Phytophthora Infestans Populations Research Articles

Abstracts, The Canadian Phytopathological Society, annual meeting, Montreacute;al, Quebec, 1999

Abstracts, The Canadian Phytopathological Society, annual meeting, Montreacute;al, Quebec, 1999

Characterization of Phytophthora infestans Populations in Western Washington.

The first detection in the United States of isolates of Phytophthora infestans having metalaxyl insensitivity and complex pathotypes occurred in western Washington during the early 1990s. To determine the genetic structure of the current population in western Washington, a total of 115 isolates of P. infestans were obtained during 1996 from infected tubers or foliage of potato, tomato, nightshade, and bittersweet throughout the region. An additional 45 isolates were collected from a single field. Based on mating type, metalaxyl-insensitivity, and molecular markers (allozymes of glucose-6-phosphate isomerase, peptidase, and RG57 DNA fingerprint), all of the isolates were A1 mating type and had the US-11 multilocus genotype. Analyses of an additional 120 isolates collected during 1997 from potato, tomato, and nightshade were performed. As in 1996, US-11 was the predominant genotype detected on all three hosts. However, three additional A2 mating type genotypes were also detected: US-7, US-8, and US-14. These three genotypes represent the first A2 mating type isolates detected in western Washington. Most of a subset of 60 isolates infected 4 to 7 of the 10 potato differentials tested. This included 90% of the isolates collected in 1996 (all US-11), plus 72% of the US-11 and 100% of the US-8 and US-14 isolates collected during 1997. Virulence phenotypes in this region are complex even without the selection pressure of R-genes in the local commercial cultivars. The apparent increase in genetic variation observed in populations of P. infestans in western Washington from 1996 to 1997 most likely occurred by migration rather than by sexual recombination.

Characterization of changes in populations of Phytophthora infestans in Canada using mating type and metalaxyl sensitivity markers

Isolates of Phytophthora infestans were obtained from individual fields of potato and tomato and from potato storages across Canada in 1994 (142 samples yielding 555 isolates), 1995 (185 samples yielding 914 isolates), and 1996 (300 samples yielding 1013 isolates). Characterization of these isolates according to mating type and sensitivity to metalaxyl revealed the changing nature of P. infestans populations in Canada. In 1994, isolates of the traditional Al, metalaxyl-sensitive (MS) phenotype were common, but by 1996, they were no longer recovered from any tissue samples. Isolates of the A2 mating type, which were predominantly insensitive to metalaxyl (MI), were prevalent in 1996, except in British Columbia (B.C.). In B.C., Al isolates highly insensitive to metalaxyl predominated. Isolates of the Al, MI phenotype were also found in a sample of tomatoes from Ontario in 1996. New populations composed of the A2 mating type were predominantly insensitive to metalaxyl, but they showed an intermediate response between the high sensitivity of the original Al population and the highly insensitive Al populations in B.C. The exception was a few A2, MS isolates obtained from B.C. in 1996. The correlation between recovery of MI isolates and metalaxyl use was poor. There was a tendency for the proportion of MI isolates from fields managed with or without metalaxyl to increase as the growing season progressed. Both mating types of P. infestans were found in the same tissue sample a maximum of three times in a given year; self-fertile isolates were not recovered. Of 627 tissue samples examined over 3 years, one was found to have oospores of P. infestans present, indicating that sexual reproduction had occurred. These results demonstrate that populations of P. infestans in Canada have changed dramatically from 1994 to 1996 and that the original Al, MS population was rapidly displaced.

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

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.

Genetic Change Within Populations of Phytophthora infestans in the United States and Canada During 1994 to 1996: Role of Migration and Recombination

