Variability in responses of US-8 and US-11 genotypes of potato and tomato isolates ofPhytophthora infestans to commercial fungicidesin vitro

Isolates ofPhytophthora infestans collected in Canada from potato and tomato plants from 1994 to 1998 were tested for their responsein vitro to equal active ingredient concentrations (1, 2.5, and 5 μg a.i/mL) of six commercial fungicides: Acrobat MZ (dimethomorph and mancozeb), Ridomil Gold (metalaxyl-m), Dithane (mancozeb), Curzate (cymoxanil), Bravo (chlorothalonil), and Tattoo C (propamocarb and chlorothalonil). Relative mycelial growth of isolates estimated on fungicideamended vs fungicide-free media was compared among isolates of the US-8 and US-11 genotypes from the two host plants. The effects of these fungicides on spore germination of isolates from the two genotypes were also estimated. Mycelial growth of US-8 and US-11 isolates was most affected by Bravo and Tattoo C, followed by Acrobat, and then Curzate and Dithane. Ridomil Gold was not effective in inhibitingin vitro growth of US-11 isolates at the concentrations tested. Variations in sensitivity to different fungicides among isolates of the same genotype collected in different years were observed. For example, based on 2.5 and 5 μg a.i./mL, potato/US-11 isolates collected from 1995 to 1997 showed increased sensitivity to Curzate and Ridomil Gold, while decreased sensitivity was recorded with Dithane. For US-8 isolates, responses to the fungicides varied according to fungicide and year. Variations were generally not significant for Acrobat, Curzate, Bravo, and Tattoo C over time. Conversely, potato/US-8 isolates from 1995 and 1998 were less sensitive to Dithane and Ridomil Gold than those from 1994, 1996, and 1997.     

Response of tubers of five potato cultivars to co-inoculation with US-1 and US-8 genotypes ofPhytophthora infestans

Summary The severity of late blight tuber rot in five potato cultivars (Green Mountain, Sebago, Dorita, AC Brador and Island Sunshine) resulting from inoculation with US-1/US-8 combinations of Canadian isolates ofPhytophthora infestans was evaluated. In addition, the potential for mixed (A1∶A2) inoculations to result in oospore formation was examined. In studies conducted in 1996 and 1997, cv. Dorita (and Island Sunshine in 1997) developed the least amount of surface or internal necrosis relative to the other cultivars following co-inoculation with US-1 and US-8 isolates ofP. infestans. Differences also were revealed among isolate combinations used for inoculation. The combination of a US-1 isolate from Prince Edward Island with a US-8 isolate from New Brunswick consistently produced the least amount of necrosis in tubers of the various cultivars. There was no evidence of oospore production in any of the tubers examined. AAFC Contribution No. 919     

Characterization ofPhytophthora infestans from Maine during 1999 and 2000

A total of 77 isolates ofPhytophthora infestans was collected in 1999 and 93 isolates were collected in 2000 from growers’ fields, home gardens, and research plots in Maine. Genotype diversity based onglucose-6-phosphate isomerase (Gpi) allozyme analysis was greater in the collections from 2000, with six genotypes identified vs two genotypes identified in 1999. The US-8 (Gpi 100/111/122) genotype constituted 88% of the isolates collected in 1999 and 75% of those collected in 2000. The US-14 (Gpi 100/122) genotype constituted 12% and 7% of the isolates collected in 1999 and 2000, respectively. Three newGpi genotypes constituted 7% of the isolates collected from potato in northern Maine in 2000. A new genotype (A1 mating type,Gpi 100/100) that was aggressive on both tomato and potato was identified from home gardens in western Maine and constituted 11% of the isolates collected in 2000. A1, A2, and A1A2 mating types as well as self-fertile isolates were collected in both years. Self-fertility in artificial culture appeared to be relatively common in field isolatesof P. infestans. Potential origins of the new genotypes are discussed.     

