encoding the SdhD subunit. Spore germination tests showed a high sensitivity (EC50 < 1 μg mL−1) of isolates lacking the substitutions. This was supported by a significant decrease in their radial growth rate, from 0.1 to 10 μg mL−1 boscalid. However, the mutated isolates had EC50 > 100 μg mL−1 and their growth rates hardly decreased at concentrations above 1–10 μg mL−1. These results add to the current knowledge of fungicide resistance development in field and indicate that early blight management in southeast Sweden should no longer rely on boscalid.
Early blight in potato, caused by Alternaria solani, is mainly controlled by frequent applications of synthetic fungicides. Reducing the use of synthetic fungicides in agriculture is desired to reach an overall sustainable development since the active components can be harmful for humans and for the ecosystem. In integrated pest management, IPM, the idea is to combine various measures, including optimized crop management, crop rotation, use of resistant cultivars, biological control agents (BCAs), plant resistance inducers, and fertilizers, to decrease the dependence on traditional chemical fungicides. In this paper, we present the results from greenhouse and field trials where we evaluated the effect of strategies aimed at reducing our reliance on synthetic fungicides including treatments with biological control agents (BCAs) (Pythium oligandrum, Polygandron®, and Bacillus subtilis, Serenade®) and plant resistance inducers (silicon products HortiStar® and Actisil®) for early blight in potato. The agents were applied separately or in combination with each other or with synthetic fungicides. In the greenhouse, trials application of these agents resulted in 50–95% reduction of infection by A. solani, but their combination did not generally improve the outcome. However, the effects were much smaller in the hand-sprayed field trials, 20–25% disease reduction and almost disappeared in full-scale field trials where application was done with tractor sprayers. In this article, we discuss possible reasons behind the drop in efficacy from greenhouse trials to full-size field evaluation.
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