Phenotypic and genotypic diversity of Phytophthora infestans populations in Scotland (1995–97)

In a survey of Scottish potato late blight ( Phytophthora infestans ) populations from 1995 to 1997, nearly 500 isolates were collected from over 80 disease outbreaks in commercial potato crops and gardens/allotments. The isolates were characterized by mating type, resistance to the fungicide metalaxyl and almost 300 were examined by DNA-based AFLP fingerprinting. These data were examined alongside cropping details to determine the population structure in the context of existing disease management strategies. A1 and A2 mating type isolates were present in both commercial potato crops and gardens or allotments although they coexisted more frequently in the latter sites. One-fifth of the isolates collected were of the A2 mating type and the frequency was similar over the 3 years and amongst sites. In 1995 the proportions of isolates that were sensitive and resistant to metalaxyl were equal (∼40%) but, over the following 2 years, the frequency of resistant isolates decreased and that of intermediate isolates increased. The mating type response to metalaxyl differed markedly, with 52% of A1 and only 5% of A2 isolates being resistant. Considerable molecular diversity was observed, with over half of the isolates having unique AFLP patterns. Analysis of the molecular and phenotypic data revealed a broad clustering of the population into three groups. Many factors point to an A2 population restricted by its sensitivity to phenylamides. The majority of the A2 isolates were found in a single AFLP group, but the presence of mixed mating type samples, an increasing frequency of isolates of intermediate metalaxyl resistance and the extent of the AFLP diversity suggest occasional sexual recombination, and thus gene flow, between groups.     

DNA fingerprinting uncovers a new sexually reproducing population ofPhytophthora infestans in the Netherlands

The oomycetous fungusPhytophthora infestans (Mont.) de Bary, which causes late blight disease in potatoes, is heterothallic with two known mating types, A1 and A2. From 1845 until 1980 only A1 mating type isolates were found in Europe. In 1980, the A2 mating type appeared permitting sexual reproduction. Here we show that virulence properties and DNA fingerprint patterns of isolates collected in the Netherlands before and after the appearance of A2 mating type isolates are different. Before 1980, eight different races were found in which virulence factors 1, 2, 3, 4 and 10 were most common. After 1980, new virulence factors (i.e. 5, 6, 7, 8 and 11) showed up and the diversity for virulence increased tremendously: 73 different races were detected among 253 isolates analyzed. DNA fingerprint analyses of isolates collected before 1980 revealed that, for at least two decades, only one RG-57 fingerprint genotype was present in Europe. Among 179 isolates collected after 1980 134 distinct RG-57 fingerprint genotypes were identified. The dramatic increase in genetic diversity strongly suggests that theP. infestans population in the Netherlands is now propagating sexually. The change from asexual to sexual reproduction, and the resulting increased adaptability and ability to survive outside the host, may interfere drastically with the regular disease control methods.     

Characteristics of Phytophthora infestans Isolates from Uruguay

Isolates of Phytophthora infestans were obtained from late blighted plants from several potato-growing regions of Uruguay in 1998 and 1999. Of these, 25 representative isolates (4 from 1998, 21 from 1999) from the main potato-growing areas of the country, were characterised in terms of mating type, metalaxyl resistance, allozyme genotype, mitochondrial haplotype, RG57 fingerprint (1999 isolates only) and pathotype. All isolates proved to be A2 mating type, monomorphic and homozygous at the loci coding for glucose-6-phosphate isomerase and peptidase (Gpi 100/100, Pep 100/100) and to possess mitochondrial haplotype IIa. Metalaxyl-resistant isolates constituted 92% of the total. All the 1999 isolates possessed the same RG57 fingerprint, which was that previously reported as associated with the clonal lineage BR-1 from Brazil and Bolivia, which is also A2, Gpi 100/100, Pep 100/100. Most of the isolates displayed broad-spectrum virulence and five carried virulence to 10 of the 11 R genes tested despite the absence of R genes in commercially grown potato cultivars. It was concluded that the Uruguayan P. infestans isolates resembled isolates from neighbouring South American countries, notably Brazil, and belong to the new populations of the pathogen now predominant in many countries.     

