Competitive Fitness of Phytophthora infestans Isolates Under Semiarid Field Conditions

Spread of US-1 and US-8 isolates of Phytophthora infestans were observed in field plots of potato (cv. Russet Burbank) grown in Pullman, WA, in 1996 and 1997. Infected greenhouse-grown potato plants with similar lesion numbers for both strains were transplanted to field plots with four replications. Spread of the pathogen was favored by sprinkler irrigation during evening hours. Diseased leaves and stems were sampled over time to determine the spread of US-1 and US-8 isolates. In 1996, late blight developed in two of the four replications (105 and 87 total isolates recovered). From those two replications, two US-1 isolates were recovered, both from the same replication. Nine isolates from one replication and six isolates from another displayed a phenotype different from the initial isolates, as determined by compatibility type, allozyme genotype, and restriction fragment length polymorphism genotype. These putative recombinant isolates may have arisen from sexual recombination between the US-1 and US-8 isolates. The remaining isolates were of the US-8 strain. In 1997, late blight developed in all four replications (123, 122, 81, and 34 total isolates recovered). One US-1 isolate was recovered (out of 123) from one replication and three (out of 122) from another, and the remaining isolates were of the US-8 strain. Isolates with phenotypes differing from the initial isolates were not recovered in 1997. In both years, oospores were not observed in the plant tissue examined. The low number of putative recombinant isolates in 1996 and their absence in 1997 suggests that sexual reproduction between US-8 and US-1 isolates in a field setting is a rare event. The predominance of US-8 isolates recovered is a measure of the increased fitness and aggressiveness of the US-8 isolates relative to the US-1 isolate used in this study. This further substantiates the increased aggressiveness of the US-8 genotype observed on excised tissues and potted plants in previous laboratory and greenhouse studies.

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Global historic pandemics caused by the FAM-1 genotype of Phytophthora infestans on six continents

Scientific Reports ◽

10.1038/s41598-021-90937-6 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Amanda C. Saville ◽

Jean B. Ristaino

Keyword(s):

Late Blight ◽

Phytophthora Infestans ◽

Herbarium Specimens ◽

Microsatellite Genotyping ◽

Irish Famine ◽

Sexual Recombination ◽

The Us ◽

Almost All ◽

British Colonies

AbstractThe FAM-1 genotype of Phytophthora infestans caused late blight in the 1840s in the US and Europe and was responsible for the Irish famine. We sampled 140 herbarium specimens collected between 1845 and 1991 from six continents and used 12-plex microsatellite genotyping (SSR) to identify FAM-1 and the mtDNA lineage (Herb-1/Ia) present in historic samples. FAM-1 was detected in approximately 73% of the historic specimens and was found on six continents. The US-1 genotype was found later than FAM-1 on all continents except Australia/Oceania and in only 27% of the samples. FAM-1 was the first genotype detected in almost all the former British colonies from which samples were available. The data from historic outbreak samples suggest the FAM-1 genotype was widespread, diverse, and spread to Asia and Africa from European sources. The famine lineage spread to six continents over 144 years, remained widespread and likely spread during global colonization from Europe. In contrast, modern lineages of P. infestans are rapidly displaced and sexual recombination occurs in some regions.

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Classifying the Relative Host Reaction in Potato Cultivars and Breeding Lines to the US-8 Strain of Phytophthora infestans in Minnesota

Plant Disease ◽

10.1094/pdis.2003.87.8.983 ◽

2003 ◽

Vol 87 (8) ◽

pp. 983-990 ◽

Cited By ~ 8

Author(s):

J. C. Jenkins ◽

R. K. Jones

Keyword(s):

