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.     

Genotypic and phenotypic characterization of Phytophthora infestans in South Korea during 2009–2016 reveals clonal reproduction and absence of EU_13_A2 genotype

In order to better understand the Phytophthora infestans population structure in South Korea, 172 isolates were collected between 2009 and 2016 from four major potato cultivation areas. Fungicide (metalaxyl and dimethomorph) response, mating type, and microsatellite (SSR) genetic fingerprints were analysed to characterize these isolates. Ten isolates collected in Gyeongnam Province, which specializes in protected winter cultivation in polytunnels, were A2 mating type. All other isolates were A1 mating type. Overall, 42% of the isolates were resistant to metalaxyl, and 43% were sensitive. All isolates were sensitive to dimethomorph. From the SSR fingerprints, 45 distinct genotypes were identified, which could be clustered into four clonal lineages: KR_1_A1, KR_2_A2, SIB-1, and US-11. KR_1_A1 was the predominant P. infestans genotype in South Korea. KR_2_A2 was only found in Gyeongnam Province; all isolates were A2 mating type and resistant to metalaxyl. SIB-1 was dominant until 2013 but its frequency has gradually decreased in more recent years. US-11 was first found in 2014, after which its frequency has increased to become codominant with KR_1_A1. The calculated standardized index of association (I_A) suggests that the South Korean P. infestans population is undergoing clonal reproduction. When compared with populations of P. infestans from the Netherlands, it has less genetic diversity and the dominant Netherlands P. infestans genotype, EU_13_A2 (Blue_13), was not found in South Korea. Such monitoring of the pathogen population contributes to a more efficient integrated pest management-based control strategy for potato late blight control in South Korea.     

Outcome of sexual reproduction in the <Emphasis Type="Italic">Phytophthora infestans</Emphasis> population in Estonian potato fields

In this study, the Estonian population of Phytophthora infestans was characterized with mating type, sensitivity to metalaxyl, virulence on 11 potato R-gene differentials and 12 SSR markers to show the outcome of potential sexual reproduction in the population. During the three years 2010–2012, 141 P. infestans isolates, collected from 23 potato fields, showed quite a high and stable frequency of the A2 mating type, 48% of the total population. In 87% of all sampled potato fields, both mating types were recorded, suggesting continuous sexual reproduction of P. infestans and possible oospore production. Metalaxyl-sensitive isolates prevailed in all three years (68 out of 99 isolates). Amongst the 95 isolates tested, 51 virulence races were found. The race structure was diverse, and most pathotypes were unique, appearing only once; the two most common pathotypes, 1.2.3.4.6.7.10.11 and 1.2.3.4.7.10.11, comprised 35% of the population. The P. infestans population was genetically highly diverse and most of the multilocus genotypes (MLGs) appeared only once. Furthermore, all of the MLGs appeared in only one of the three sampling years. Our results confirm that the high diversity in the Estonian P. infestans population is most likely the result of frequent sexual reproduction, which benefits the survival, adaptability and diversity of the pathogen in the climate of North-Eastern Europe.     

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.     

Dominance of a single clonal lineage in the Phytophthora infestans population from northern Shaanxi,China revealed by genetic and phenotypic diversity analysis

Late blight caused by Phytophthora infestans is the most serious disease of potato worldwide. To understand the P. infestans population structure in northern Shaanxi, an emerging potato production region in China, 125 single‐lesion isolates were randomly collected from farmers' fields in 2009 and characterized phenotypically and genotypically. A mating type assay showed that 94 isolates were A1 mating type. Virulence determination of selected isolates on a set of differential potato lines containing R1 to R11, respectively, showed the presence of two pathotypes, of which the pathotype lacking avirulence genes Avr3, Avr4 and Avr10 was dominant. Isolates lacking all avirulence factors Avr1 to Avr11 were detected but at lower frequency (13·6%). Analysis for mtDNA haplotype showed all 61 examined isolates were IIa. A total of seven multilocus genotypes were distinguished among 125 isolates, as determined with seven polymorphic microsatellite markers. The genotype SG‐1 was dominant in the population with a frequency of 75·2% and was present throughout the region. Analysis of the phenotypic and genotypic structures of P. infestans populations indicated strict clonal reproduction of the pathogen and suggested that sexual reproduction probably does not occur. Potential implications for disease management are discussed.     

