Gene RB cloned from Solanum bulbocastanum confers broad spectrum resistance to potato late blight

 Song J., Bradeen J. M., Naess S. K., Raasch J. A., Wielgus S. M., Haberlach G. T., Liu, J., Kuang H., Austin-Phillips S., Buell C. R., Helgeson J. P. and Jiang J. 2003. PNAS:100: 9128-9133. © 2003 National Academy of Sciences, U.S.A.

 Late blight, caused by the oomycete pathogen Phytophthora infestans, is the most devastating potato disease in the world. Control of late blight in the United States and other developed countries relies extensively on fungicide application. We previously demonstrated that the wild diploid potato species Solanum bulbocastanum is highly resistant to all known races of P. infestans. Potato germplasm derived from S. bulbocastanum has shown durable and effective resistance in the field. Here we report the cloning of the major resistance gene RB in S. bulbocastanum by using a map-based approach in combination with a long-range (LR)-PCR strategy. A cluster of four resistance genes of the CC-NBS-LRR (coiled coil–nucleotide binding site–Leu-rich repeat) class was found within the genetically mapped RB region. Transgenic plants containing a LR-PCR product of one of these four genes displayed broad spectrum late blight resistance. The cloned RB gene provides a new resource for developing late blight-resistant potato varieties. Our results also demonstrate that LR-PCR is a valuable approach to isolate genes that cannot be maintained in the bacterial artificial chromosome system.

Corresponding authors: John P. Helgeson or Jiming Jiang; Email: jphelges(at)wisc.edu or  jjiang1(at)wisc.edu.

Resistance to Phytophthora infestans in somatic hybrids of Solanum nigrum L. and diploid potato

 Zimnoch-Guzowska E., Lebecka R., Kryszczuk A., Maciejewska U., Szczerbakowa A., and Wielgat B. 2003. Theoretical and Applied Genetics 107:43-48. © Springer-Verlag 2003.

 In breeding for resistance to late blight, (Phytophthora infestans) Mont. De Bary), an economically important disease affecting potatoes, the search for new sources of durable resistance includes the non-host wild Solanum species, The aim of this work was to evaluate the resistance to P. infestans in the somatic hybrids between S. nigrum L. and diploid potato clone ZEL-1136. Sixteen somatic hybrids, their fusion parents, and three standard potato cultivars were screened for resistance to P. infestans in two types of tests—on whole plants and detached leaves—with two highly aggressive and virulent isolates of P. infestans, US8 and MP322. In the whole plant assay, the foliage of the somatic hybrids showed no symptoms of infection, while the foliage of the potato fusion parent and the standard cultivars was infected with P. infestans, In the detached leaflet assay, the breaking-down of resistance of the S.nigrum L. parent and the variable response of individual hybrid clones were noted. Nine S. nigrum L. (+) ZEL-1136 hybrids showed a resistance that was significantly higher than that of S. nigrum, while six clones expressed a resistance to P. infestans similar to that of  S. nigrum. The results confirm the effective transfer of late blight resistance of  S. nigrum into its somatic hybrids with potato.

 Corresponding author: Ewa Zimnoch-Guzowska, Email: mlochow(at)ihar.edu.pl

Stability of partial resistance in potato cultivars exposed to aggressive strains of Phytophthora infestans.

 Flier W G, van den Bosch G B M and Turkensteen L J. 2003. Plant Pathology 52:326–337.

 Potato cultivars were evaluated for their resistance responses to aggressive strains of Phytophthora infestans in field and laboratory experiments. Analysis of variance revealed differential cultivar-by-isolate interactions for both foliar and tuber blight resistance. Differential responses occur as revealed by specific susceptibilities of cultivars to certain pathogen genotypes and changing rank order. In general, severity of late blight epidemics as observed in the haulms did not correlate well with foliar blight resistance ratings as presented in the National List of Recommended Potato Varieties. No significant correlation was found between tuber blight incidence under field conditions and the tuber blight rating in the National List. Also, there was no relation between the field and laboratory tuber blight resistance assessments. A significant association was demonstrated between late blight infection in the foliage and tuber blight incidence under field conditions. The presence of differential interaction, independent of R-gene-based resistance, indicates some adaptation of P. infestans to partial resistance and consequently adverse effects on the stability and durability of partial resistance to potato late blight.

