Phenotypic and Genotypic Characterization of Phytophthora infestans Isolates Associated with Tomato and Potato Crops in Colombia.

Late blight disease, caused by the plant pathogen Phytophthora infestans, is one of the major threats for tomato and potato crops. Monitoring the populations of P. infestans is important to determine if there are changes in the sensitivity to fungicides and host preference. In this study, microsatellite markers and mitochondrial haplotypes were used to assess the genotype of isolates of P. infestans collected from tomato and potato plants in Colombia. Furthermore, sensitivity to the three fungicides cymoxanil (penetrant fungicide), mefenoxam, and fluopicolide (systemic fungicides), and tomato-potato host preference, were evaluated. Mitochondrial haplotyping showed that isolates collected on tomato were from the genetic groups Ia and Ib, while isolates collected on potatoes belonged to group IIa. Microsatellite analyses showed that isolates from tomato form two groups, including the Ib mitochondrial haplotype (which is genetically close to the US-1 clonal lineage) and the Ia haplotype (related to the EC-3 lineage), whereas Colombian isolates from potato formed a separate group. Furthermore, differences in sensitivity to fungicides were observed. Eighty-one percent of the isolates tested were resistant to mefenoxam with an EC50 >10 µg ml-1. Forty-two percent of the isolates showed an intermediate resistance to cymoxanil. The EC50 values ranged between 1 and 10 µg ml-1. For fluopicolide, 90% of the isolates were sensitive, with EC50 <1 µg ml-1. Host preference assays showed that potato isolates infected both host species. Thus, isolates that infect potatoes may pose a risk for tomato crops nearby.

Differential Susceptibility of Tree Tomato (Solanum betaceum) Cultivars to Late Blight Caused by Phytophthora betacei.

Host-pathogen interactions of a new species of Phytophthora, causal agent of late blight of tree tomato (Solanum betaceum Cav.), identified as Phytophthora betacei, were investigated with four different cultivars. Thirty-six P. betacei isolates, collected from southern Colombia between 2008 and 2009, were used to inoculate common tree tomato cultivars, Común, Híbrido, Injerto, and Holandés. Data on incubation and latent periods as well as infection efficiency, lesion development, and total sporulation were collected via detached leaf assays. Significant differences in susceptibility, based on the parameters measured, were observed. Común was the most susceptible cultivar, followed by Injerto, Híbrido, and Holandés. The mean incubation period was lowest for Común at 125.6 h post-inoculation (hpi) and highest for Híbrido at 139.4 hpi. No significant differences in latent period were observed. All 36 isolates produced necrotic lesions on Común, and 33, 24, and 21 caused infection on Injerto, Híbrido, and Holandés, respectively. Two isolates were able to cause infection only on Común, and 13 isolates were able to infect all four cultivars. Infection efficiency was significantly higher for the cultivar Común, followed by Injerto, Híbrido, and Holandés. Average lesion size was larger on Común than on any other cultivar. An inverse relationship of lesion size and total sporulation was observed. Común had significantly lower total sporulation than Híbrido and Holandés, which had the smallest average lesion sizes. These data show variation in pathogenicity of P. betacei isolates, under controlled conditions, and differential susceptibility of four distinct S. betaceum cultivars.

Phytophthora infestans Sporangia Produced in Culture and on Tomato Leaflet Lesions Show Marked Differences in Indirect Germination Rates, Aggressiveness, and Global Transcription Profiles.

Sporangia of Phytophthora infestans from pure cultures on agar plates are typically used in lab studies, whereas sporangia from leaflet lesions drive natural infections and epidemics. Multiple assays were performed to determine if sporangia from these two sources are equivalent. Sporangia from plate cultures showed much lower rates of indirect germination and produced much less disease in field and moist-chamber tests. This difference in aggressiveness was observed whether the sporangia had been previously incubated at 4°C (to induce indirect germination) or at 21°C (to prevent indirect germination). Furthermore, lesions caused by sporangia from plates produced much less sporulation. RNA-Seq analysis revealed that thousands of the >17,000 P. infestans genes with a RPKM (reads per kilobase of exon model per million mapped reads) >1 were differentially expressed in sporangia obtained from plate cultures of two independent field isolates compared with sporangia of those isolates from leaflet lesions. Among the significant differentially expressed genes (DEGs), putative RxLR effectors were overrepresented, with almost half of the 355 effectors with RPKM >1 being up- or downregulated. DEGs of both isolates include nine flagellar-associated genes, and all were down-regulated in plate sporangia. Ten elicitin genes were also detected as DEGs in both isolates, and nine (including INF1) were up-regulated in plate sporangia. These results corroborate previous observations that sporangia produced from plates and leaflets sometimes yield different experimental results and suggest hypotheses for potential mechanisms. We caution that use of plate sporangia in assays may not always produce results reflective of natural infections and epidemics.