Replacement of the European wheat yellow rust population by new races from the centre of diversity in the near‐Himalayan region

Isolates of recently spreading races of yellow rust from wheat and triticale in Europe were analysed using virulence phenotypic data of 2605 isolates sampled in 12 countries between 2000 and 2014. A subset of 239 isolates was investigated by microsatellite markers. At least three races of non‐European origin, termed ‘Warrior’, ‘Kranich’ and ‘Triticale aggressive’, were identified in the post‐2011 population. The Warrior race was already present in high frequencies in the first year of detection in most European countries and to a large extent it replaced the pre‐2011 European population. In contrast, the two other exotic races were localized to certain regions and/or crop type. The presence already of at least six multilocus genotypes of the Warrior race and five genotypes of the Kranich race in the first year of detection and across large areas is consistent with a hypothesis of aerial spread from genetically diverse source populations. A comparison with reference isolates sampled from six continents suggested that the Warrior and Kranich races originated from sexually recombining populations in the centre of diversity of the yellow rust fungus in the near‐Himalayan region of Asia. However, the Triticale aggressive race was most similar to populations in the Middle East/Central Asia. The study illustrated the potential role of sexual Puccinia striiformis populations as a reservoir for new races replacing distant clonal populations.     

Population genetic analysis of Phytophthora infestans in northwestern China

Late blight caused by Phytophthora infestans is the most devastating disease of potato worldwide. To understand the P. infestans population structure and dynamics in northwestern China, 959 single‐lesion isolates were purified in three consecutive years (2009–2011) and were characterized for mating type, pathotype, mtDNA haplotype and molecular variation at eight SSR loci. The results showed that the distribution of mating types changed significantly over years, with self‐fertile isolates dominant in 2010 and 2011. SSR genotyping distinguished 959 isolates into 151 genotypes, and association analysis indicated that P. infestans populations in 2010 and 2011 were strictly asexual while in 2009 they showed signs of sexual reproduction. Population analysis showed that the majority of genetic variation was within P. infestans populations. Isolates sharing identical SSR genotypes were detected in distant regions, indicating that migration of P. infestans could have occurred between regions. Pathogenicity assays on a set of potato differential lines containing R1 to R11 resistance genes detected four pathotypes from 74 selected isolates, with the pathotype virulent against all 11 R genes being dominant. Three mtDNA haplotypes (Ia, IIa, IIb) were detected with Ia being dominant among 507 isolates examined. Phylogenetic analysis indicated that P. infestans populations in northwestern China are distant from European lineages including 13‐A2 (blue‐13) at the time of this survey. The results have implications for the trade of healthy seed tubers as a means of managing late blight.     

What is the evidence for sexual reproduction of Phytophthora infestans in Europe?

The biology of late blight of potato and tomato, caused by Phytophthora infestans, changed when sexual reproduction by the pathogen became possible in many parts of the world, including Europe. In northern Europe, especially Scandinavia, there is increasing evidence that the pathogen is reproducing sexually on a regular basis, although in other regions further south or to the west it appears to reproduce primarily in a clonal manner. The presence of both mating types, the production of viable oospores, and observations of fields with soilborne sources of inoculum are consistent with sexual reproduction. Studies with different marker systems have revealed a population structure without any dominating clonal lineages in Scandinavia, and that is most easily explained by sexual reproduction. Phytophthora infestans recovered from the soil can also be linked to parental genotypes using likelihood‐based methods when codominant markers are used. A synthesis of all the available data points to a second centre of sexual reproduction in northern Europe.     

Large subclonal variation in Phytophthora infestans populations associated with Ecuadorian potato landraces

The population of Phytophthora infestans on potato landraces in three provinces (Carchi, Chimborazo and Loja) of Ecuador was analysed. All isolates (n = 66) were of the A1 mating type. Simple sequence repeats (SSR) were used to assess the genetic diversity of the isolates. The P. infestans isolates from the potato landraces grouped in a single clade together with reference isolates belonging to the clonal lineage EC‐1. In the 66 SSR profiles obtained, 31 multilocus genotypes were identified. The 66 isolates constituted 49 different races according to the Solanum demissum differential set ( R1 to R11). The P. infestans population was complex and virulent on 4 to 11 R genes. Analysis showed that the subclonal variation in the Ecuadorian EC‐1 clone is increasing over time and is much larger than clonal variation in lineages in the Netherlands and Nicaragua, suggesting high mutation rates and little or no selection in Ecuador.     