Novel genotypes ofPhytophthora infestans in Canada during 1994 and 1995

Populations ofPhytophthora infestans, causing late blight of potato, changed significantly in Canada in the mid-1990s. The US-1 genotype (A1, metalaxylsensitive [MS]) was displaced by novel genotypes (mainly US-8) in much of the country in 1994 and 1995. Many of the new genotypes were insensitive to metalaxyl and were of the A2 mating type, although metalaxyl-insensitive (MI) Al isolates of the gll (or US-11) genotype were found in British Columbia. A total of 14 unique multilocus genotypes were described over the two years of the study based on mating type (Al or A2), metalaxyl sensitivity (MS or MI), allozyme banding patterns (Gpi andPep), and random amplified polymorphic DNA (RAPD) markers. Six of these genotypes could be distinguished by allozyme banding patterns at theGpi locus alone; RAPD analysis revealed additional variation within allozyme groupings and defined additional genotypes. Among 82 isolates collected from various Canadian provinces in 1994, US-1 and US-8 constituted 30% and 49% of isolates, respectively; other genotypes occurred in minor frequency. Among 50 isolates collected in 1995, US-1 and US-8 constituted 10% and 74%, respectively. Isolates of gll (US-11) were found in British Columbia in 1995. Diversity of genotypes was greater in 1994 (11 genotypes) than in 1995 (six genotypes). We hypothesize that sexual reproduction may have been responsible for generating some of the genetic variability inP. infestans populations, during a time period in which A2 isolates first migrated into many provinces in Canada.     

Impact of seed potatoes infected withPhytophthora infestans (US-1 or US-8 genotypes) on crop growth and disease risk

As the surface area with late blight symptoms of seed tubers increased, the incidence of plant emergence decreased. This relationship was more dramatic with the US-8 genotype than the US-1 genotype ofhytophthora infestans, causal agent of late blight. However, when seed tubers were inoculated with both US-1 and US-8 isolates, the plant growth responses were generally intermediate to those obtained with the two genotypes alone. More resistant cultivars (eg. Dorita, Island Sunshine, and Sebago) had greater plant emergence and less seed rot than more susceptible cultivars (eg. Green Mountain and Shepody) but the response differences between resistant and susceptible cultivars was less clear with US-8 as most cultivars had 100% seed rot. Seed handling treatments affected crop growth and disease risk; as the inoculum levels and in some cases, the pre-planting storage period increased, plant emergence decreased generally due to greater seed rot incidences. Of the one registered seed treatment fungicide and several non-registered and/or experimental fungicides tested, only two significantly improved plant emergence. During three years of study, “diseased” seed treatments generally led to seed rot except in 1996 when one plant in one experimental plot of infected seed had subsequent spread of the disease to the foliage. Furthermore, within one week, a field epidemic developed outwardly from the single infected plant due to the occurrence of favourable weather conditions. Further research opportunities are discussed relative to the impact of infected seed on crop growth and disease risk     

Relationships between pathotypes and RAPDs, Gpi-allozyme patterns, mating types, and resistance to metalaxyl ofPhytophthora infestans in Canada in 1997

Relationships between pathotypes and other markers such as mating types, metalaxyl resistance, Gpi-allozyme and random amplified polymorphic DNA (RAPD) patterns, were investigated among 33 isolates ofPhytophthora infestans obtained from both potato and tomato across Canada in 1997. Pathogenicity testings, based on virulence against potato differentials carrying single-resistance genes and the size of lesions on leaves, revealed higher pathogenic diversity in 1997 than in the few past years, with more pathotypes and more race-complex isolates. A higher pathogenic diversity was observed within A2 (US-8) than within A1 (US-11) isolates. There was no correlation between the level of resistance to metalaxyl of an isolate and its pathogenicity. Nevertheless, when grouped into metalaxyl-sensitive (MS), -intermediate (MI), and -resistant (MR) classes, both average pathotype complexity and average lesion size were lowest for MS group. RAPD data revealed a good distinction between A1 (US-11) and A2 (US-8) mating type groups, and between isolates from British Columbia and those from other provinces. The detection in the last few years of both A1 and A2 mating types in Canada and the occurrence in 1997 of some isolates sharing different characteristics of both of these two groups suggest that sexual reproduction may have occurred locally.     