Phytophthora infestans RNA virus 2, a novel RNA virus from Phytophthora infestans, does not belong to any known virus group.

Phytophthora infestans is the causal agent of potato and tomato late blight. In this study, we obtained the complete genome sequence of a novel RNA virus from this plant pathogen, tentatively named "Phytophthora infestans RNA virus 2" (PiRV-2). The PiRV-2 genome is 11,170 nt in length 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. Phylogenetic analysis demonstrated that it did not cluster with any known virus group. We conclude that PiRV-2 belongs to a new virus family yet to be described. This virus was found to be faithfully transmitted through asexual reproduction.

The Irish potato famine pathogen Phytophthora infestans originated in central Mexico rather than the Andes.

Phytophthora infestans is a destructive plant pathogen best known for causing the disease that triggered the Irish potato famine and remains the most costly potato pathogen to manage worldwide. Identification of P. infestan's elusive center of origin is critical to understanding the mechanisms of repeated global emergence of this pathogen. There are two competing theories, placing the origin in either South America or in central Mexico, both of which are centers of diversity of Solanum host plants. To test these competing hypotheses, we conducted detailed phylogeographic and approximate Bayesian computation analyses, which are suitable approaches to unraveling complex demographic histories. Our analyses used microsatellite markers and sequences of four nuclear genes sampled from populations in the Andes, Mexico, and elsewhere. To infer the ancestral state, we included the closest known relatives Phytophthora phaseoli, Phytophthora mirabilis, and Phytophthora ipomoeae, as well as the interspecific hybrid Phytophthora andina. We did not find support for an Andean origin of P. infestans; rather, the sequence data suggest a Mexican origin. Our findings support the hypothesis that populations found in the Andes are descendants of the Mexican populations and reconcile previous findings of ancestral variation in the Andes. Although centers of origin are well documented as centers of evolution and diversity for numerous crop plants, the number of plant pathogens with a known geographic origin are limited. This work has important implications for our understanding of the coevolution of hosts and pathogens, as well as the harnessing of plant disease resistance to manage late blight.

Identification of the Dominant Genotypes of Phytophthora infestans in Canada Using Real-Time PCR with ASO-PCR Assays.

Phytophthora infestans, a pathogenic oomycete that is the causal agent of potato and tomato late blight, has devastating effects worldwide. The genetic composition of P. infestans populations in Canada has changed considerably over the last few years, with the appearance of several new genotypes showing different mating types and sensitivity to the fungicide metalaxyl. Genetic markers allowing for a rapid assessment of genotypes from small amounts of biological material would be beneficial for the early detection and control of this pathogen throughout Canada. Mining of the P. infestans genome revealed several regions containing single-nucleotide polymorphisms (SNP) within both nuclear genes and flanking sequences of microsatellite loci. Allele-specific oligonucleotide polymerase chain reaction (ASO-PCR) assays were developed from 14 of the 50 SNP found by sequencing. Nine optimized ASO-PCR assays were validated using a blind test comprising P. infestans and other Phytophthora spp. The assays revealed diagnostic profiles unique to each of the five dominant genotypes present in Canada. The markers developed in this study can be used with environmental samples such as infected leaves, and will contribute to the genomic toolbox available to assess the genetic diversity of P. infestans at the intraspecific level. For late blight management, early warning about P. infestans genotypes present in potato and tomato fields will help growers select the most appropriate fungicides and application strategies.

Evaluation of the BlightPro Decision Support System for Management of Potato Late Blight Using Computer Simulation and Field Validation.

The objective of this study was to evaluate the utility of the BlightPro decision support system (DSS) for late blight management using computer simulation and field tests. Three fungicide schedules were evaluated: (i) calendar-based (weekly) applications, (ii) applications according to the DSS, or (iii) no fungicide. Simulation experiments utilized 14 years of weather data from 59 locations in potato-producing states. In situations with unfavorable weather for late blight, the DSS recommended fewer fungicide applications with no loss of disease suppression; and, in situations of very favorable weather for late blight, the DSS recommended more fungicide applications but with improved disease suppression. Field evaluation was conducted in 2010, 2011, 2012, and 2013. All experiments involved at least two cultivars with different levels of resistance. DSS-guided and weekly scheduled fungicide treatments were successful at protecting against late blight in all field experiments. As expected, DSS-guided schedules were influenced by prevailing weather (observed and forecast) and host resistance and resulted in schedules that maintained or improved disease suppression and average fungicide use efficiency relative to calendar-based applications. The DSS provides an interactive system that helps users maximize the efficiency of their crop protection strategy by enabling well-informed decisions.