Characterization of some Asian isolates of Phytophthora infestans

A total of 401 isolates of Phytophthora infestans were collected from eight Asian regions (Korea, India, Taiwan, Indonesia, Thailand, Nepal, China and Japan) between 1992 and 2000 – 318 from potato and 83 from tomato. The isolates were analysed for mating type, metalaxyl resistance, RG57 fingerprinting, mitochondrial DNA (mtDNA) haplotype and the polymorphism of three allozyme loci, i.e. glucose-6-phosphate isomerase ( Gpi ), peptidase ( Pep ) and malic enzyme ( Me ). The isolates were multilocus-genotyped based on RFLP (RG57) fingerprint, dilocus allozyme genotype, mtDNA haplotype and mating type. Twenty multilocus genotypes were identified among 125 isolates. Of these genotypes, 14 had not been previously reported. Some of the multilocus genotypes were common to isolates from several geographical regions, suggesting migration. The metalaxyl-resistant isolates belonged to the multilocus genotypes JP-1, JP-2, and JP-3. Multilocus genotypes coexisting in a single field were found in following regions: Thailand (1994), central China (1996), Nepal (1997) and Japan (1998 and 2000). The possible origins of certain genotypes are discussed, including the possibility of sexual recombination within the P. infestans populations in Nepal and perhaps Thailand.     

Diversity of Phytophthora infestans from Poland

A collection of 96 Polish isolates of Phytophthora infestans sampled in the years 2006, 2008 and 2009 were analysed using phenotypic and genotypic markers. Mating type, virulence, resistance to metalaxyl, mitochondrial haplotype and polymorphism at 12 simple sequence repeat (SSR) loci were determined. The majority of isolates were of the A1 mating type, mitochondrial haplotype Ia and sensitive to metalaxyl. Virulence factors against potato R genes R1, R3, R4, R7, R10 and R11 were present in most isolates. Genotyping using SSR markers revealed high genetic diversity within the Polish P. infestans population. Amongst the 96 isolates 66 unique genotypes were identified, 49 of which were observed only in single isolates. Eight isolates of the genotype 13_A2 lineage that has been reported in other parts of Europe were also found in Poland. The implications of these results are discussed.     

The role of aggressiveness and competition in the selection of Phytophthora infestans populations

Selection within populations of Phytophthora infestans was investigated by comparing the aggressiveness of single‐lesion isolates on detached leaflets of four potato cultivars with differing levels of race‐nonspecific resistance to P. infestans. The isolates included 23 representative of Northern Ireland genotypes from the early 2000s, used to inoculate previously reported field trials on competitive selection (2003–2005), plus 12 isolates recovered from the 2003 trial. The cultivars were those planted in the previous trials: Atlantic (blight‐susceptible) and Santé, Milagro and Stirling (partially resistant). Very highly significant variation for latent period, infection frequency and lesion area was found between genotypes and cultivars; differences between genotypes were more marked on the more resistant cultivars, but no one genotype was the most aggressive across all. Detached leaflets were also inoculated with mixtures of isolates from each genotype group at three sporangial concentrations: differences in aggressiveness between genotypes were more apparent at lower concentrations and on the more resistant cultivars. Genotype groups that were the most aggressive on the more resistant cultivars tended to be those selected by the same cultivars in the field. A mixture of all isolates of all genotypes was used to inoculate detached leaflets of the same cultivars. With one exception, single spore isolates recovered from any one leaflet belonged to a single genotype, but different genotypes were recovered from different cultivars. Phytophthora infestans isolates from Northern Ireland showed significant variation for foliar aggressiveness, and pathogen genotypes exhibited differential aggressiveness to partially resistant cultivars and interacted competitively in genotype selection.     

Displacement of US‐1 clonal lineage by a new lineage of Phytophthora infestans on potato in Kenya and Uganda

Phytophthora infestans, the pathogen responsible for causing late blight in potato and tomato, is a rapidly evolving and highly adaptable pathogen. Following the appearance of a new clonal genotype in Kenya (KE‐1 lineage) in 2007, there was a need to document possible displacement of the US‐1 genotype by the newer lineage. Samples from Kenya (260) collected on potato and from Uganda collected on potato (134) and tomato (32) were genotyped with simple sequence repeat (SSR) and mitochondrial DNA haplotype (mtDNA) markers. Results show that, four years after its discovery, the KE‐1 lineage has completely displaced the US‐1 lineage on potato in Kenya. The KE‐1 lineage has also migrated into Uganda through the eastern part of the country that borders western Kenya. All potato samples from eastern Uganda were of the KE‐1 lineage. Moreover, samples from eight fields in western Uganda were also of the KE‐1 lineage, which is in agreement with an ongoing westward displacement. However, the new lineage has not displaced the old lineage on tomato, as all samples from tomato were US‐1. Principal coordinate analysis (PCA) scatter plots revealed clustering that indicated separation between the lineages and also between the subpopulations of the US‐1 lineage on potato and tomato.     