Population studies on <Emphasis Type="Italic">Phytophthora infestans</Emphasis> on potatoes and tomatoes in southern Germany

Fifty-seven isolates of Phytophthora infestans from blighted potato foliage were collected in 1995 in southern Germany and analysed for mating type and sensitivity to metalaxyl. Fifty-six of them were characterised as A1 and one as A2 mating types. Resistance to metalaxyl was observed frequently: 53 isolates were resistant, three were partially sensitive, and one was sensitive. In a subsequent field study in 1999, 84 isolates collected from blighted potato and tomato foliage were analysed for mating type. Seventy-two were characterised as A1 and twelve as A2 mating types. The response of 76 isolates to metalaxyl and to propamocarb was tested. The majority (42) of the 76 isolates was classified as resistant to metalaxyl; 31 were partially sensitive and only three isolates were sensitive. The results with propamocarb were less discrete; 10 isolates were classified as resistant and three were clearly sensitive. AFLP fingerprinting was used to examine the genetic structure of the southern German P. infestans population collected in 1999 and indicated that the tested population can be sub-divided into a tomato group, a potato group and a mixed group containing isolates collected from both crops. The presence of Ia and IIa mitochondrial DNA haplotypes indicates that the German P. infestans isolates belong to the new pathogen population that has also been reported in neighbouring regions of Europe. The present study indicates that at the beginning of the season only a few genotypes were present, and the population became genetically more variable at the end of the growing season.     

The occurrence of the A2 mating type of Phytophthora infestans in the Netherlands; significance and consequences

Phytophthora infestans (Mont.) de Bary, the causal agent of potato late blight, was first discovered in Europe in 1845. Until 1980, only A1 mating type isolates were known to occur in Europe. The absence of A2 mating type isolates restrained the fungus from sexual reproduction. In the early 1980s, A2 mating type isolates were discovered in Europe. Presumably, a new introduction ofP. infestans isolates originating from Mexico had taken place. In this paper, the significance of the presence of A1 and A2 mating type isolates in the Netherlands is reviewed. Now that both mating types are present, sexual reproduction can occur and its consequences for the control of potato late blight are discussed.     

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.     

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.     

Evidence for selection pressure from resistant potato genotypes but not from fungicide application within a clonal Phytophthora infestans population

Insight into pathogen population dynamics provides a key input for effective disease management of the potato late blight pathogen Phytophthora infestans. Phytophthora infestans populations vary from genetically complex to more simple with a few clonal lineages. The presence or absence of certain strains of P. infestans may impact the efficacy of fungicides or host resistance. Current evidence indicates that genetically, the Irish populations of P. infestans are relatively simple with a few clonal lineages. In this study, P. infestans populations were genetically characterized based on samples collected at the national centre for potato breeding during the period 2012–16. The dominance of clonal lineages within this P. infestans population was confirmed and the potential selection pressure of fungicide treatment (2013–15) and host resistance (2016) on this clonal P. infestans population was then investigated. It was found that fungicide products did not notably affect the genetic structure of sampled populations relative to samples from untreated control plants. In contrast, samples taken from several resistant potato genotypes were found to be more often of the EU_13_A2 lineage than those taken from control King Edward plants or potato genotypes with low resistance ratings. Resistant potato varieties Sarpo Mira and Bionica, containing characterized R genes, were found to strongly select for EU_13_A2 strains.     