Metalaxyl Resistance in Phytophthora infestans: Assessing Role of RPA190 Gene and Diversity Within Clonal Lineages.

Prior work has shown that the inheritance of resistance to metalaxyl, an oomycete-specific fungicide, is complex and may involve multiple genes. Recent research indicated that a single nucleotide polymorphism (SNP) in the gene encoding RPA190, the largest subunit of RNA polymerase I, confers resistance to metalaxyl (or mefenoxam) in some isolates of the potato late blight pathogen Phytophthora infestans. Using both DNA sequencing and high resolution melt assays for distinguishing RPA190 alleles, we show here that the SNP is absent from certain resistant isolates of P. infestans from North America, Europe, and Mexico. The SNP is present in some members of the US-23 and US-24 clonal lineages, but these tend to be fairly sensitive to the fungicide based on artificial media and field test data. Diversity in the level of sensitivity, RPA190 genotype, and RPA190 copy number was observed in these lineages but were uncorrelated. Controlled laboratory crosses demonstrated that RPA190 did not cosegregate with metalaxyl resistance from a Mexican and British isolate. We conclude that while metalaxyl may be used to control many contemporary strains of P. infestans, an assay based on RPA190 will not be sufficient to diagnose the sensitivity levels of isolates.

Acquired Resistance to Mefenoxam in Sensitive Isolates of Phytophthora infestans.

The systemic fungicide mefenoxam has been important in the control of late blight disease caused by Phytophthora infestans. This phenylamide fungicide has a negative effect on the synthesis of ribosomal RNA; however, the genetic basis for inherited field resistance is still not completely clear. We recently observed that a sensitive isolate became tolerant after a single passage on mefenoxam-containing medium. Further analyses revealed that all sensitive isolates tested (in three diverse genotypes) acquired this resistance equally quickly. In contrast, isolates that were "resistant" to mefenoxam in the initial assessment (stably resistant) did not increase in resistance upon further exposure. However, there appeared to be a cost associated with acquired resistance in the initially sensitive isolates, in that isolates with acquired resistance grew more slowly on mefenoxam-free medium than did the same isolates that had never been exposed to mefenoxam. The acquired resistance of the sensitive isolates declined slightly with subsequent culturing on medium free of mefenoxam. To investigate the mechanism of acquired resistance, we employed strand-specific RNA sequencing. Many differentially expressed genes were genotype specific, but one set of genes was differentially expressed in all genotypes. Among these were several genes (a phospholipase "Pi-PLD-like-3," two ATP-binding cassette superfamily [ABC] transporters, and a mannitol dehydrogenase) that were up-regulated and whose function might contribute to a resistance phenotype.

Fungicide Sensitivity of U.S. Genotypes of Phytophthora infestans to Six Oomycete-Targeted Compounds.

Phytophthora infestans causes potato late blight, an important and costly disease of potato and tomato crops. Seven clonal lineages of P. infestans identified recently in the United States were tested for baseline sensitivity to six oomycete-targeted fungicides. A subset of the dominant lineages (n = 45) collected between 2004 and 2012 was tested in vitro on media amended with a range of concentrations of either azoxystrobin, cyazofamid, cymoxanil, fluopicolide, mandipropamid, or mefenoxam. Dose-response curves and values for the effective concentration at which 50% of growth was suppressed were calculated for each isolate. The US-8 and US-11 clonal lineages were insensitive to mefenoxam while the US-20, US-21, US-22, US-23, and US-24 clonal lineages were sensitive to mefenoxam. Insensitivity to azoxystrobin, cyazofamid, cymoxanil, fluopicolide, or mandipropamid was not detected within any lineage. Thus, current U.S. populations of P. infestans remained sensitive to mefenoxam during the displacement of the US-22 lineage by US-23 over the past 5 years.

A member of the virus family Narnaviridae from the plant pathogenic oomycete Phytophthora infestans.