Late Blight ◽

Phytophthora Infestans ◽

Rank Correlation ◽

The United States ◽

Spearman Rank Correlation ◽

Breeding Lines ◽

Spearman Rank ◽

Progress Curve ◽

Tuber Blight ◽

The Us

A total of 32 commercial cultivars grown in the United States and 15 potato breeding lines and non-U.S. cultivars were evaluated at Rosemount, MN for their reaction to the US-8 strain of Phytophthora infestans. Commercial red-, russet-, and white-skinned cultivars tested in the commercial cultivar trial (COMC) in 1996 and 1997 were susceptible (S) to moderately susceptible (MS) to this organism, except for Elba, which ranked as moderately resistant (MR). Yellow-fleshed cvs. Hertha, Santé, and Agria were screened in the late blight nursery (LB1) in 1997 and 1998 and classified as S to MS while Island Sunshine, Brador, and Aziza were classified as MR. The Scottish breeding line G6582-3 and U.S. breeding lines A90586-11, AWN86514-2, AWN85624-5, B0692-4, B0718-3, and B0767-2 were classified as resistant (R). Comparison among entries was based on the area under the disease progress curve (AUDPC). Spearman rank correlation for AUDPC in the 1996 and 1997 COMC trials at 14 to 18 days after inoculation (r = 0.65, P < 0.01) was greater than any other interval tested. The Spearman rank correlation for AUDPC in the 1997 and 1998 LB1 trials at 14 to 18 days after inoculation was r = 0.87, P < 0.01 and increased only slightly in successive assessment dates, suggesting that, in Minnesota, effective evaluation of the foliar infection of late blight can occur at 18 days after inoculation or later. The average tuber blight incidence for the COMC trials at harvest was 10.0% in 1996 and 9.7% in 1997. The average tuber blight incidence for the LB1 trials at harvest was 4.3% in 1997 and 14.6% in 1998. Pearson correlations between tuber blight incidence and foliar disease for the COMC trials was very low; however, for the LBl trials, it was significant in both 1997 (r = 0.53, P < 0.01) and 1998 (r = 0.53, P < 0.01). Asymptomatic tubers harvested from the COMC trials developed additional tuber blight when stored 28 days at ambient temperatures and still more when stored for another 5 months at 5°C. Surviving tubers of nine entries were planted in field trials during 1997 and 1998 to determine if plants that develop from tubers exposed to P. infestans could manifest late blight in the subsequent season. Late blight failed to develop throughout the trials in either year.

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Did a novel virus contribute to late blight epidemics?

10.1101/383653 ◽

2018 ◽

Cited By ~ 2

Author(s):

Guohong Cai ◽

Kevin Myers ◽

William E. Fry ◽

Bradley I. Hillman

Keyword(s):

North America ◽

Late Blight ◽

Phytophthora Infestans ◽

Molecular Mechanisms ◽

Rna Virus ◽

Causal Agent ◽

Clonal Lineage ◽

Significant Similarity ◽

Tomato Production ◽

The Us

AbstractPhytophthora infestansis the causal agent of potato and tomato late blight. In this study, we characterized a novel RNA virus, Phytophthora infestans RNA virus 2 (PiRV-2). The PiRV-2 genome is 11,170 nt and lacks a polyA tail. It contains a single large open reading frame (ORF) with short 5’- and 3’-untranslated regions. The ORF is predicted to encode a polyprotein of 3710 aa (calculated molecular weight 410.94 kDa). This virus lacks significant similarity to any other known viruses, even in the conserved RNA-dependent RNA polymerase region. Comparing isogenic strains with or without the virus demonstrated that the virus stimulated sporangia production inP. infestansand appeared to enhance its virulence. Transcriptome analysis revealed that it achieved sporulation stimulation likely through down-regulation of ammonium and amino acid intake inP. infestans. This virus was faithfully transmitted through asexual reproduction. Survey of PiRV-2 presence in aP. infestanscollection found it in most strains in the US-8 lineage, a very successful clonal lineage ofP. infestansin North America. We suggest that PiRV-2 may have contributed to its success, raising the intriguing possibility that a potentially hypervirulent virus may contribute to late blight epidemics.Author SummaryPotato late blight, the notorious plant disease behind the Irish Potato Famine, continues to pose a serious threat to potato and tomato production worldwide. While most studies on late blight epidemics focuses on pathogen virulence, host resistance, environmental factors and fungicide resistance, we present evidence in this study that a virus infecting the causal agent,Phytophthora infestans, may have played a role. We characterized a novel RNA virus, Phytophthora infestans RNA virus 2 (PiRV-2) and examined its effects on its host. By comparing identicalP. infestansstrains except with or without the virus, we found that PiRV-2 stimulated sporulation ofP. infestans(a critical factor in late blight epidemics) and increased its virulence. We also profiled gene expression in these strains and identified potential molecular mechanisms through which PiRV-2 asserted its sporulation stimulation effect. In a survey of PiRV-2 presence in aP. infestanscollection, we found PiRV-2 in most isolates of the US-8 clonal lineage, a very successfull ineage that dominated potato fields in North America for several decades. We suggest that PiRV-2 may have contributed to its success. Our findings raise the intriguing possibility that a potentially hypervirulent virus may contribute to late blight epidemics.