Changes in specific virulence in Polish populations ofPhytophthora infestans: 1985–1991

Ninety-five isolates ofPhytophthora infestans collected throughout Poland during 1985–1991 and characterized for multilocus genotypes based on mating type, allozymes and DNA fingerprint, were analyzed for specific virulence to differential potato cultivars carrying ten major resistance genes. The multilocus analysis led to three groupings. The first group contained 22 isolates of a clonal lineage (PO-1) that is postulated to have been present in Europe during most of the twentieth century, but PO-1 isolates were recovered in Poland only during 1985–1988. This group contained, on average, virulence to 5.5 specific resistance genes per isolate. The second group consisted of 30 isolates in a clonal lineage (PO-4) that had not been detected before 1988. PO-4 isolates had virulence to a mean of 6.5 resistance genes per isolate. The third group was composed of 43 isolates representing 38 multilocus genotypes also not detected before 1988. These diverse genotypes had virulence to an average of 6.7 specific resistance genes per isolate. More than half (53%) of the PO-4 isolates shared a single pathotype. The group of 43 isolates was dominated by two pathotypes: the most common one (47% of the isolates) was the same pathotype that dominated PO-4 isolates; the next most common one (21%) differed from the most common one by the absence of virulence to resistance gene R5. The recent immigrant isolates (not detected before 1988) generally had virulence to a greater number of specific resistance genes than did isolates in the previous population [detected before 1988 (PO-1)]. Recent immigrant populations were dominated by one or two pathotypes, so their pathotypic diversity values were somewhat less than that of the previous population.     

Phytophthora infestans in a single field in southwest Sweden early in spring: symptoms, spatial distribution and genotypic variation

Sixty-eight isolates of Phytophthora infestans were collected from an early planted, fleece-protected potato field in southwest Sweden in May 2001. No infection was observed in the neighbouring fields. In the field investigated, infected plants were found in six discrete foci, with symptoms almost exclusively observed in the lower part of the canopy and numerous infections found on stems and on leaves touching the ground. The structure of the population was analysed based on mating type, mitochondrial haplotype and microsatellites (SSRs) as markers. Both mating types were present and haplotypes Ia and IIa were detected. Among 61 isolates analysed with microsatellite markers, 14 multilocus genotypes were distinguished based on six polymorphic loci. Out of the six foci, three included 3–5 genotypes each. There were unique genotypes in all foci, except two closely situated to each other. These findings strongly suggest that soilborne oosporic inoculum contributed significantly to initiate the late-blight epidemic in the investigated field.     

Modelling the spatiotemporal dynamics of Phytophthora infestans at a regional scale

Potato late blight caused the Irish Potato Famine and still causes billions of dollars of annual crop damage. Tracking and predicting its spread remains problematic. Growers overspray potato fields with fungicide as a precaution, irrespective of the prevalence or actual risk of the disease. This study created a new weather‐based mathematical model for the spread of late blight at a regional scale using empirical data, and validated it using a novel approach for presence‐only data. The model was tested using a contrapositive ‘proof’ by comparing predicted to actual weather patterns to examine its accuracy. The model was then used to create risk maps showing the likelihood of future outbreaks in the region. Such risk maps can help growers optimize late blight suppression and fungicide use by alerting them to the most probable day at which they are at the most risk, using real‐time weather data from the previous few days. These risk maps would be updated daily to account for conditions needed for sporulation. Overall, this work offers a methodology to understand and model a disease's spread in time and space.     