Identification of late blight, Colorado potato beetle, and blackleg resistance in three Mexican and two South American wild 2x (1EBN)Solanum species

Wild potatoes are important sources of genes for resistance to disease and insect pests. A collection of wild Mexican and South AmericanSolarium species from the US potato Genebank was evaluated under laboratory and/or field conditions for their reaction to late blight (Phytophthora infestans), Colorado potato beetle (CPB,Leptinotarsa decemlineata Say), and blackleg (Erwinia carotovora subsp.atroseptica (van Hall) Dye) in order to identify individual genotypes with multiple resistance genes. Late blight inoculations using aggressive isolates (US-8/A2 and US-11/A1 mating types) of P.infestans revealed a wide range of variation for resistance between and within the accessions of the wild species tested. For late blight, susceptible as well as moderately to highly resistant genotypes were observed in all the species tested. However, at least one accession from the three Mexican and one South American wild diploid species tested showed a relatively uniform high level of resistance toP. infestans. These includedS. bulbocastanum, S. pinnatisectum, S. cardiophyllum, andS. circaeifolium. Two accessions from South American speciesS. commersonii were highly susceptible to late blight. For the Colorado potato beetle test, only one species,S. pinnatisectum appeared uniformly resistant to CPB under field conditions. Results of screening for blackleg resistance showed that there were major differences between genotypes in the wild species. Accessions ofS. circaeifolium PI 498119 andS. bulbocastanum PI 243504 were identified as having significantly higher blackleg resistance than cultivated potato and the other wild species tested. However, genotypes from these two accessions were more susceptible to late blight and CPB. Characterization of theP. infestans isolate P1801C.16 used for late blight evaluation and multi-locus isolate tests using US-8/A2 and US-11/A1 races revealed that the resistance inS. pinnatisectum genotypes tested corresponded to a race-non-specific genetic system, which was different from any existing R genes.Solanum pin-natisectum genotypes with both high levels of late blight and CPB resistance as well as blackleg resistance genotypes identified in the present study represent a diverse gene pool that may be useful for development of new potato cultivars with multiple disease and insect resistance. The potential utilization of these valuable sources for improvement of cultivated potato is discussed.     

Changes in race structure of Canadian populations ofPhytophthora infestans based on specific virulence to selected potato clones

Summary A collection of 80 isolates ofPhytophthora infestans, representing multilocus genotypes of the pathogen found in Canada from 1994 to 1996, was screened on a series of nine differential potato host cultivars to determine the physiological race of the isolates. A total of 28 pathotypes were found among the 11 genotypes present in the collection. There was a significant increase in the complexity of pathotypes from 1994 to 1996 which reflected the displacement of the US-1 genotype (mean of 2.2 host differentials infected) by the US-8 genotype (mean of 8.1 host differentials infected). Eighteen of the US-8 isolates could overcome all nine of the resistance genes tested. Virulence genes which overcame host resistance conferred by R8 were the rarest in the populations studied. The predominance of complex populations ofP. infestans in Canada implies that the introduction of cultivars with specific R genes for resistance would be an ineffective control measure. AAFC Contribution No. 901     

Differences in tuber rot development for North American clones ofPhytophthora infestans

The relative aggressiveness ofPhytophthora infestons clones in potato tubers was compared in three trials using 7 to 24 isolates of 2 to 4 clones. Visible rot developed slowly at 13C with isolates of the US-1 genotype, the only significant clone found in North America prior to 1979, but substantially faster with most isolates of the newer clonal genotypes US-6, US-7 and US-8. Certain US-7 isolates were similar to US-1, and US-6 isolates also had a broad range of aggressiveness. Secondary infection byFusarium sp. increased rot development in many instances, but this effect was not clone-related. Differences in rot development may affect potato storage or late blight disease transmission.     