A virus that has properties consistent with inclusion in the virus family Narnaviridae was described in Phytophthora infestans, the oomycete that caused the Irish potato famine. The genome of phytophthora infestans RNA virus 4 (PiRV-4) is 2,984 nt with short complementary terminal sequences and a single open reading frame predicted to encode an RNA-dependent RNA polymerase (RdRp) most closely related to saccharomyces cerevisiae narnavirus 20S (ScNV-20S) and ScNV-23S, the members of the genus Narnavirus, family Narnaviridae. This report constitutes the first description of a member of the family Narnaviridae from a host taxon outside of the kingdom Fungi.

Recent Genotypes of Phytophthora infestans in the Eastern United States Reveal Clonal Populations and Reappearance of Mefenoxam Sensitivity.

Isolates of Phytophthora infestans (n = 178) were collected in 2002 to 2009 from the eastern United States, Midwestern United States, and eastern Canada. Multilocus genotypes were defined using allozyme genotyping, and DNA fingerprinting with the RG-57 probe. Several previously described and three new mulitilocus genotypes were detected. The US-8 genotype was found commonly on commercial potato crops but not on tomato. US-20 was found on tomato in North Carolina from 2002 through 2007 and in Florida in 2005. US-21 was found on tomato in North Carolina in 2005 and Florida in 2006 and 2007. US-22 was detected on tomato in 2007 in Tennessee and New York and became widespread in 2009. US-22 was found in 12 states on tomato and potato and was spread on tomato transplants. This genotype accounted for about 60% of all the isolates genotyped. The US-23 genotype was found in Maryland, Virginia, Pennsylvania, and Delaware on both tomato and potato in 2009. The US-24 genotype was found only in North Dakota in 2009. A1 and A2 mating types were found in close proximity on potato and tomato crops in Pennsylvania and Virginia; therefore, the possibility of sexual reproduction should be monitored. Whereas most individuals of US-8 and US-20 were resistant to mefenoxam, US-21 appeared to be intermediately sensitive, and isolates of US-22, US-23, and US-24 were largely sensitive to mefenoxam. On the basis of sequence analysis of the ras gene, these latter three genotypes appear to have been derived from a common ancestor. Further field and laboratory studies are underway using simple sequence repeat genotyping to monitor current changes in the population structure of P. infestans causing late blight in North America.

Methyl esterase 1 (StMES1) is required for systemic acquired resistance in potato.

Whether salicylic acid (SA) plays a role in systemic acquired resistance (SAR) signaling in potato is currently unclear because potato, unlike tobacco and Arabidopsis, contains highly elevated levels of endogenous SA. Recent studies have indicated that the SA derivative methyl salicylate (MeSA) serves as a long-distance phloem-mobile SAR signal in tobacco and Arabidopsis. Once in the distal, uninfected tissue of these plant species, MeSA must be converted into biologically active SA by the esterase activity of SA-binding protein 2 (SABP2) in tobacco or members of the AtMES family in Arabidopsis. In this study, we have identified the potato ortholog of tobacco SABP2 (StMES1) and shown that the recombinant protein converts MeSA to SA; this MeSA esterase activity is feedback inhibited by SA or its synthetic analog, 2, 2, 2, 2'-tetra-fluoroacetophenone (tetraFA). Potato plants (cv. Désirée) in which StMES1 activity was suppressed, due to either tetraFA treatment or silencing of StMES1 expression, were compromised for arachidonic acid (AA)-induced SAR development against Phytophthora infestans. Presumably due to the inability of these plants to convert MeSA to SA, the SAR-defective phenotype correlated with elevated levels of MeSA and reduced expression of pathogenesis-related (PR) genes in the untreated distal tissue. Together, these results strongly suggest that SAR signaling in potato requires StMES1, its corresponding MeSA esterase activity, and MeSA. Furthermore, the similarities between SAR signaling in potato, tobacco, and Arabidopsis suggest that at least certain SAR signaling components are conserved among plants, regardless of endogenous SA levels.

Quantitative resistance to late blight from Solanum berthaultii cosegregates with R(Pi-ber): insights in stability through isolates and environment.