 Corresponding author: Wilbert Flier, Email: Wilbert.flier(at)wur.nl

The effect of the presence of R-genes for resistance to late blight (Phytophthora infestans) of potato (Solanum tuberosum) on the underlying level of field resistance

Stewart H E, Bradshaw J E, Pande B. 2003. Plant Pathology 52:193–198.

The differential genotypes R1, R10 and R11, as originally defined by Black, were crossed with R-gene-free cultivars and the progenies divided into two subpopulations comprising those which had inherited the R-gene and those which had not. The underlying level of field resistance of the two groups was compared in a field trial in which they were inoculated with an isolate that could overcome the relevant R-genes. The R-gene-bearing group was significantly (P<0·001) more resistant than the R-gene-free group, with mean scores over four dates in 2000 of 4·86 and 4·09, respectively, on a 1–9 scale of increasing resistance, and of 4·10 and 2·35 on one date in 2001. However, the magnitude of the effect depended on the R-gene and the year of the trial. Data from a progeny of cv. Stirling (with an R-gene and a high level of field resistance) were examined and the same effect of an R-gene found. Fewer of the R-gene-bearing group of clones were highly susceptible, and more were resistant. The most resistant clones always bore the R-gene. It is concluded that increased resistance is conferred by the defeated R-gene or linked genes for field resistance.

Corresponding author: John E. Bradshaw, Email: jbrads(at)scri.sari.ac.uk

Characterization of isolates of Phytophthora infestans from southern and southeastern Brazil from 1998 to 2000

 Reis A, Smart C D, Fry W E, Maffia L A and Mizubuti E S G.2003. Plant Disease 87:896–900.

The population of Phytophthora infestans in Brazil was first characterized 12 years ago. In this research, isolates of P. infestans from potato (n = 184) and tomato (n = 267) collected in southern and southeastern Brazil were characterized to provide more detailed analysis of the current structure of the population. All 451 isolates were analyzed for mating type, and subsets of the isolates were analyzed for allozymes, restriction fragment length polymorphism fingerprint, mtDNA haplotypes, and metalaxyl resistance. Tomato isolates were all of A1 mating type, mtDNA Ib, and US-1 genotype or some variant within this clonal lineage. Of the potato isolates, 82% were A2 mating type, mtDNA IIa, BR-1 genotype, which is a new lineage of P. infestans. All A2 isolates were found on potato, whereas 91% of the A1 isolates were from tomato. A1 and A2 isolates were never found in the same field. The frequency of resistance to metalaxyl was higher in isolates from tomato (55%) than in isolates from potato (38%). After more than a decade of coexistence of isolates of the A1 and A2 mating types, the population was highly clonal, dominated by the BR-1 and US-1 clonal lineages.

 Corresponding author: Eduardo Mizubuti, Email: mizubuti(at)ufv.br

 Phenotypic and genotypic diversity of Phytophthora infestans populations in Scotland (1995-97)

Cooke D E L, Young V, Birch P R J, Toth R,.Gourlay F,.Day J P, Carnegie S F and Duncan J M. Plant Pathology 52:181–192.

 In a survey of Scottish potato late blight (Phytophthora infestans) populations from 1995 to 1997, nearly 500 isolates were collected from over 80 disease outbreaks in commercial potato crops and gardens/allotments. The isolates were characterized by mating type, resistance to the fungicide metalaxyl and almost 300 were examined by DNA-based AFLP fingerprinting. These data were examined alongside cropping details to determine the population structure in the context of existing disease management strategies. A1 and A2 mating type isolates were present in both commercial potato crops and gardens or allotments although they coexisted more frequently in the latter sites. One-fifth of the isolates collected were of the A2 mating type and the frequency was similar over the 3 years and amongst sites. In 1995 the proportions of isolates that were sensitive and resistant to metalaxyl were equal ( 40%) but, over the following 2 years, the frequency of resistant isolates decreased and that of intermediate isolates increased. The mating type response to metalaxyl differed markedly, with 52% of A1 and only 5% of A2 isolates being resistant. Considerable molecular diversity was observed, with over half of the isolates having unique AFLP patterns. Analysis of the molecular and phenotypic data revealed a broad clustering of the population into three groups. Many factors point to an A2 population restricted by its sensitivity to phenylamides. The majority of the A2 isolates were found in a single AFLP group, but the presence of mixed mating type samples, an increasing frequency of isolates of intermediate metalaxyl resistance and the extent of the AFLP diversity suggest occasional sexual recombination, and thus gene flow, between groups.