Production, survival and infectivity of oospores of Phytophthora infestans

The formation of oospores of Phytophthora infestans was studied in tomato and potato crops and volunteer plants under field conditions, and in laboratory tests with leaf discs of potato cultivars differing in their level of race-nonspecific resistance. Oospores were readily detected in blight-affected tomato leaflets and fruits, and in leaflets of field crops and volunteer potato plants. Oospores extracted from blighted potato leaflets yielded 13 oospore-derived progeny. Oospores were also produced following inoculation of leaf discs of eight potato cultivars expressing different levels of race-nonspecific resistance with a mixture of sporangia of A1 and A2 isolates. The highest numbers of oospores were produced in cvs Bintje (susceptible) and Pimpernel (resistant), and the lowest in Nicola (intermediate resistance). The relationship between lesions per leaflet and oospore incidence, affected by varying A1 : A2 ratios, was explored using a simple mathematical model, and validated by comparing actual oospore production in leaflets with multiple lesions of the race-nonspecific-resistant potato clone Lan 22-21 with the predictions generated by the model. Survival of oospores was investigated after their incorporation in either a sandy or a light clay soil in buried clay pots exposed to the local weather conditions. Over 6 years these soils were regularly assessed for their infection potential using floating leaflets in a spore-baiting bioassay. Sandy and clay soils contaminated with oospores remained infectious for 48 and 34 months, respectively, when flooded. Infections of floating potato leaflets occurred within 84–92 h and ceased after 11 days. Soil samples remained infective if dried and re-flooded on two, but not more, occasions.     

Development of a species-specific and sensitive detection assay for Phytophthora infestans and its application for monitoring of inoculum in tubers and soil

A specific and sensitive PCR assay for the detection of Phytophthora infestans , the cause of late blight of potato, in soil and plant tissues was developed. A P. infestans -specific primer pair (INF FW2 and INF REV) was designed by comparing the aligned sequences of rDNA internal transcribed spacer regions of most of the known Phytophthora species. PCR amplification of P. infestans DNA with primers INF FW2 and INF REV generated a 613 bp product, and species specificity was demonstrated against DNA from nine other Phytophthora species and seven potato-blemish pathogens. In a single-round PCR assay, 0·5 pg pure P. infestans DNA was detectable. Sensitivity was increased to 5 fg DNA in a nested PCR assay using Peronsporales-specific-primers in the first round. As few as two sporangia or four zoospores of P. infestans could be detected using the nested assay. Procedures are described for detection of P. infestans in leaves, stem and seed potato tubers before expression of symptoms. A soil assay in which 10 oospores per 0·5 g soil were detectable was developed and validated using samples of field soil. The PCR assay was used to examine the long-term survival of sexual (oospores) and asexual (sporangia and mycelium) inoculum of P. infestans in leaf material buried in a replicated experiment under natural field conditions. Oospores were consistently detected using the PCR assay up to 24 months (total length of the study) after burial in soil, whereas the sporangial inoculum was detected for only 12 months after burial. Sporangial inoculum was shown to be nonviable using a baiting assay, whereas leaf material containing oospores remained viable up to 24 months after burial.     

宁夏马铃薯晚疫病菌交配型和生理小种研究

为明确宁夏回族自治区马铃薯主产区晚疫病菌的交配型种类以及生理小种的类型、组成和分布,为宁夏有针对性地选育马铃薯抗晚疫病品种提供科学依据。利用A1、A2交配型标准菌株和含有11个主效抗性基因的鉴别寄主,对2018年-2019年从原州区、泾源县、彭阳县、隆德县、西吉县、盐池县、海原县采集得到的130个马铃薯晚疫病菌菌株进行交配型和生理小种鉴定。结果表明,宁夏马铃薯晚疫病菌群体的交配型和生理小种存在多样性,采集地交配型有A1、A2、SF(自育型)3类,分别占被测菌株的30%、63.8%、6.2%;生理小种有8种类型,其中生理小种1.2.3.4.5.6.7.8.9.10.11发生频率最高,占被测菌株总数的44.62%,是宁夏马铃薯主栽区的晚疫病菌优势小种,在各测试地均有分布;次优势小种为3.4.10和1.2.3.5.6.7.8.11,发生频率分别为12.31%和10.77%。宁夏不同种植区域马铃薯晚疫病菌交配型组成复杂,采集地组成差异较大,但西吉县、原州区交配型年度间的变化基本趋于一致。宁夏马铃薯晚疫病菌群体组成日趋复杂,生产中要合理布局已有抗病品种,挖掘培育抗病水平高的新品种,综合防控马铃薯...     

Production and Germination of Oospores of Phytophthora infestans (Mont.) de Bary in Morocco

One hundred and eight isolates of Phytophthora infestans were collected from infected potato and tomato crops in the middle-north of Morocco during 1997–2000. Pairings of these isolates with tester isolates of mating type A1 and A2 revealed that 60% of the isolates were mating type A2 (65/108) and 40% were mating type A1. After 10 days incubation at 20 °C and a 16-h photoperiod, approximately 25% and 18% of the oospores produced in-vitro germinated in potato soil extract and potato root extract, respectively. Oospores were observed in potato leaf tissues in pairings that were fertile in-vitro. Maximum production of oospores was obtained in potato leaves of cultivars that were moderately susceptible (Desirée, Nicola) after 10 days of incubation at 15 °C and a 16-h photoperiod. These results confirm the presence of P. infestans strains that are sexually compatible under Moroccan climatic conditions. Production of oospores constitutes a threat for these crops because of the occurrence of recombinants with new virulences which may be difficult to control and as a consequence survival of oospores in absence of the host plant in the soil.     