Genetic resistance is a valuable tool in the fight against late blight of potatoes but little is known about the stability and specificity of quantitative resistance including the effect of defeated major resistance genes. In the present study we investigated the effect of different isolates of Phytophthora infestans on the mode of action of R(Pi-ber), an R-gene originating from Solanum berthaultii. The experiments were conducted on progenies derived from two reciprocal inter-specific backcrosses of Solanum tuberosum and S. berthaultii. The plant-pathogen interaction was tested in diverse environments including field, greenhouse and growth chamber conditions. The R(Pi-ber) gene provided complete resistance against a US8 isolate of P. infestans in all trials. When isolates compatible with R(Pi-ber) were used for inoculation, a smaller, but significant resistance effect was consistently detected in the same map position as the R-gene. This indicates that this R-gene provides a residual resistance effect, and/or that additional resistance loci are located in this genomic region of chromosome X. Additional quantitative resistance loci (QRL) were identified in the analyzed progenies. While some of the QRL (such as those near TG130 on chromosome III) were effective against several isolates of the pathogen, others were isolate specific. With a single exception, the S. berthaultii alleles were associated with a decrease in disease severity. Resistance loci reported in the present study co-locate with previously reported R-genes and QRL to P. infestans and other pathogens.

Characterization of Phytophthora infestans populations in Colombia: first report of the A2 mating type.

Phytophthora infestans, the causal agent of late blight in crops of the Solanaceae family, is one of the most important plant pathogens in Colombia. Not only are Solanum lycopersicum, and S. tuberosum at risk, but also several other solanaceous hosts (Physalis peruviana, S. betaceum, S. phureja, and S. quitoense) that have recently gained importance as new crops in Colombia may be at risk. Because little is known about the population structure of Phytophthora infestans in Colombia, we report here the phenotypic and molecular characterization of 97 isolates collected from these six different solanaceous plants in Colombia. All the isolates were analyzed for mating type, mitochondrial haplotypes, genotype for several microsatellites, and sequence of the internal transcribed spacer (ITS) region. This characterization identified a single individual of A2 mating type (from Physalis peruviana) for the first time in Colombia. All isolates had an ITS sequence that was at least 97% identical to the consensus sequence. Of the 97 isolates, 96 were mitochondrial haplotype IIa, with the single A2 isolate being Ia. All isolates were invariant for the microsatellites. Additionally, isolates collected from S. tuberosum and P. peruviana (64 isolates) were tested for: aggressiveness on both hosts, genotype for the isozymes (glucose-6-phosphate isomerase and peptidase), and restriction fragment length polymorphism fingerprint pattern as detected by RG57. Isolates from S. tuberosum were preferentially pathogenic on S. tuberosum, and isolates from P. peruviana were preferentially pathogenic on P. peruviana. The population from these two hosts was dominated by a single clonal lineage (59 of 64 individuals assayed), previously identified from Ecuador and Peru as EC-1. This lineage was mating type A1, IIa for mitochondrial DNA, invariant for two microsatellites, and invariant for both isozymes. The remaining four A1 isolates were in lineages very closely related to EC-1 (named EC-1.1, CO-1, and CO-2). The remaining lineage (the A2 mating type) had characteristics of the US-8 lineage (previously identified in Mexico, the United States, and Canada). These results have important epidemiological implications for the production of these two crops in Colombia.

PiRV-2 stimulates sporulation in Phytophthora infestans.

Phytophthora infestans is the causal agent of potato and tomato late blight. This pathogen, which caused the Irish potato famine, is of profound historical significance and still poses a major threat in today's agroecosystems. Research on late blight epidemics usually focuses on pathogen virulence, host resistance, environmental factors and fungicide resistance. In this study, we examined the effect of PiRV-2, an RNA virus harbored by some P. infestans isolates, on its host. Comparing isogenic isolates with or without the virus demonstrated that the virus stimulated sporangia production in P. infestans. Transcriptome analysis suggested that it achieved sporulation stimulation likely through down-regulation of ammonium and amino acid intake in P. infestans. Survey of a limited P. infestans collection found PiRV-2 presence in most strains in the US-8 lineage, a very successful clonal lineage of P. infestans in North America. We suggest that PiRV-2 may affect the ecological fitness of P. infestans and thus could contribute to late blight epidemiology.

Insights into evolving global populations of Phytophthora infestans via new complementary mtDNA haplotype markers and nuclear SSRs.