Using chemical and biological treatments to reduce the potential of potato leak caused byPythium aphanidermatum in Tunisia

Summary The effect of fungicides and the antagonistic fungusTrichoderma harzianum (Th) on the potato leak pathogenPythium aphanidermatum (Pa) was investigated in vitro. Rot was reduced by 73% to 89% when tubers inoculated withPa were treated by immersion in solutions (0.5% w/v) of the fungicides Ridomil MZ58. Dithane M45. Manebe 80 and Tachigaren 360. Complete protection was achieved by dusting inoculated tubers with Ridomil MZ58 diluted in kaolin at a final concentration of 0.1% (w/w). The biological treatments of dipping inoculated tubers in a conidial water suspension ofTh at 10⁸ conidia/ml or of dusting with a mixture of a barley culture ofTh and kaolin to a final concentration of 2×10⁹ conidia/g were as effective as the chemical treatments. Control was still effective when inoculated tubers were treated with Ridomil MZ58. Tachigaren 360 or the conidial suspension ofTh after 24h incubation at 25°C.     

A detached-leaf method to evaluate late blight resistance in potato and tomato

We developed a detached leaf method for evaluating potato (Solanum tuberosum L.) and tomato (Lycopersicon esculentum Mill.) germplasm for reactions toPhytophthora infestans, the causal organism of late blight. Primary leaves from the third to the sixth node of potato plants, and the fourth to sixth node of tomato plants were excised at the stem. Their petioles were inserted into 14 mm × 100 mm floral aqua tubes containing 9 ml of sterilized distilled water. The leaves in the aqua tubes were placed with abaxial sides down on galvanized metal hardware cloth (12.5 × 12.5 mm mesh). A 12.5 mm sensi-disc containing 50μl of 2 × 10⁴ zoospores was placed in the centers of the terminal leaflet and the second leaflet pair of the potato leaf. A single disc was placed on the center of the tomato leaf. The supporting hardware cloth was placed 2.5 cm above distilled water (2.5 cm deep) in 31 cm × 17 cm × 8 cm clear plastic boxes with tight fitting lids. Leaves on intact tomato and potato plants were inoculated in a similar manner and placed in a mist chamber. Lesion growth was determined 4, 5, and 6 days following inoculation. There were no significant differences in reactions to isolates ofP. infestans on detached and intact leaves of potato cultivars Green Mountain and Kennebec and the tomato cultivar Bonnie Best.     

Fungicidal control ofPhytophthora erythroseptica: The cause of pink rot on potato

Two shade house and six field experiments were undertaken to evaluate fungicides and timing of application for the control of pink rot of potatoes caused byPhytophthora erythroseptica. Pink rot developed in up to 60% of tubers grown in artificially inoculated soil and 21% of the tubers in naturally infected soil. Ridomil (metalaxyl) and Ridomil Gold (mefenoxan) granules applied to the soil at planting completely or nearly completely controlled pink rot throughout the season in both pot and field experiments. Foliar applications of Ridomil Gold MZ (mefenoxan plus mancozeb) applied when tubers were around 20 mm diameter and another one or two sprays 12 to 28 days later also provided more than 80% control of pink rot in most experiments. Foliar applications of Shirlan (fluazanim) or Acrobat MZ (dimethomorph plus mancozeb) showed some activity against pink rot, but were significantly less effective than Ridomil Gold MZ. Foli-R-Fos 200 (phosphonic acid) or Dithane M45 (mancozeb) applied to the canopy showed little activity against pink rot in most experiments. Yield increases of between 6 and 17 tons ha^?1 resulted from the application of either Ridomil granules at planting or two to three Ridomil Gold MZ foliar sprays around “tuber set.”     