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Survival of Isolates of the US-22, US-23, and US-24 Clonal Lineages of Phytophthora infestans by Asexual Means in Tomato Seed at Cold Temperatures

Plant Disease ◽

10.1094/pdis-04-15-0462-re ◽

2016 ◽

Vol 100 (1) ◽

pp. 180-187 ◽

Cited By ~ 6

Author(s):

Kenneth E. Frost ◽

Anna C. Seidl Johnson ◽

Amanda J. Gevens

Keyword(s):

Late Blight ◽

Phytophthora Infestans ◽

Winter Season ◽

Tomato Seed ◽

Tomato Production ◽

Cold Temperatures ◽

Production Region ◽

Degree Day ◽

Management Concepts ◽

The Us

Survival of Phytophthora infestans, causal agent of potato and tomato late blight, is thought to be negligible when exposed to freezing conditions typical of a Wisconsin winter. However, the persistence of relatively new P. infestans clonal lineages US-22, US-23, and US-24 within a production region during 2010 to 2014 warranted further investigation. We used tomato seed as a culture medium to determine the survival of P. infestans isolates representing the three lineages under temperatures of 18, 4, 0, −3, and −5°C for 11 time points (1 to 112 days postincubation). Survival varied interactively with temperature, duration of time at a temperature, and clonal lineage of the P. infestans isolate. US-22, -23, and -24 isolates survived for 112 days at 18 and 4°C, 84 days at 0°C, and 14 days at −3°C. US-23 survived longer at −3 and −5°C than did US-22 or US-24. The vigor of US-22 and US-24 isolates decreased with increasing exposure to cold temperatures, a trend that was not observed for the US-23 isolate. By calculating the length of time needed to kill the lineage isolates on infested tomato seed at five temperatures, we predicted that P. infestans would survive in 5% of tomato seed for 99, 25, and 16 days at 0, −3, and −5°C, respectively. We further applied a degree-day model to our empirical data to describe P. infestans survival as a function of cooling degree-day accumulations using archived soil temperatures at 5- and 10-cm depths at four Wisconsin locations over 27 years. The model indicated that survival of P. infestans in 5% of infested tomato seed would occur at 35 and 39% of the location–year combinations at 5- and 10-cm soil depths, respectively. Together, these data suggested that P. infestans has the potential to survive over the winter season by asexual means in infested tomato seed in Wisconsin and other Northern latitudes. Our cooling degree-day model for late blight in the tomato production system offers a tool for anticipating and mitigating disease based on integrated pest management concepts previously utilized for insects.

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Investigating the Host Range of the US-22, US-23, and US-24 Clonal Lineages of Phytophthora infestans on Solanaceous Cultivated Plants and Weeds

Plant Disease ◽

10.1094/pdis-09-13-0924-re ◽

2014 ◽

Vol 98 (6) ◽

pp. 754-760 ◽

Cited By ~ 5

Author(s):

Anna C. Seidl Johnson ◽

Amanda J. Gevens

Keyword(s):