不同栽培模式下葡萄霜霉病菌遗传结构的时间动态分析

为明确不同栽培模式下葡萄霜霉病菌Plasmopara viticola的遗传结构、遗传多样性及遗传分化水平,于2014-2015年定期采集露地和避雨2种栽培模式下的葡萄霜霉病菌菌株,利用6对SSR引物对该病菌基因型、遗传多样性及遗传分化进行对比分析。结果表明,露地和避雨栽培模式下葡萄霜霉病菌群体的Nei’s基因多样性指数大于0.14,香农多样性指数大于0.31,2种栽培模式下群体具有丰富的遗传多样性,但避雨栽培模式可显著降低群体等位基因数和等位基因频率。露地栽培模式下该病菌群体的流行模式呈现中等水平无性繁殖,2年初侵染和再侵染对病害流行的贡献率分别约占26.1%和73.9%;避雨栽培模式下葡萄霜霉病菌群体的流行模式则呈现高等水平无性繁殖,初侵染和再侵染对病害流行的贡献率分别约占4.3%和95.7%。卵孢子的形成对于葡萄霜霉病菌种群遗传变异和有效越冬起着关键的作用。2014-2015年露地栽培模式下葡萄霜霉病菌群体的主效流行基因型对病害流行的贡献率分别为44.5%和51.8%;而其在避雨栽培模式下葡萄霜霉病菌群体的贡献率分别可达84.2%和87.1%。同一年份的露地和避雨栽培模式下葡萄霜霉病菌群体的主效基因型种类相同,2个群体间的等位基因频率呈现显著正相关性,且二者之间存在频繁的基因交流,推测避雨栽培模式下葡萄霜霉病的初侵染源自于避雨设施附近的露地栽培病株上再侵染形成的飞散传播孢子囊。     

Phenotypic and genotypic variation in Michigan populations of Phytophthora infestans from 2008 to 2010

Severe potato and tomato late blight epidemics in Michigan since 2008 prompted characterization of Phytophthora infestans isolates from the region. From eight counties in Michigan, 124 isolates were collected from infected potato and tomato plants from 2008 to 2010 and characterized using ‘classical’ markers and microsatellites. The classical markers included mating type, Gpi allozyme, mitochondrial DNA haplotype, sensitivity to metalaxyl‐M and tuber pathogenicity. All isolates from 2008 to 2010 were A2 mating type and Ia mtDNA haplotype (124 isolates), 105 isolates had Gpi profile 100/122, 17 isolates had the profile 100/100/111 and the remaining two isolates had 100/111/122. Sensitivity to metalaxyl‐M, expressed as EC₅₀ for mycelial growth in vitro, ranged from <0·1 to 91 μg mL^?1, where 95 and 96% of isolates were classified as either sensitive or intermediate in 2008–2009 and 2010 respectively. The metalaxyl‐M sensitivity and dominant Gpi profile were typical of clonal lineage US‐22, first isolated in 2008 in North America from tomato plants. Tuber pathogenicity, characterized as severity of tuber late blight, was also variable among isolates; however, isolates were less aggressive than previous genotypes present in Michigan, such as US‐8. Microsatellites (simple sequence repeats; SSRs) revealed a shift in the population, characterized by two clusters differentiated over time. These results suggested displacement of the US‐8 genotype by US‐22 from 2008 to 2010 in Michigan. Continuous tracking of changes within P. infestans populations provides evidence of genetic shifts due to migration, prompting modification of management strategies based on the phenotypic characteristics of causal genotypes.     

SSR assessment of Phytophthora infestans populations on tomato and potato in British gardens demonstrates high diversity but no evidence for host specialization

Phytophthora infestans populations can differ in composition as a result of host specialization on tomato and potato hosts. In Great Britain many amateur gardeners grow outdoor tomatoes but there is little or no commercial tomato production outdoors. This study analysed isolates of P. infestans from British gardens with 12 multiplexed simple sequence repeat markers that are used to monitor the disease on commercial potato crops. Samples of P. infestans from tomato hosts were collected in 3 years and from potato in 1 year from across Great Britain. Seven previously unreported clonal lineages were detected in garden populations and higher frequencies of unique clonal lineages (28–40%) were present compared with populations from British commercial potato crops reported elsewhere. Garden populations had a lower proportion (11–48% less) of the most common lineages (13_A2 and 6_A1) that together made up at least 86% of the commercial potato populations during the sampling period. Host species accounted for only 2·0% of molecular variance detected between garden potato‐ and tomato‐hosted samples. No significant difference in clonal lineage composition was found between host species in Great Britain and this could be due to the whole P. infestans population overwintering on potato. British garden populations on both hosts were much more diverse than those on commercial potato crops; this finding may be influenced by less frequent fungicide use by gardeners and a higher diversity of unsprayed susceptible potato cultivars, enabling metalaxyl‐sensitive and less aggressive genotypes to survive in gardens.     