Testing for resistance to metalaxyl inPhytophthora infestans isolates from northwestern Washington

Outbreaks of late blight occurred in commercial potato fields in northwestern Washington in 1989 and reached epidemic proportions in 1990. Since most blighted fields reportedly had received 2–4 late-season applications of metalaxyl, the possibility existed that pathogenic strains ofPhytophthora infestans had developed resistance to this systemic fungicide. In testing this hypothesis,P. infestans was isolated consistently from diseased leaves, stems, and tubers of potato when small tissue pieces were surface-disinfected in 0.5% NaOCl for 3 min. Virulence of isolates was assessed on detached leaflets of eight differential (R-gene) potato genotypes. Although simple races (single and double) were recovered, there was also a high frequency of complex race combinations. When tested in metalaxyl-amended media, 81% of the 73 isolates recovered from diseased tissues were highly resistant to 10 μg metalaxyl/ml and 19% yielded intermediate resistance. Sensitive isolates were not recovered. Fifty-five percent of 40 isolates sporulated on potato tuber disks in the presence of ≥10 ug metalaxyl/ml. On leaf tissues, 85% of 20 isolates sporulated in the presence of 10 μg metalaxyl/ml, and 50% sporulated in the presence of 100 μg/ml. This is the first report ofP. infestans resistance to metalaxyl in the U.S.     

Changes in metalaxyl resistance among glucose phosphate isomerase genotypes ofPhytophthora infestans in Canada during 1997 and 1998

A total of 930 isolates ofPhytophthora infestans collected during 1997 and 1998 from commercial potato and tomato fields, home gardens, and potato storages in Canada was analyzed for glucose phosphate-isomerase (Gpi) genotype, mating type, and resistance to metalaxyl. Both mating types and diversity in resistance to metalaxyl were detected within and amongGpi-genotypes. Pathogen populations were dominated by the 100:111:122Gpi-genotype (characteristic of US-8) in all the provinces where late blight was detected, except in 1997 in British Columbia (BC) where the 100:100:111Gpi-genotype (characteristic of US-11) predominated.Gpi-genotypes 100:111, 111:122, and 100:122 (characteristic of US-7, US-10 and US-14/US-17, respectively) were less common. Nearly all of the isolates with the 100:100:111Gpi-genotype were the A1 mating type, except for a few collected in BC in 1997. A majority of isolates with the 100:111:122Gpi-genotype were A2 except for a few found in Québec in 1998. Based on colony growth on media amended with 1,10, or 100 μg/ml metalaxyl, the 100:100:111Gpi-genotype isolates had higher levels of resistance to the fungicide in 1998 compared to 1997. In 1998, level of metalaxyl resistance was higher within the 100:100:111Gpi-genotype whereas it was lower within the 100:111: 122Gpi-genotype, when compared to 1997. BC was the only Canadian province where an increase in metalaxyl resistance within the 100:111:122Gpi-genotype was recorded between 1997 and 1998. Isolates from leaves had higher metalaxyl resistance levels than stem or tuber isolates, regardless of genotype.     

Characterization and Distribution of Three New Clonal Lineages of Phytophthora infestans Causing Late Blight in Wisconsin from 2009 to 2012