Novel microsatellite markers for the analysis of Phytophthora infestans populations

Co-dominant microsatellite molecular markers for Phytophthora infestans were developed and their potential for monitoring the genetic variation in populations was demonstrated in the UK, across Europe and worldwide. Markers were developed according to two strategies. First, several thousand P. infestans expressed sequence tag (EST) and bacterial artificial chromosome (BAC) sequences were screened for the presence of simple sequence repeat (SSR) motifs, and, of these, 100 candidate loci were selected for further investigation. Primer pairs developed to these loci were tested against a panel of 10 P. infestans isolates and approximately 10% were shown to be polymorphic and therefore appropriate for further testing. Secondly, the construction and screening of a partial genomic library resulted in the development of one additional polymorphic marker. The resulting 12 SSR markers were converted to higher-throughput fluorescence-based assays and used in combination with two previously published markers to characterize a wider collection of 90 P. infestans isolates from the UK and six other countries. Several isolates from the closely related species P. mirabilis , P. ipomoea and P. phaseoli collected from around the world were also genotyped using these markers. Amongst the 90 isolates of P. infestans examined, considerable SSR diversity was observed, with 68 different genotypes and an average of 3·9 (range 2–9) alleles per locus. When other Phytophthora species were genotyped, all loci were successfully amplified and the majority were polymorphic, indicating their transferability for the potential study of other closely related taxa.     

Genotypic and phenotypic characterization of Phytophthora infestans populations on Java,Indonesia

Phytophthora infestans is endemic to Indonesia and can infect potato crops at any stage in the growing season. Little is known about P. infestans populations in Indonesia. The objectives of this study were first to identify the genotypes causing late blight in the main potato-growing regions on Java in Indonesia, and secondly to examine genotypic diversity in the P. infestans populations in those regions. Samples were collected on FTA cards (n = 140) or in tubers (n = 6) from 15 locations in nine regencies over four years (2016–2019). Microsatellite analysis revealed that late blight outbreaks in these regencies were caused by EU_2_A1 and other genotypes that are unique to Indonesia. Eighty percent of the samples that amplified with CAPS markers were the A1 mating type. Cultures of six isolates were determined to be the A1 mating type based on the pairing test, and of these, two isolates were intermediate and four were sensitive to metalaxyl-M (mefenoxam). The mode of reproduction of the P. infestans population on Java, Indonesia, was found to be clonal. However, as the sample size in this study was small, more isolates need to be tested to confirm this. Microsatellite analysis revealed that 90% of Indonesian samples had trisomic loci. A high number of multilocus genotypes (MLGs) were found in all nine regencies (131 MLGs out of 146 samples). Results indicate that there is ongoing polyploidization in these populations due to a high mutation rate and no selection pressure from the susceptible potato hosts that are being grown in Indonesia.     

Influence of competition and host plant resistance on selection in Phytophthora infestans populations in Michigan, USA and in Northern Ireland

Competition between genotypes of Phytophthora infestans was studied by inoculating potato cultivars with differing susceptibility to late blight in field experiments over three years in Northern Ireland, UK, and Michigan, USA. Multiple isolates of six genotype groups of P. infestans were chosen from the local populations in both N. Ireland and Michigan for inoculation of separate field trials planted in 2003, 2004 and 2005. Four cultivars were used in each trial; two (susceptible cv. Atlantic and the partially resistant cv. Stirling) were common to both locations, whereas the two additional cultivars (with partial resistance to late blight) were cvs Santé and Milagro in N. Ireland and cvs Pike and Jacqueline Lee in Michigan. Single-lesion isolates of P. infestans were obtained from leaves at 1% level of infection, characterized using pre-assigned markers and re-assigned to their respective genotype groups. Extreme selection occurred within the population of genotypes of P. infestans in N. Ireland in each year, with different genotype groups dominating the infection of different cultivars. Selection was observed on all cultivars tested, but was greatest on the more resistant cultivars. Over the 3 years, all of the 114 isolates obtained from cv. Milagro belonged to a single group, whereas among the 118 isolates from cv. Atlantic all six groups were represented. By contrast, in Michigan, the US-8 genotype dominated infection in all cultivars in each year; only 12 of 374 isolates characterized belonged to other genotypes (11 US-14 and a single US-10 isolate).     