ABSTRACT Dramatic changes occurred within populations of Phytophthora infestans in the United States and Canada from 1994 through 1996. Occurrence of the US-8 genotype, detected rarely during 1992 and 1993, increased rapidly and predominated in most regions during 1994 through 1996. US-7, which infected both potato and tomato and made up almost 50% of the sample during 1993, was detected only rarely among 330 isolates from the United States analyzed during 1994. It was not detected at all in more limited samples from 1996. Thus, ability to infect both potato and tomato apparently did not increase the fitness of this genotype relative to US-8, as predicted previously. US-1, the previously dominant genotype throughout the United States and Canada, made up 8% or less of the samples analyzed during 1994 through 1996. A few additional genotypes were detected, which could indicate the beginnings of sexual reproduction of P. infestans within the United States and Canada. However, clonal reproduction still predominated in all locations sampled; opportunities for sexual reproduction probably were limited, because the A1 and A2 mating types usually were separated geographically. The high sensitivity of the US-1 genotype to the fungicide metalaxyl also could have reduced opportunities for contact between the mating types in fields where this compound was applied. The previous correlation between metalaxyl sensitivity and genotype was confirmed and extended to a new genotype, US-17: all US-1 isolates tested were sensitive; all isolates of the US-7, US-8, and US-17 genotypes tested to date have been resistant. Isolates of P. capsici and P. erythroseptica, two other species often found on tomato and potato, could be easily distinguished from each other and from P. infestans using a simple allozyme assay for the enzyme glucose-6-phosphate isomerase. This technique could be useful for rapid identification of species, in addition to genotype of P. infestans. It generally was not possible to predict which genotypes would be present in a location from 1 year to the next. Long-distance movement of US-8 in seed tubers was documented, and this was probably the primary means for the rapid spread of this genotype from 1993 through 1996.

Distribution of sensitivity responses to cymoxanil within global populations of Phytophthora infestans

In-vitro and in-vivo results show no decrease in Phytophthora infestans sensitivity to cymoxanil after more than 16 years of intensive use, and that a single, sensitive population exists within the countries sampled. © 1998 SCI

Sensitivity profiles ofMycosphaerella graminicolaandPhytophthora infestanspopulations to different classes of fungicides

Prior to the use of fungicides, the baseline sensitivity of individuals in a pathogen population may already differ by a factor of 10 to 100 between the least and the most sensitive isolates. In Mycosphaerella graminicola populations, this factor, measured in vitro, was 5 to 20 for both the strobilurin analogue azoxystrobin (baseline) and the triazole cyproconazole which has been in use for several years. In Phytophthora infestans populations, this factor, measured in a leaf disc assay, was about 100 for azoxystrobin (baseline), up to 1000 for the cyanoacetamide cymoxanil and >10000 for the phenylamide oxadixyl; both of the latter have been used for many years. In M. graminicola, cross-sensitivity was present between all azole fungicides for the majority of the isolates, whereas no correlation was found between triazoles and azoxystrobin. Despite the existence of cross-sensitivity between azoles, ‘box-and-whiskers’ plots revealed large variations in the sensitivity profiles of some triazoles; isolates resistant to triazoles have not been detected in M. graminicola populations. In P. infestans populations, the proportion of the phenylamide-resistant sub-population increased during the season more rapidly in treated than in untreated fields, but it was low at the beginning of the next season in all fields. During disease epidemics, the fitness of phenylamide-resistant P. infestans isolates, as characterised by lesion size, was higher than that of the sensitive isolates, but after the overwintering period, the recovery of resistant isolates was apparently lower. The presence of both A1 and A2 mating types of P. infestans in European populations, although at different frequencies, allows sexual recombination and increased genetic diversity, affecting sensitivity and fitness. Such mixed populations can still be adequately controlled by using sound anti-resistance strategies. ©1997 SCI

Sensitivity to Protectant Fungicides and Pathogenic Fitness of Clonal Lineages of Phytophthora infestans in the United States

