Erland Liljeroth

The actual yield losses due to early blight disease in potato worldwide are not well described, and therefore, we analysed a decade-long series of field trials performed in Sweden. The series involved 18 trials with starch potato and six ware potato trials where early blight infection rate and yield were studied in untreated and fungicide-treated plots. Yield loss was estimated as the difference between the best fungicide strategy with respect to infection rate and untreated. The potential yield loss was calculated from the relationship between infection rate and yield. Yield and yield loss due to early blight were analysed in relation to weather and soil type. In starch potato, the variation in starch yield increase of fungicide-treated potato ranged from 1.9 to 29.9%, with an average of 11.9%. The calculated potential starch yield loss due to early blight was higher, 14.3% on average, reflecting minor infections in fungicide-treated plots. The large range of variation among trials indicates complex interactions among factors influencing infection rate and yield. We did not find any yield loss due to early blight in the ware potato trials. We found positive correlations between average season air temperature and infection rate in untreated plots. There was a negative correlation between average season air temperature and starch yield in both untreated and fungicide-treated plots. We also found a negative relationship between high soil sand content and infection rate. The yield increase due to fungicide treatment was higher on soils with high sand content. The calculated yield increase needed to compensate for the cost of early blight fungicide product sprayed in starch potato was approximately 250 kg starch per ha, corresponding to an approximately 2.7% yield increase. However, it is generally not economically feasible in Sweden to use fungicides against early blight in ware potato ready for harvest before the end of August.

Based on discussions within the Northern Tubers of Potato network (N’TOP-net), this review highlights northern Scandinavia’s potential for sustainable, low pest seed potato production. While long transport distances currently limit large-scale supply for consumption or processing, low pest pressure and stricter EU plant protection regulations increase its value for seed production. Climate change is expected to extend the growing season, driving renewed interest in Northern Scandinavia’s role in European food security. Finland exemplifies this potential, and parts of northern and central Sweden—historically suppliers of disease-free seed potatoes, even exported to Brazil—offer expansion opportunities. Nordic potato production, key biotic stressors, and opportunities for regional cooperation are examined, with a focus on novel farming practices, breeding innovations, and disease surveillance to improve resilience and sustainability. Despite shared values in cultivar selection, certification, and potato preferences, Nordic production strategies remain uncoordinated for long-term sustainability. We advocate for transnational, interdisciplinary collaboration to enhance Europe’s food security through joint efforts in three key areas: (1) soil-conserving farming, (2) breeding for adaptation to longer day length and resistance traits, and (3) transnational pest and disease surveillance. A Nordic potato initiative can strengthen European cooperation on sustainable production amid climate change. However, as policies must balance the benefits of longer growing seasons with emerging risks such as pests, droughts, and flooding, coordinated research, regulatory adaptation, and climate resilience investments are essential for safeguarding seed potato quality, food security, and supply chain stability.

Integrated pest management (IPM) is an important tool for sustainable crop production. IPM includes a diversity of methods, e.g., the use of biological control agents (BCAs) for disease control or growth promotion. While there is an increasing interest in the use of BCAs, less is known about their environmental costs and benefits on wild species, such as wild crop relatives. For example, a BCA may have the positive effect of controlling disease in wild relatives, but could also have the negative effect of growth promotion on wild relatives that act as weeds. In this study, we investigated if three wild potato relatives—the perennial climber Solanum dulcamara , and the annual weeds S. nigrum and S. physalifolium —could be infected by Alternaria solani , the causal agent of early potato blight in Sweden, and studied how two BCAs, Pythium oligandrum (a laboratory strain) and Bacillus amyloliquefaciens (Serenade), affected the disease and growth promotion in a series of greenhouse and field experiments. Our studies confirmed the semantic knowledge that A. solani can infect all three wild species, in particular the two annual species often growing as weeds in potato fields. We also found a disease-controlling effect of B. amyloliquefaciens , but not P. oligandrum , in the greenhouse. Some growth effects were found for both BCAs, but whether these were positive or negative varied with trait, plant species, and genotype. In conclusion, BCAs can confer both environmental costs and benefits on the three wild relatives of potato investigated in the current study, which should be taken into consideration for development of sustainable potato cultivation.

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.

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.