Late Blight ◽

Phytophthora Infestans ◽

Host Range ◽

Management Strategies ◽

The United States ◽

Cultivated Plants ◽

Production Region ◽

Black Nightshade ◽

The Us ◽

Bittersweet Nightshade

Phytophthora infestans causes late blight, one of the most important diseases of potato and tomato worldwide. Recently in the United States, three newly identified clonal lineages, US-22, US-23, and US-24, have become widespread. While potato and tomato are the most commonly infected solanaceous hosts for P. infestans, new lineages may have a broader or different host range. Under controlled conditions, we determined the host range of isolates representing US-22, US-23, and US-24 genotypes of P. infestans on detached tissues of cultivated solanaceous plants and solanaceous weeds common to the upper midwestern production region. None of the isolates representing the clonal lineages produced late blight symptoms or signs on foliage of selected cultivars of eggplant, pepper, tomatillo, or ground cherry in a detached leaf assay. Symptoms and signs were evident on the potato and tomato cultivars tested, although with the US-24 isolate, infection on tomato was limited. None of the isolates sporulated on the common weed black nightshade, but some sporulation and necrosis was observed with all representatives of the lineages on bittersweet nightshade and petunia. Hairy nightshade supported abundant sporulation and symptoms, and sporangial production was not significantly different than that on tomato for each of the isolates representing the three lineages, indicating the potential for this weed to be a source of inoculum and contribute substantially to late blight epidemics. Interestingly, black nightshade had the highest incidence of sporulation on berries, but the lowest on leaves, suggesting the importance of testing multiple plant organs when determining susceptibility of a species. Our results update knowledge of the host range of the ever-changing P. infestans populations and will help to improve late blight management strategies by targeting these additional hosts.

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Effect of Temperature on Growth and Sporulation of US-22, US-23, and US-24 Clonal Lineages of Phytophthora infestans and Implications for Late Blight Epidemiology

Phytopathology ◽

10.1094/phyto-03-14-0064-r ◽

2015 ◽

Vol 105 (4) ◽

pp. 449-459 ◽

Cited By ~ 7

Author(s):

Anna C. Seidl Johnson ◽

Kenneth E. Frost ◽

Douglas I. Rouse ◽

Amanda J. Gevens

Keyword(s):

Growth Rate ◽

Late Blight ◽

Phytophthora Infestans ◽

Growth Rates ◽

Weather Data ◽

Effect Of Temperature ◽

Clonal Lineage ◽

Tomato Production ◽

The Us ◽

Lesion Growth

Epidemics of late blight, caused by Phytophthora infestans (Mont.) de Bary, have been studied by plant pathologists and regarded with great concern by potato and tomato growers since the Irish potato famine in the 1840s. P. infestans populations have continued to evolve, with unique clonal lineages arising which differ in pathogen fitness and pathogenicity, potentially impacting epidemiology. In 2012 and 2013, the US-23 clonal lineage predominated late blight epidemics in most U.S. potato and tomato production regions, including Wisconsin. This lineage was unknown prior to 2009. For isolates of three recently identified clonal lineages of P. infestans (US-22, US-23, and US-24), sporulation rates were experimentally determined on potato and tomato foliage and the effect of temperature on lesion growth rate on tomato was investigated. The US-22 and US-23 isolates had greater lesion growth rates on tomato than US-24 isolates. Sporulation rates for all isolates were greater on potato than tomato, and the US-23 isolates had greater sporulation rates on both tomato and potato than the US-22 and US-24 isolates. Experimentally determined correlates of fitness were input to the LATEBLIGHT model and epidemics were simulated using archived Wisconsin weather data from four growing seasons (2009 to 2012) to investigate the effect of isolates of these new lineages on late blight epidemiology. The fast lesion growth rates of US-22 and US-23 isolates resulted in severe epidemics in all years tested, particularly in 2011. The greater sporulation rates of P. infestans on potato resulted in simulated epidemics that progressed faster than epidemics simulated for tomato; the high sporulation rates of US-23 isolates resulted in simulated epidemics more severe than simulated epidemics of isolates of the US-22 and US-24 isolates and EC-1 clonal lineages on potato and tomato. Additionally, US-23 isolates consistently caused severe simulated epidemics when lesion growth rate and sporulation were input into the model singly or together. Sporangial size of the US-23 isolates was significantly smaller than that of US-22 and US-24 isolates, which may result in more efficient release of sporangia from the tomato or potato canopy. Our experimentally determined correlates of fitness and the simulated epidemics resulting from their incorporation into the LATEBLIGHT model suggest that US-23 isolates of P. infestans may have the greatest fitness among currently prevalent lineages and may be the most likely lineage to persist in the P. infestans population. The US-23 clonal lineage has been documented as the most prevalent lineage in recent years, indicating its overall fitness. In our work, US-23 had the highest epidemic potential among current genotypes. Given that epidemic potential is a component of fitness, this may, in part, explain the current predominance of the US-23 lineage.