Population structure of Phytophthora infestans in China – geographic clusters and presence of the EU genotype Blue_13

The population structure of Phytophthora infestans in China was studied and three mitochondrial haplotypes (Ia, IIa, IIb) were observed. Genetic analysis with 10 highly informative SSR markers identified 68 different genotypes, including three dominant clonal lineages. In the Chinese P. infestans population, the genotypes were strongly clustered according to their geographic origin. One of dominant clonal lineages was genetically similar to Blue_13, a dominant clonal lineage found in Europe since 2004. This is the first report of Blue_13 outside Europe. Only one mating type (A1) was found in the northern and southeastern provinces, but in southern and northwestern China both mating types were observed. The mating type ratio and SSR allele frequencies indicate that in China the sexual cycle of P. infestans is rare. These results emphasize that the migration of asexual propagules and the generation of subclonal variation are the dominant driving factors of the population structure of P. infestans in China. They may also have implications for the role of monitoring P. infestans populations in potato late blight management strategies in China.     

Distribution of major clonal lineages EU_13_A2, EU_2_A1, and EU_23_A1 of Phytophthora infestans associated with potato late blight across crop seasons and regions in Algeria

Potato is one of the most important agricultural crops in Algeria and worldwide. Each year, potato late blight, caused by Phytophthora infestans, is responsible for significant damage that leads to large production losses, and is thus a direct threat to food security in Algeria. In this study, 131 isolates of P. infestans and 92 DNA fingerprints captured on FTA cards were sampled from commercial and seed production fields in three major potato production regions (western, eastern, and central) during the main-season and late-season in Algeria over six cropping seasons (2010–2016). Genotypes of P. infestans and population genetic diversity were analysed using a 17-plex simple-sequence repeat (SSR) marker assay, and the mating type of all isolates was characterized. Both mating types (A1 and A2) were found, and often occurred in the same field. Differences in mating type proportion were observed between regions and between sampling periods. Analysis with SSR markers showed the prevalence of the EU_13_A2 lineage (70%) over EU_2_A1 (16%), EU_23_A1 (10%), and 4% of unknown multilocus lineage (MLL). The EU_13_A2 showed differentiation within the group. EU_23_A1 was found mainly in late-season crops. However, the cropping region did not influence the distribution of lineages due to the dispersal of the pathogen in Algeria by seeds. Genetic structure did not reveal a clear variation in distribution of the three lineages throughout the sampling regions. These data provide important new information on the composition and change over time of P. infestans populations in Algeria and open the way for a better understanding of the local epidemiology of this important pathogen.     

Characterisation of <Emphasis Type="Italic">Phytophthora infestans</Emphasis> isolates collected from potato in Estonia during 2002–2003

A collection of 101 isolates of Phytophthora infestans, obtained from seven sampling sites representing central, east and south-east Estonia during 2002 and 2003 were assessed for several phenotypic and genotypic markers. All 101 isolates were assessed for virulence and resistance to metalaxyl. Virulence to each of the 11 classic resistance genes was found among the tested isolates. The mean number of virulences per isolate was 6.3, with a very low frequency of virulence against resistance genes R5 (5%) and R9 (14%). The most common pathotypes were 1.3.4.7.8.10.11 and 1.3.4.7.10.11, representing altogether 12% of the studied strains. In terms of metalaxyl resistance, 30 resistant, 52 intermediate and 19 sensitive isolates were found. A subgroup of 50 isolates was assessed for mating type, allozymes [glucose-6-phosphate isomerase (Gpi) and peptidase (Pep)], DNA fingerprints with probe RG57 and mtDNA haplotype. Of this subset, 30 were A1 and 20 were A2. Collections from three of the seven fields contained both mating types. Allozyme analysis did not reveal any polymorphism. However, 19 diverse RG57 fingerprints were detected, and two mitochondrial DNA haplotypes, Ia and IIa, were detected. By combining the mating type, mtDNA haplotype and RG57 fingerprint data, 26 multilocus genotypes were identified, of which 18 were detected only once. Genotypic diversity measured by the normalised Shannon diversity index was high (0.76). The large number of multilocus genotypes and the presence of both mating types in some fields indicate that sexual reproduction may take place in Estonian populations of P. infestans.