Stability and variability of virulence of Phytophthora infestans assessed in a ring test across European laboratories

Determining virulence towards race‐specific resistance genes is a prerequisite to understanding the response of pathogen populations to resistant cultivars, and therefore to assess the durability of these resistance genes and the performance of resistance management strategies. In Phytophthora infestans, virulence testing began shortly after the introduction of R‐genes from Solanum demissum into S. tuberosum cultivars. However, the characteristics of R‐gene expression, the sensitivity of the phenotype to environmental and physiological parameters, and the diversity of experimental protocols make the comparison of data from different studies problematic. This prompted European teams working on P. infestans diversity to: (i) design a joint protocol, using detached leaflets from greenhouse‐grown plants of a shared set of differential cultivars inoculated with standardized suspensions of inoculum, and (ii) assess the performance of this protocol in a blind ring test involving 12 laboratories and 10 European isolates of the pathogen. A high level of consensus in the determination of virulence/avirulence to R1, R3, R4, R7, R8, R10 and R11 was achieved among the collaborators, showing that the protocol could be robustly applied across a range of laboratories. However, virulence to R2, R5 or R9 was detected more frequently in some laboratories, essentially from northern Europe; these genes are known to be highly sensitive to host and environmental conditions. The consensus determination was often markedly different from the original virulence phenotype of the isolates, suggesting virulence instability in stored P. infestans isolates. This indicates that creating reliable core collections of pathogen isolates with known virulences could be difficult.     

Aggressiveness and host specificity of Brazilian isolates of Phytophthora infestans

The population of Phytophthora infestans in Brazil consists of two clonal lineages, US-1 associated with tomatoes and BR-1 associated with potatoes. To assess whether host specificity in these lineages resulted from differences in aggressiveness to potato and tomato, six aggressiveness-related epidemiological components – infection frequency (IF), incubation period (IP), latent period (LP), lesion area (LA), lesion expansion rate (LER) and sporulation at several lesion ages (SSLA) – were measured on detached leaflets of late blight-susceptible potato and tomato plants. Infection frequency of US-1 was similar on potato and tomato leaflets, but IF of BR-1 was somewhat reduced on tomato. Incubation period was longer on both hosts with US-1, although this apparent lineage affect was not significant. Overall there was no host effect on IP. On potato, BR-1 had a shorter LP (110·3 h) and a larger LA (6·5 cm²) than US-1 (LP = 162·0 h; LA = 2·8 cm²). The highest LER resulted when isolates of BR-1 (0·121 cm² h⁻¹) and US-1 (0·053 cm² h⁻¹) were inoculated on potato and tomato leaflets, respectively. The highest values of the area under the sporulation capacity curve (AUSC) were obtained for isolates of US-1 inoculated on tomato leaflets (6146) and for isolates of BR-1 on potato leaflets (3775). In general, higher values of LA, LER, SSLA and AUSC, and shorter values of LP were measured when isolates of a clonal lineage were inoculated on their original host than with the opposite combinations. There is evidence that there are quantitative differences in aggressiveness components between isolates of US-1 and BR-1 clonal lineages that probably contribute to host specificity of P. infestans populations in Brazil.     

Tracking Phytophthora infestans with SSR markers within and between seasons – a field study in Sweden

The dynamics of a late blight epidemic and sexual reproduction in Phytophthora infestans were studied in an experimental field in mid‐Sweden. The field was inoculated with six isolates of P. infestans taken from another potato field where sexual reproduction of the pathogen was suspected. Three weeks after inoculation single‐lesion leaflets were sampled and the resulting isolates characterized using microsatellites (SSRs) and mating type as markers. Among the 151 isolates analysed, the inoculum genotypes constituted more than 80% of the genotypes found, with three other genotypes making up the remainder. The following year, P. infestans obtained from soil samples taken from this field were analysed, and six novel genotypes were identified. Genotypes from the previous summer’s population were not detected. Analysis of the genotypes recovered was consistent with them being recombinants, with the previous summer’s population acting as parents. These findings are consistent with the hypothesis that oospores produced during a summer epidemic in Sweden can overwinter and cause infection the next year.