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First Report of Late Blight Caused by Phytophthora infestans Clonal Lineage US-24 on Potato (Solanum tuberosum) in Wisconsin

Plant Disease ◽

10.1094/pdis-09-12-0825-pdn ◽

2013 ◽

Vol 97 (1) ◽

pp. 152-152 ◽

Cited By ~ 6

Author(s):

A. J. Gevens ◽

A. C. Seidl

Keyword(s):

Solanum Tuberosum ◽

Late Blight ◽

Phytophthora Infestans ◽

Weather Conditions ◽

Clonal Lineage ◽

Banding Patterns ◽

First Report ◽

The Us ◽

Epidemiological Characteristics ◽

The U.S

Potato (Solanum tuberosum) crops are grown on over 25,090 ha in Wisconsin annually. Late blight, caused by Phytophthora infestans (Mont.) deBary, is a potentially devastating disease that affects tomato and potato crops in Wisconsin every few years when inoculum is introduced and weather conditions favor disease. Incidence and severity of late blight are highly variable in these few years due to differences in pathogen clonal lineages, their timing and means of introduction, and weather conditions. Prevention of this disease through prophylactic fungicide application can cost producers millions of dollars annually in additional chemical, fuel, and labor expenses. Populations of P. infestans in the U.S. have recently undergone significant genetic change, resulting in isolates with unique clonal lineages and epidemiological characteristics (1). In 2010, late blight epidemics were of low severity in discrete portions of a few fields and were seen exclusively on potato in two counties of central Wisconsin. Symptoms included water-soaked to dark brown circular lesions with pale green haloes accompanied by white fuzzy pathogen sporulation typically on leaf undersides in high humidity conditions. Infected plants were collected by professional crop consultants and submitted to the authors at the University of Wisconsin Vegetable Pathology Laboratory in Madison, Wisconsin. Eight isolates of P. infestans were generated from individual leaf samples, representing separate fields, by removing sporangia from sporulating lesions and placing onto Rye A agar amended with rifampicin and ampicillin. Axenic, single zoospore-derived cultures of isolates were generated from parent cultures and maintained on Rye A agar for further characterization. Mycelium was coenocytic with hyphal diameter of 5 to 8 μm (n = 50). Sporangia were limoniform to ovoid, semi- to fully papillate, caducous, had short pedicels, and were 36.22 × 19.11 μm (height × width; n = 50). The average length-width ratio was 1.91. Allozyme banding patterns at the glucose-6-phosphate isomerase (Gpi) locus indicated a 100/100/111 profile, consistent with the US-24 clonal lineage (3,4). Mating type assays confirmed the isolates to be A1 and intermediate insensitivity to mefenoxam was observed in vitro (4). Genomic DNA was extracted with a phenol:chloroform:isoamyl alcohol solution and restriction fragment length polymorphism (RFLP) analysis was performed using the RG-57 probe on a representative isolate and resulted in banding patterns consistent with US-24 (2,3). Clonal lineages of P. infestans documented in Wisconsin in previous epidemics included US-8 in the mid-1990s and US-1 in the 1970s. The US-24 (A1) clonal lineage was very widespread in the U.S. in 2010 and its presence in Wisconsin in the same year as identification of US-22 (A2) posed great concern for potential sexual recombination, oospore production, and soil persistence. Fortunately, the opposite mating types were separated spatiotemporally. To the best of our knowledge, this is the first report of the P. infestans clonal lineage US-24 causing late blight on potato in Wisconsin. References: (1) K. Deahl. (Abstr.) Phytopathology 100:S161, 2010. (2) S. B. Goodwin et al. Curr. Genet. 22:107, 1992. (3) Hu et al. Plant Dis. 96:1323, 2012. (4) A. C. Seidl and A. J. Gevens. (Abstr.) Phytopathology 101:S162, 2011.