ABSTRACT Since 1991, dramatic changes have occurred in the genetic composition of populations of Phytophthora infestans in the United States. Clonal lineages recently introduced into the United States (US-7 and US-8) are more common now than the previously dominant lineage (US-1). To help determine why these changes occurred, four clonal lineages of P. in-festans common during the early 1990s in the United States and Canada were evaluated for sensitivity to the protectant fungicides mancozeb and chlorothalonil using amended agar assays for isolates collected from 1990 to 1994. No isolate or lineage was resistant to either mancozeb or chlorothalonil. There were significant differences among isolates for degree of sensitivity to one fungicide individually, but there were no significant (P = 0.05) differences among the US-1, US-6, US-7, and US-8 clonal lineages for degree of sensitivity to both fungicides. Therefore, resistance to protectant fungicides cannot explain the rapid increase in frequency of the US-7 and US-8 clonal lineages. Three components of pathogenic fitness (latent period, lesion area, and sporulation after 96 h) were tested for the three clonal lineages that were detected most commonly during 1994 (US-1, US-7, and US-8). All but one of the isolates in this analysis were collected during 1994 and evaluated within 10 months of collection by inoculating detached leaflets of the susceptible potato cultivar Norchip. There were significant differences between the US-1 and US-8 clonal lineages for lesion area and sporulation, and between US-1 and US-7 for latent period. The US-6 clonal lineage was excluded from the pathogenic fitness experiments, because no isolates of this lineage were collected during 1994. Compared with US-7 and US-8, US-1 had the longest latent period and the smallest lesions with the least sporulation. Incorporation of the differences between US-1 and US-8 in computer simulation experiments revealed that significantly more protectant fungicide (e.g., 25%) would be required to suppress epidemics caused by the US-8 clonal lineage compared with US-1. These differences in pathogenic fitness components probably contribute to the general predominance of the "new" clonal lineages (especially US-8) relative to the "old" US-1 lineage.

The germplasm release of B0718-3 and B0767-2: Two late blight resistant potato clones

Two potato breeding selections were released by the United States Department of Agriculture, Agricultural Research Service on November 14, 1996, as germplasm that is highly resistant to current populations ofPhytophthora infestans (Mont.) deBary, the causal organism of potato late blight: B0718-3 and B0767-2. These two clones have been evaluated for their reaction toP. infestans in replicated plots at Presque Isle, Maine since 1990. B0718-3 is a netted white-skinned selection with good yields but low specific gravity. It is both male and female fertile and crosses readily with tetraploid Tuberosum. B0767-2 is a russet-skinned selection that is low yielding and low in specific gravity. It has not been successfully crossed; however, it may have some potential use in somatic hybridizations. Neither B0718-3 nor B0767-2 process into acceptable colored chips. Genetic material of this release has been deposited in the National Plant Germplasm System where it is available for research purposes, including development and commercialization of new cultivars.

Another Look at the Origin of Late Blight of Potatoes, Tomatoes, and Pear Melon in the Andes of South America.