Phytophthora infestans causes late blight of potato and tomato, a disease that has been estimated to cost U.S. potato growers $287.8 million annually. We collected isolates of P. infestans from Wisconsin from 2009 to 2012 and determined distribution of clonal lineages and mating types and sensitivity to the systemic fungicide mefenoxam. We also sought to evaluate the current utility of an analysis of the Glucose-6-phosphate isomerase (Gpi) allozyme locus for predicting mefenoxam sensitivity with the aim of delivering timely information to growers. Overall, 143 isolates were collected from 52 locations in 20 Wisconsin counties from 2009 to 2012. Three clonal lineages, US-22, US-23, and US-24, were identified and were novel to Wisconsin and the U.S. US-22 is of the A2 mating type and sensitive to mefenoxam, with Gpi 100/122. US-23 and US-24 are of the A1 mating type and primarily intermediately sensitive to mefenoxam, with Gpi 100/100 and 100/100/111, respectively. Because of this close correlation and the unique Gpi patterns for each lineage present, we were able to predict mefenoxam sensitivity directly from samples using the allozyme assay and quickly deliver management information to growers. Both mating types were present in Wisconsin in 2009 and 2010 but were spatially separated and no evidence of sexual recombination or soil persistence was detected. The presence of new clonal lineages of P. infestans in Wisconsin indicates a need for continued close monitoring of late blight to facilitate generation of timely information for enhanced short-term and long-term late blight management.     

Assessment of mating types and resistance to metalaxyl of Canadian populations ofPhytophthora infestans in 1997

The distribution of A1 and A2 mating types, and of metalaxyl-sensitive (MS), metalaxyl-intermediate (MI), and metalaxyl-resistant (MR) strains ofPhytophthora infestans, the causal pathogen of late blight, were determined in Canada in 1997. Potato plants or tubers and tomato plants or fruits, suspected to have late blight infection, were received from seven Canadian provinces in 1997, and were assessed for the presence ofP. infestans. Among 220 samples suspected to have late blight,P. infestans was recovered from 132 samples (60%) received from British Columbia (BC), Manitoba, New Brunswick, Prince Edward Island, and Quebec. From the 132 samples, 364 isolates were obtained and tested for their mating type and response to metalaxyl. Populations ofP. infestans within Canadian provinces were homogeneous for mating types, except within BC, where populations comprised both A1 and A2 mating types both within fields and sometimes within samples. The A2 mating type was found in all provinces where late blight was present. The A1 mating type was found only in Manitoba (1 isolate) and in BC (38 isolates). Unlike the old A1 isolates, those found in BC in 1997 were highly resistant to metalaxyl. These A1 isolates are even more resistant than A2 isolates. Based on assays with 10 ug.ml^?1 of metalaxyl, 14% of all tested isolates were MS, 69.8% were MI and 16.2% were MR. In addition, all provinces contained MS, MI, and MR isolates. No specific mating type or metalaxyl sensitivity level were specific to tomato or potato.     

Impact of Different US Genotypes of <Emphasis Type="Italic">Phytophthora infestans</Emphasis> on Potato Seed Tuber Rot and Plant Emergence in a Range of Cultivars and Advanced Breeding Lines

Seed pieces of different potato cultivars and advanced breeding lines (ABLs) from north central US breeding programmes were inoculated with different genotypes of Phytophthora infestans (US-1, US-1.7, US-8, US-11 and US-14). The effect of these genotypes of P. infestans on seed piece rot severity after re-storage was assessed using an image analysis technique. P. infestans genotypes demonstrated variable ability to cause seed piece rot and to reduce plant emergence measured as final plant stand (%) and the relative area under the plant emergence curve (RAUEPC). The US-8 genotype of P. infestans was the most aggressive genotype, as indicated by tuber rot severity across all cultivars/ABLs tested, followed by US-14 in both years. The US-1, US-1.7 and US-11 genotypes were the least aggressive, causing only moderate seed piece rotting across cultivars/ABLs tested. Similar trends were observed in two field experiments, where the US-8 and US-14 genotypes delayed or reduced emergence. Values of final plant stand (%) and RAUEPC demonstrated that the cultivars/ABLs Atlantic, MSJ453-4Y and Torridon were the least susceptible across all P. infestans genotypes. In both experiments cv. Pike was the most susceptible. Other cultivars/ABLs demonstrated variable responses to different genotypes of P. infestans. No symptoms of P. infestans were observed on emerged plants up to 60 days after planting. The variability of susceptibility of tubers to different genotypes of P. infestans has implications for plant breeding efforts in that the major emphasis in the past has been to breed for foliar resistance, with limited emphasis on the reaction of the tuber. Results from this study suggest that highly aggressive genotypes of P. infestans such as US-8 may lead to severe tuber rotting and deterioration of tubers before emergence, but despite this observation the US-8 genotype is still predominant in North America.     