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Global historic pandemics caused by the FAM-1 genotype of the Irish potato pathogen Phytophthora infestans

10.1101/2020.08.25.266759 ◽

2020 ◽

Author(s):

Amanda C. Saville ◽

Jean B. Ristaino

Keyword(s):

Late Blight ◽

Phytophthora Infestans ◽

Irish Potato ◽

Herbarium Specimens ◽

Microsatellite Genotyping ◽

Irish Famine ◽

The Us ◽

Almost All ◽

British Colonies

AbstractThe FAM-1 genotype of Phytophthora infestans caused late blight in the 1840s in the US and Europe and was responsible for the Irish famine. We examined 140 herbarium specimens collected between 1845 and 1991 from six continents and used 12-plex microsatellite genotyping (SSR) to identify FAM-1 and the mtDNA lineage (Herb-1/ Ia) present in historic samples. FAM-1 was detected in approximately 73% of the historic specimens and was found on 6 continents. The US-1 genotype was found in only 27% of the samples and was found later on all continents except Australia/Oceania. FAM-1 was the first genotype detected in almost all the former British colonies from which samples were available. The data from historic samples suggest the FAM-1 genotype was widespread, diverse, and spread more widely than US-1. The famine lineage spread to six continents over 140 years, and likely spread during global colonization from Europe.

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Genotyping of Phytophthora infestans in Eastern Africa Reveals a Dominating Invasive European Lineage

Phytopathology ◽

10.1094/phyto-07-18-0234-r ◽

2019 ◽

Vol 109 (4) ◽

pp. 670-680 ◽

Cited By ~ 5

Author(s):

Anne W. Njoroge ◽

Björn Andersson ◽

Alison K. Lees ◽

Collins Mutai ◽

Gregory A. Forbes ◽

...

Keyword(s):

Late Blight ◽

Phytophthora Infestans ◽

Potato Production ◽

Eastern Africa ◽

Population Changes ◽

Management Options ◽

Planting Material ◽

The Us ◽

Specialized Form ◽

Late Blight Of Potato

Strains of Phytophthora infestans, the pathogen causing late blight of potato and tomato, are thought to be moved around the world through infected planting material. Since its first appearance in 1941, late blight has caused important losses to potato production in the eastern-Africa region (EAR). In the current study, the genetic structure of the population in Kenya, Uganda, Tanzania, Burundi, and Rwanda was characterized using 12-plex microsatellite markers with the aim of testing the hypothesis that a strain originating from Europe, 2_A1, has recently dominated the population in EAR. Analyses of 1,093 potato and 165 tomato samples collected between 2013 and 2016 revealed the dominance on potato in all countries of the 2_A1 clonal lineage. On tomato, a host-specialized form of the US-1 lineage appears to persist in Rwanda, Uganda, and Tanzania whereas, in Kenya, most samples from tomato (72.5%) were 2_A1. The US-1 lineage in Tanzania had two private alleles at the Pi02 marker, suggesting a possible independent introduction into the region. US-1 had higher genetic variability than 2_A1, consistent with the earlier establishment of the former. Continuous tracking of P. infestans population changes should help identify new virulent and aggressive strains, which would inform strategic disease management options.