HomePlant DiseaseVol. 81, No. 6Another Look at the Origin of Late Blight of Potatoes, Tomatoes, and Pear Melon in the Andes of South America PreviousNext Special Report OPENOpen Access licenseAnother Look at the Origin of Late Blight of Potatoes, Tomatoes, and Pear Melon in the Andes of South AmericaZ. Gloria Abad and Jorge A. AbadZ. Gloria AbadCorresponding author: Z. Gloria Abad E-mail: E-mail Address: [email protected]Search for more papers by this author and Jorge A. AbadSearch for more papers by this authorAffiliationsAuthors and Affiliations Z. Gloria Abad Jorge A. Abad , Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616 Published Online:23 Feb 2007https://doi.org/10.1094/PDIS.1997.81.6.682AboutSectionsPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat DetailsFiguresLiterature CitedRelated Vol. 81, No. 6 June 1997SubscribeISSN:0191-2917e-ISSN:1943-7692 Metrics Article History Issue Date: 25 Jan 2008Published: 23 Feb 2007Accepted: 20 Mar 1997 Pages: 682-688 Information© 1997 The American Phytopathological SocietyPDF downloadCited byGlobal historic pandemics caused by the FAM-1 genotype of Phytophthora infestans on six continents11 June 2021 | Scientific Reports, Vol. 11, No. 1The Importance of Mycological and Plant Herbaria in Tracking Plant Killers4 February 2020 | Frontiers in Ecology and Evolution, Vol. 7Two different R gene loci co-evolved with Avr2 of Phytophthora infestans and confer distinct resistance specificities in potatoStudies in Mycology, Vol. 89, No. 1Modulation of Human Immune Response by Fungal Biocontrol Agents3 February 2017 | Frontiers in Microbiology, Vol. 8Historic Late Blight Outbreaks Caused by a Widespread Dominant Lineage of Phytophthora infestans (Mont.) de Bary28 December 2016 | PLOS ONE, Vol. 11, No. 12Genomic Characterization of a South American Phytophthora Hybrid Mandates Reassessment of the Geographic Origins of Phytophthora infestans17 November 2015 | Molecular Biology and Evolution, Vol. 33, No. 2Mitochondrial genome sequences reveal evolutionary relationships of the Phytophthora 1c clade species10 March 2015 | Current Genetics, Vol. 61, No. 4Molecular Analysis of Phytophthora infestans (Mont.) de Bary from Chapingo, Mexico. Phylogeographic Referential25 February 2014 | American Journal of Potato Research, Vol. 91, No. 5Estudios preliminares para la producción de un biosurfactante bacteriano activo contra Phytophthora infestans (Mont.) De Bary7 January 2008 | Ciencia & Tecnología Agropecuaria, Vol. 8, No. 2Tuber Blight Development in Potato Cultivars in Response to Different Genotypes of Phytophthora infestans30 July 2013 | Journal of Phytopathology, Vol. 162, No. 1Worldwide Migrations, Host Shifts, and Reemergence of Phytophthora Infestans , the Plant Destroyer26 October 2013Ecofriendly control of potato late blight causative agent and the potential role of lactic acid bacteria: a review4 August 2012 | Applied Microbiology and Biotechnology, Vol. 96, No. 1Caracterización de razas fisiológicas de Phytophthora infestans (Mont.) de Bary en lulo (Solanum quitoense Lam.)8 June 2012 | Corpoica Ciencia y Tecnología Agropecuaria, Vol. 13, No. 1Species Tree Estimation for the Late Blight Pathogen, Phytophthora infestans, and Close Relatives17 May 2012 | PLoS ONE, Vol. 7, No. 5Characterization of Phytophthora infestans Populations in Colombia: First Report of the A2 Mating TypeAngela M. Vargas, Lina M. Quesada Ocampo, Maria Catalina Céspedes, Natalia Carreño, Adriana González, Alejandro Rojas, A. Paola Zuluaga, Kevin Myers, William E. Fry, Pedro Jiménez, Adriana J. Bernal, and Silvia Restrepo4 December 2008 | Phytopathology®, Vol. 99, No. 1Phylogenetic relationships of Phytophthora andina , a new species from the highlands of Ecuador that is closely related to the Irish potato famine pathogen Phytophthora infestans20 January 2017 | Mycologia, Vol. 100, No. 4An Andean origin of Phytophthora infestans inferred from mitochondrial and nuclear gene genealogies27 February 2007 | Proceedings of the National Academy of Sciences, Vol. 104, No. 9The Northern Ireland Phytophthora infestans population 1998-2002 characterized by genotypic and phenotypic markersPlant Pathology, Vol. 55, No. 3Genetic diversity and host differentiation among isolates of Phytophthora infestans from cultivated potato and wild solanaceous hosts in PeruPlant Pathology, Vol. 54, No. 6Genetic Diversity of Phytophthora infestans sensu lato in Ecuador Provides New Insight Into the Origin of This Important Plant PathogenN. E. Adler, L. J. Erselius, M. G. Chacón, W. G. Flier, M. E. Ordoñez, L. P. N. M. Kroon, and G. A. Forbes5 February 2007 | Phytopathology®, Vol. 94, No. 2Phenotypic and genotypic diversity of Phytophthora infestans populations in Scotland (1995-97)Plant Pathology, Vol. 52, No. 2Tracking historic migrations of the Irish potato famine pathogen, Phytophthora infestansMicrobes and Infection, Vol. 4, No. 13Genetic Structure of Peruvian Populations of Phytophthora infestansWillmer G. Perez, J. Soledad Gamboa, Yesenia V. Falcon, Mario Coca, Rubi M. Raymundo, and Rebecca J. Nelson22 February 2007 | Phytopathology®, Vol. 91, No. 10One stop mycologyMycological Research, Vol. 103, No. 3The Importance of Archival and Herbarium Materials in Understanding the Role of Oospores in Late Blight Epidemics of the PastJean B. Ristaino22 February 2007 | Phytopathology®, Vol. 88, No. 11Re-emergence of Potato and Tomato Late Blight in the United StatesWilliam E. Fry and Stephen B. Goodwin22 February 2007 | Plant Disease, Vol. 81, No. 12

Aggressiveness and other factors relating to displacement of populations of Phytophthora infestans in England and Wales