不同杀菌剂防治马铃薯晚疫病田间药效试验

试验研究了7种杀菌剂对马铃薯晚疫病的防治作用。结果表明:68.75%银法利悬浮剂1125 g/hm2、53%金雷多米尔水分散颗粒剂1500g/hm2、60%氟吗啉.代森锰锌可湿性粉剂1225g/hm2、72%克露可湿性粉剂2000g/hm2对马铃薯晚疫病防效分别为88.38%、86.15%、84.55%、81.68%,是防治马铃薯晚疫病较理想的药剂,且增产效果明显,建议在生产上交替使用。     

Evaluation of wildSolanum species for resistance to the US-8 genotype ofPhytophthora infestans utilizing a fine-screening technique

Greenhouse assays were carried out to characterizeSolanum accessions previously reported to be late blight resistant and to identify individuals within the accessions with high levels of resistance to late blight. Foliage of wildSolanum species accessions or hybrid cultivated potato x wild species (diploid or tetraploid) from Mexico, Russia, or South America was inoculated withPhytophthora infestans (US-8) and was scored for severity of foliar symptoms at 7 and 14 days after inoculation (DAI). Mexican accessions (S. brachycarpum,S. pinnatisectum,S. guerreroense, andS.fendleri) were significantly more resistant (1.1% infection at 14 DAI) than either Russian (59.1% infection at 14 DAI) or South American (53.4% infection at 14 DAI) accessions. Moreover, the genotypes within the Mexican accessions tended to be more uniformly resistant whereas genotypes within the Russian and South American accessions tended to segregate for resistance. The more resistant genotypes of Russian and South American accessions (176/546) were retested, and 56 genotypes were identified as having potential for use in potato breeding programs for resistance to late blight. Fifty percent of the selected genotypes were fromS. microdontum accessions PI498124, PI595510, and PI595511. The potential of these species for incorporation into breeding programs for late blight resistance is discussed.     

Amplification of the <Emphasis Type="Italic">Phytophthora infestans</Emphasis> RG57 loci Facilitates <Emphasis Type="Italic">in planta</Emphasis> T-RFLP Identification of Late Blight Genotypes

Late blight, caused by the oomycete Phytophthora infestans (Mont.) de Bary, is a devastating disease in potato and tomato and causes yield and quality losses worldwide. The disease first emerged in central America and has since spread in North America including the United States and Canada. Several new genotypes of P. infestans have recently emerged, including US-22, US-23 and US-24. Due to significant economic and environmental impacts, there has been an increasing interest in the rapid identification of P. infestans genotypes. In addition to providing details regarding the various phenotypic characteristics such as fungicide resistance, host preference, and pathogenicity associated with various P. infestans genotypes, information related to pathogen movement and potential recombination may also be determined from the genetic analyses. Restriction fragment length polymorphism (RFLP) analysis with the RG57 loci is one of the most reliable procedures used to genotype P. infestans. However, the RFLP procedure requires propagation and isolation of the pathogen and relatively large amounts of DNA. Isolation of the late blight pathogen is sometimes impossible due to the poor condition of the infected tissues or the presence of fungicide residues. In this study, we describe a procedure to identify P. infestans at the molecular level in planta using terminal restriction fragment length polymorphism (T-RFLP) of the RG57 loci. This T-RFLP assay is sufficiently sensitive to detect and differentiate P. infestans genotypes directly in planta without propagation and isolation of the pathogen, to facilitate the timely implementation of best management practices.