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First Report of Late Blight Caused by Phytophthora infestans Clonal Lineage US-23 on Potato in Idaho

Plant Disease ◽

10.1094/pdis-02-14-0196-pdn ◽

2015 ◽

Vol 99 (3) ◽

pp. 417-417 ◽

Cited By ~ 1

Author(s):

P. S. Wharton ◽

P. Nolte ◽

W. W. Kirk ◽

S. Dangi ◽

A. J. Gevens

Keyword(s):

United States ◽

Late Blight ◽

Phytophthora Infestans ◽

Agar Medium ◽

The United States ◽

Eastern United States ◽

Clonal Lineage ◽

Predominant Genotype ◽

The Us ◽

Pure Cultures

Late blight, caused by Phytophthora infestans (Mont.) de Bary, is a destructive disease of potato (Solanum tuberosum) and tomato (S. lycopersicum) in the United States. Prior to 2007, the US-8 clonal lineage was the predominant genotype in the United States (4). Since 2007, a significant genetic change in the population of P. infestans occurred in the eastern United States with the appearance of new isolates with unique genotypes and epidemiological characteristics (3). These new genotypes US-22, US-23, and US-24 are sensitive to metalaxyl and represent mating types A2, A1, and A1, respectively (1,2). Prior to 2012, only US-8 had been documented in Idaho (5). In 2013, late blight was discovered in late August on potato crops (cv. Russet Norkotah) in Bingham and Madison counties, ID. Infected foliage (four samples from Bingham County and five from Madison) was sent to Michigan State University and the University of Wisconsin for confirmation of P. infestans and characterization of the isolates. Five sections from the leading edge of lesions were excised with a sterilized scalpel and placed on potato tuber slices (‘Dark Red Norkotah’). Pathogen sporulation on the excised lesions was enhanced by incubation in plastic boxes lined with moistened paper towels for 5 days at 18°C in the dark. The sporulating lesions were transferred onto pea agar medium (160 g peas, 5 g sucrose, 15 g agar, 700 ml distilled water) amended with 50 mg/ml vancomycin. Ten pure cultures were obtained for each of 4 isolates per county by hyphal tipping. Cellulose acetate electrophoresis was conducted to determine Gpi allozyme genotype of the 4 isolates (4). The allozyme banding patterns were 100/100 at the Gpi locus, consistent with previously reported analyses of the US-23 genotype (1,2). Genomic DNA was extracted from 10 pure cultures using the DNeasy Plant Mini Kit (Qiagen, Germantown, MD), and SSR analyses were performed. Microsatellite markers Pi02, Pi4B, Pi63, PiG11, and D13 were used in SSR analyses. Pi02, Pi4B, and Pi63 had alleles of 162/164, 213/217, and 270/279 bp in size, respectively which is consistent with the reference US-23 genotype (1). However, heterozygosity was detected at locus D13 in the Idaho genotype with allele size of 134/210 bp and an additional allele of 140/155/176 bp at locus PiG11. This is different from the standard US-23 genotype (homozygous alleles 134/134 at locus D13 and two alleles 140/155 at locus PiG11). These allele changes indicate the isolates may be variants of US-23 isolates as all other phenotypic characteristics were similar to those of reference US-23 isolates. The Idaho genotypes were sensitive to metalaxyl both in vitro on rye A agar medium amended with metalaxyl at <0.1 ppm, and in vivo on Ridomil treated foliage tests at <0.1 ppm (1,2). Mating type assays confirmed the pathogen to be the A1 mating type. In the 2009 and 2010 late blight epidemics in the eastern United States, US-23 was the predominant genotype, but to our knowledge this genotype has never been reported previously in Idaho. Thus, this is the first known report of P. infestans genotype US-23 causing late blight on potato in Idaho, indicating a change in the population of P. infestans. In Idaho, the source of this genotype remains unknown, although infected tomatoes have been implicated in the widespread dissemination of this genotype of P. infestans in the eastern United States. References: (1) G. Danies et al. Plant Dis. 97:873, 2013. (2) C. Hu et al. Plant Dis. 96:1323, 2012. (3) K. Deahl. (Abstr.) Phytopathology 100:S161, 2010. (4) S. B. Goodwin et al. Plant Dis. 79:1181, 1995. (5) USAblight. Recent US Genotypes. Online: www.usablight.org/node/52 , retrieved 3 January 2014.

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