Mating type, in vitro sensitivity to the phenylamide fungicide metalaxyl, and mitochondrial (mtDNA) haplotype were determined in some or all of 618 isolates of Phytophthora infestans from the years between 1978 and 1995. A2 mating type occurred infrequently in most but not all years and insensitivity to metalaxyl increased over time. After 1982, the mtDNA Ib haplotype was largely replaced (except for one isolate in 1986 and one in 1995) by two new haplotypes, Ia and IIa. Type Ia was much more common than type IIa. Approximately one quarter of these isolates (165) were compared using two components of fitness associated with aggressiveness (infection frequency × number of sporangia per lesion) on detached leaves of cultivars Maris Piper, Cara and Stirling, which were chosen as exhibiting increasing levels of race non-specific resistance. Isolates were compared with three ‘standard’ isolates of low, intermediate or high aggressiveness, and the data standardised for comparison between experiments. On cvs. Cara and Stirling, but not on Maris Piper, mtDNA Ia and IIa haplotypes were more aggressive than type Ib in several separate experiments. Similarly, metalaxyl sensitive phenotypes were more aggressive than insensitive phenotypes on Cara and Stirling but not on Maris Piper. The displacement of mtDNA type Ib by types Ia and IIa over this period may have been a result of the lower aggressiveness and lack of complete insensitivity to metalaxyl in type Ib isolates.

The origin of Phytophthora infestans populations present in Europe in the 1840s: a critical review of historical and scientific evidence

A critical review of available historical and scientific evidence related to the question of the origin of the first European populations of the potato late blight pathogen Phytophthora infestans is presented. It shows that the bases for the current theories of a direct introduction of the fungus into Europe and North America from either a Mexican or an Andean centre of origin and of diversity are questionable. An alternative theory, involving a three‐step process: (i) migration from central Mexico to South America several centuries ago; (ii) migration from South America to the US in 1841–1842, and (iii) migration to Europe from either South America, US or both in 1843–1844, is in good agreement with both the historical records and with the genetics and structure of current populations.

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.

Direct Detection of Gene Flow and Probable Sexual Reproduction ofPhytophthora infestansin Northern North America

Gene flow into populations of Phytophthora infestans in the United States and Canada was detected directly in 1992 and 1993. In total, 384 isolates were analyzed for mating type and dilocus allozyme genotype. Of these, 130 isolates were analyzed for nuclear DNA fingerprint variation using probe RG57. Only 11 multilocus genotypes were identified, nine of which had never been detected previously in the United States or Canada. The A2 mating type and isolates with the Glucose-6-phosphate isomerase 111 allele increased dramatically in frequency compared with samples in previous years. The only likely explanation for these changes is that there was massive immigration from outside the United States or Canada. Northwestern Mexico was the probable source population for these recent migrations; the two most common migrant genotypes were probably direct clonal descendants of isolates collected previously in northwestern Mexico. Populations in 118 of the 122 sites sampled appeared to be monomorphic. Thus, the genetic structure of epidemic populations in most sites was very simple and probably exclusively asexual. The two mating types were usually separated geographically. However, both mating types were detected together in three fields in British Columbia. Some isolates from these fields had unique genotypes that could have arisen by sexual recombination between the most common A1 and A2 genotypes in British Columbia. This is the first evidence for the probable occurrence of sexual reproduction of P. infestans in North America north of Mexico

Identification of mating types and metalaxyl resistance in North American populations ofPhytophthora infestans

The A2 mating type ofPhytophthora infestans was first reported in the United States in 1990. Concurrently,P. infestans strains resistant to metalaxyl ere found in the Pacific Northwest. Collaborative surveys were undertaken during 1991–1993 to investigate the frequency of occurrence of A2 mating types and metalaxyl resistant strains in populations ofP. infestans isolated from outbreaks of late blight in potato and tomato crops in North America.In vitro testing indicated that isolates from the northeastern U.S. and Atlantic Canada were primarily (52/55) metalaxyl sensitive and all were A1 mating types. Among 85 isolates from late blight epidemics in Florida and Texas, greater than 61% were both metalaxyl resistant and A2 mating type. Metalaxyl resistance and A2 mating types were identified also in a few tomato isolates from North Carolina. Although the majority of 134 isolates from the Pacific Northwest (British Columbia and Washington) were metalaxyl resistant, only 2 isolates from Washington were A2 mating types. Among 111 isolates from 2 sites in central Mexico, 63% and 77% were both metalaxyl resistant and A2 mating types. The data indicate also a higher frequency of metalaxyl resistance in A2 isolates, than in A1 isolates, among isolates from Florida and Texas. Highest metalaxyl resistance levels were found, however, in A1 isolates from California, where no A2 isolates were recovered.