Potato early blight is caused by the necrotrophic fungus Alternaria solani and can result in yield losses of up to 50% if left uncontrolled. At present, the disease is controlled by chemical fungicides, yet rapid development of fungicide resistance renders current control strategies unsustainable. On top of that, a lack of understanding of potato defences and the quantitative nature of resistance mechanisms against early blight hinders the development of more sustainable control methods. Necrotrophic pathogens, compared to biotrophs, pose an extra challenge to the plant, since common defence strategies to biotic stresses such as the hypersensitive response and programmed cell death are often beneficial for necrotrophs. With the aim of unravelling plant responses to both the early infection stages (i.e., before necrosis), such as appressorium formation and penetration, as well as to later responses to the onset of necrosis, we present here a transcriptome analysis of potato interactions with A. solani from 1 h after inoculation when the conidia have just commenced germination, to 48 h post inoculation when multiple cell necrosis has begun. Potato transcripts with putative functions related to biotic stress tolerance and defence against pathogens were upregulated, including a putative Nudix hydrolase that may play a role in defence against oxidative stress. A. solani transcripts encoding putative pathogenicity factors, such as cell wall degrading enzymes and metabolic processes that may be important for infection. We therefore identified the differential expression of several potato and A. solani transcripts that present a group of valuable candidates for further studies into their roles in immunity or disease development.

Currently available fungicides against potato late blight are effective but there are concerns about the sustainability of frequent applications and the risks of fungicide resistance. Therefore, we investigated how potassium phosphite can be integrated into late blight control programs with reduced fungicides in field trials. Phosphite was somewhat less effective than the conventional fungicides at suppressing late blight in the foliage, and the tubers contained less starch. However, when we reduced the amount of phosphite and combined it with reduced amounts of conventional fungicides, we observed no differences in disease suppression, total yields, and tuber starch contents compared with the full treatments with conventional fungicides. The amount of phosphite detected in the harvested tubers was linearly associated with the amount of phosphite applied to the foliage. Our analyses indicate that phosphite could replace some fungicides without exceeding the current European Union standards for the maximum residue levels in potato tubers. No phosphite was detected in the starch from the tubers. In 1 of 2 years, early blight (caused by Alternaria solani) was less severe in the phosphite treatments than in the treatments without phosphite. The integration of phosphite into current treatment strategies would reduce the dependence on conventional fungicides.

Section Key Message Using disease bioassays and transcriptomic analysis we show that intact SA-signalling is required for potato defences against the necrotrophic fungal pathogen Alternaria solani. AbstractSection Abstract Early blight, caused by the necrotrophic fungus Alternaria solani, is an increasing problem in potato cultivation. Studies of the molecular components defining defence responses to A. solani in potato are limited. Here, we investigate plant defence signalling with a focus on salicylic acid (SA) and jasmonic acid (JA) pathways in response to A. solani. Our bioassays revealed that SA is necessary to restrict pathogen growth and early blight symptom development in both potato foliage and tubers. This result is in contrast to the documented minimal role of SA in resistance of Arabidopsis thaliana against necrotrophic pathogens. We also present transcriptomic analysis with 36 arrays of A. solani inoculated SA-deficient, JA-insensitive, and wild type plant lines. A greater number of genes are differentially expressed in the SA-deficient mutant plant line compared to the wild type and JA- insensitive line. In wild type plants, genes encoding metal ion transporters, such as copper, iron and zinc transporters were upregulated and transferase-encoding genes, for example UDP-glucoronosyltransferase and Serine-glyoxylate transferase, were downregulated. The SA-deficient plants show upregulation of genes enriched in GO terms related to oxidoreductase activity, respiratory chain and other mitochondrial-related processes. Pathogenesis-related genes, such as genes encoding chitinases and PR1, are upregulated in both the SA-deficient and wild type plants, but not in the JA-insensitive mutants. The combination of our bioassays and the transcriptomic analysis indicate that intact SA signalling, and not JA signalling, is required for potato defences against the necrotrophic pathogen A. solani.

Early blight of potato, caused by Alternaria solani, is an economically important foliar disease in most potato-growing regions. Growing cultivars with higher levels of resistance to early blight can reduce tuber yield losses and the need for fungicide applications. In this research, a bi-parental tetraploid potato population, segregating for resistance to early blight in leaves and tubers, was characterized to identify novel quantitative trait loci (QTL) associated with foliar and tuber early blight resistance. Assessment of the disease resistance in the foliage was performed by field evaluation and in tuber under controlled conditions. Results from this study revealed significant differences (P < 0.001) in resistance to A. solani among potato clones both in the leaves and in tubers. There was no statistically significant correlation (r = 0.06, P = 0.35) between the resistance scores from leaves and tubers. Several clones exhibited; however, high levels of resistance both in leaves and tubers and are; thus, promising candidates for breeding for early blight resistance. Linkage mapping revealed several QTL for early blight affecting both foliage and tubers. QTL associated with disease resistance in the tuber were found on chromosomes 1, 2, 3, 4, 8, 11 and 12. QTL associated with disease resistance in foliage were also examined for independence from defoliation, and independent QTL were; thus, found on chromosomes 5 and 11.