In many parts of the world the damaging potato late blight pathogen, Phytophthora infestans, is spread as a succession of clonal lineages. The discrimination of genetic diversity within such evolving populations provides insights into the processes generating novel lineages and the pathways and drivers of pathogen evolution and dissemination at local and global scales. This knowledge, in turn, helps optimise management practices. Here we combine two key methods for dissecting mitochondrial and nuclear diversity and resolve intra and inter-lineage diversity of over 100 P. infestans isolates representative of key clonal lineages found globally. A novel set of PCR primers that amplify five target regions are provided for mitochondrial DNA sequence analysis. These five loci increased the number of mtDNA haplotypes resolved from four with the PCR RFLP method to 37 (17, 6, 8 and 4 for Ia, Ib, IIa, and IIb haplotypes, respectively, plus 2 Herb-1 haplotypes). As with the PCR RFLP method, two main lineages, I and II were defined. Group I contained 25 mtDNA haplotypes that grouped broadly according to the Ia and Ib types and resolved several sub-clades amongst the global sample. Group II comprised two distinct clusters with four haplotypes corresponding to the RFLP type IIb and eight haplotypes resolved within type IIa. The 12-plex SSR assay revealed 90 multilocus genotypes providing accurate discrimination of dominant clonal lineages and other genetically diverse isolates. Some association of genetic diversity and geographic region of contemporary isolates was observed; US and Mexican isolates formed a loose grouping, distinct from isolates from Europe, South America and other regions. Diversity within clonal lineages was observed that varied according to the age of the clone. In combination, these fine-scale nuclear and maternally inherited mitochondrial markers enabled a greater level of discrimination among isolates than previously available and provided complementary perspectives on evolutionary questions relating to the diversity, phylogeography and the origins and spread of clonal lineages of P. infestans.

Large sub-clonal variation in Phytophthora infestans from recent severe late blight epidemics in India.

The population structure of the Phytophthora infestans populations that caused the recent 2013-14 late blight epidemic in eastern India (EI) and northeastern India (NEI) was examined. The data provide new baseline information for populations of P. infestans in India. A migrant European 13_A2 genotype was responsible for the 2013-14 epidemic, replacing the existing populations. Mutations have generated substantial sub-clonal variation with 24 multi-locus genotypes (MLGs) found, of which 19 were unique variants not yet reported elsewhere globally. Samples from West Bengal were the most diverse and grouped alongside MLGs found in Europe, the UK and from neighbouring Bangladesh but were not linked directly to most samples from south India. The pathogen population was broadly more aggressive on potato than on tomato and resistant to the fungicide metalaxyl. Pathogen population diversity was higher in regions around the international borders with Bangladesh and Nepal. Overall, the multiple shared MLGs suggested genetic contributions from UK and Europe in addition to a sub-structure based on the geographical location within India. Our data indicate the need for improved phytosanitary procedures and continuous surveillance to prevent the further introduction of aggressive lineages of P. infestans into the country.

Resistance to Phytophthora infestans in Lycopersicon pennellii.

To determine if the desert tomato, Lycopersicon pennellii, possesses resistance to late blight, caused by Phytophthora infestans, two plant populations were analyzed. Resistance was identified through assessments of disease progress in an F2 mapping population (L. esculentum × L. pennellii) and in a series of introgression lines (L. pennellii into L. esculentum). Levels of resistance varied widely among individuals within each population. However, the response of individuals to different strains of P. infestans was consistent. In the mapping population, a quantitative trait locus (QTL) was detected near marker T1556 on chromosome 6. This QTL accounted for 25% of the phenotypic variance in the population. The occurrence of this QTL was confirmed from analysis of the introgression lines (ILs), where IL 6-2 (containing marker T1556) was the most resistant IL in 2002 and the second most resistant IL in 2001. The identification of an additional QTL for resistance to late blight in tomato will aid in the development of durable resistance to this devastating disease.

Selection for Fungicide Resistance Within a Growing Season in Field Populations of Phytophthora infestans at the Center of Origin.

ABSTRACT The central highlands of Mexico should provide an optimal testing ground for evaluating the potential threat of selection for resistance to fungicides in the population of Phytophthora infestans. We evaluated the hypotheses that exposure to the fungicides azoxystrobin, cymoxanil, dimethomorph, fluazinam, mancozeb, metalaxyl, and propamocarb hydrochloride would lead to (i) a shift in the sensitivity distributions (i.e., selection) and (ii) a lower genotypic diversity of the population. We compared populations from unsprayed plots with populations that had been exposed to several applications of each of the fungicides within a single field season. This study provides novel baseline data and shows that the Toluca valley P. infestans population has a wide range of sensitivities to the fungicides fluazinam, cymoxanil, dimethomorph, metalaxyl, and propamocarb. Directional selection toward resistance combined with a reduction in genetic diversity of the P. infestans population was observed only for the fungicide metalaxyl. The results obtained provide direct experimental support for continuing vigilance regarding further introductions of exotic strains of P. infestans into the United States.