Increased Genotypic Diversity via Migration and Possible Occurrence of Sexual Reproduction ofPhytophthora infestansin Poland

Genetic changes in populations of Phytophthora infestans in Poland were determined by analyzing 247 isolates collected from 1985 through 1991. All isolates collected from 1985 through 1987 were Al mating type and consisted of a single clonal lineage based on allozyme and DNA fingerprint analyses. The appearance of new genotypes in Poland, presumably due to migration, was first detected in 1988 with the identification of the A2 mating type, three allozyme, and a number of DNA fingerprint alleles that had not been detected previously. This migration resulted in dramatic changes. Gene and genotype diversity and the frequency of uniquely detected genotypes all increased beginning in 1988, and reached a peak in 1990 [...]

Clonal Diversity and Genetic Differentiation ofPhytophthora infestansPopulations in Northern and Central Mexico

Two Phytophthora infestans populations in northern Mexico (north-western and northeastern) were analyzed for genetic variation by use of mating type, two allozyme loci, and two DNA fingerprinting probes, and were compared to two populations from central Mexico. The genetic structure of the populations varied widely, even though both mating types occurred in all locations sampled. There was very little genetic diversity in northwestern Mexico; four genotypes were detected among 88 isolates tested, and only two of these were common. All A2 isolates in this population appeared to represent a single clone and were exclusively on potatoes, whereas the A1 isolates were primarily on tomatoes (...)

A second world‐wide migration and population displacement of Phytophthora infestans?

The appearance of the A2 mating type (previously restricted to central Mexico) in Europe during the 1980s prompted an investigation of the genetic make‐up of European populations using allozyme loci as genetic markers. The investigation shows that major genetic changes have occurred in populations of Phytophthora infestans in the Netherlands, Poland, and the British Isles. It now appears that a new type of population has been introduced into several locations, and has displaced or is displacing the original populations in these locations. The new and old population types are characterized by unique allozyme alleles and genotypes. The mechanism for displacement of the‘old’by the‘new’population is not yet known.

Synergistic interactions between phenylamide fungicides and respiration inhibitors in controlling phenylamide-resistant populations ofPhytophthora infestans on potato tuber disks

The efficacy of mixtures of respiration inhibitors and phenylamide fungicides (oxadixyl and metalaxyl) in controlling late blight was investigated using potato tuber disks. Results showed that uncoupling agents(e.g. DNP), inhibitors of ATP formation(e.g. oligomycin), ionophores(e.g, valinomycin) and inhibitors that block specific carriers in the electron transport chain(e.g. rotenone, antimycin A) increased significantly the control efficacy of both metalaxyl and oxadixyl towards populations ofPhytophthora infestons when applied to tuber slices pretreated with sublethal doses of both fungicides. When applied alone at the same doses, respiration inhibitors did not inhibit the development of the late blight fungus. Increased control efficacy of mixtures was not noted against phenylamide-sensitive populations of the fungus.

Monitoring of Virulence in Populations of Pbytophthora infestans Using a Schwarzbach Spore Trap

AbstractA modified Sehwarzbach jet spore trap was used to sample Phytophthora infestans populations in small crops of potato within polyethylene tunnels and in the field. Airborne sporangia were eaptured in the trap and deposited upon potato leaf discs, of a cultivar lacking race‐specific resistance genes, contained within the trap. Virulence characters of isolates captured in the spore trap were determined by subculturing the isolates on leaf discs of a range of potato genotypes with race‐specific resistance (R‐genes), incubating them and then recording compatible and mcompatible reactions, reactions. For four isolates of P. infestans the levels of sporulation on the differential leaf discs were noted and although cultivar × isolate interactions were inconsistent, variable light and temperature conditions had no significant effect on the reactions of the cultivars.