 Corresponding author: Dave Cooke, Email: dcooke(at)scri.sari.ac.uk

Characteristics of Phytophthora infestans Isolates from Uruguay

Deahl K L, Pagani MC, Vilaro F L, Perez F M, Morovec B and Cooke LR. 2003. European Journal of Plant Pathology 109:277–281. ©Kluwer Academic Publishers 2003.

 Isolates of Phytophthora infestans were obtained from late blighted plants from several potato-growing regions of Uruguay in 1998 and 1999. Of these, 25 representative isolates (4 from 1998, 21 from 1999) from the main potato-growing areas of the country, were characterised in terms of mating type, metalaxyl resistance, allozyme genotype, mitochondrial haplotype, RG57 fingerprint (1999 isolates only) and pathotype. All isolates proved to be A2 mating type, monomorphic and homozygous at the loci coding for glucose-6-phosphate isomerase and peptidase (Gpi 100/100, Pep 100/100) and to possess mitochondrial haplotype IIa. Metalaxyl-resistant isolates constituted 92% of the total. All the 1999 isolates possessed the same RG57 fingerprint, which was that previously reported as associated with the clonal lineage BR-1 from Brazil and Bolivia, which is also A2, Gpi 100/100, Pep 100/100. Most of the isolates displayed broad-spectrum virulence and five carried virulence to 10 of the 11 R genes tested despite the absence of R genes in commercially grown potato cultivars. It was concluded that the Uruguayan P. infestans isolates resembled isolates from neighbouring South American countries, notably Brazil, and belong to the new populations of the pathogen now predominant in many countries.

Corresponding author: Ken Deahl, Email: deahlk(at)ba.ars.usda.gov

Genetic variation among asexual progeny of Phytophthora infestans detected with RAPD and AFLP markers

 Abu-El Samen F M, Secor G A and Gudmestad N C. 2003. Plant Pathology 52:314–325.

 Genotypic variation among 32 single-zoospore isolates (SZI) of Phytophthora infestans, derived asexually from two hyphal-tip parental isolates (PI-105 and PI-1) of the US-8 genotype, was assessed with 80 random amplified polymorphic DNA (RAPD) primers and 18 amplified fragment length polymorphic DNA (AFLP) primer pairs. In previous investigations, the SZIs from parental isolate PI-105 showed high levels of virulence variability and were differentiated into 14 races, whereas the SZIs from PI-1 showed identical virulence to the parent. The purpose of this investigation was to determine if phenotypic variation observed among SZIs of P. infestans could be detected at the DNA level in these isolates. Polymorphism was detected with 51 RAPD primers and with all 18 AFLP primer pairs in PI-105 SZIs. In SZIs from PI-1, polymorphism was also detected with 25 RAPD primers and 17 AFLP primer pairs. Cluster analysis using the unweighted pair-group method with arithmetic averages (UPGMA) separated the SZIs from parent PI-105 into six virulence groups, 11 RAPD groups and three AFLP groups. Cluster analysis of PI-1 SZIs, which all belong to the same virulence group, differentiated them into four RAPD groups and six AFLP groups. No close correlation among RAPD, AFLP and virulence groups could be established within the two progenies of SZIs. Results of this study suggest that there is a considerable level of inherent genetic variability among SZIs derived asexually from the same parental isolate. The possible mechanisms and implications of this genetic variation are discussed.

 Corresponding author: Ned Gudmestad, Email: Neil.Gudmestad(at)ndsu.nodak.edu

 Identification and characterization of isolates of Phytophthora infestans using fatty acid methyl ester (FAME) profiles

 Larkin R P and Groves C L. 2003. Plant Disease 87:1233–1243.

 The utility of fatty acid profiles for characterization and differentiation of isolates of P. infestans was investigated. Two libraries of fatty acid methyl ester (FAME) profiles (one representing average genotype characteristics and one representing individual isolate characteristics) were established from at least eight replicate samples of each of 25 different isolates of P. infestans, including representative isolates of US-1, US-6, US-7, US-8, US-11, US-14, and US-17 genotypes. These libraries then were used to identify and characterize additional unknown isolates. Fatty acid profile characteristics also were compared with cultural and genetic characteristics of the isolates. FAME profiles for isolates of P. infestans were consistent over multiple extractions and distinctly different from profiles for isolates of other Phytophthora species, such as P. capsici and P. erythroseptica, as well as isolates of Pythium spp. and various other fungal groups. Overall, profiles from different isolates within the same genotype shared similar characteristics, although there was overlap among some genotypes. Incubation temperature, growth medium, and prolonged storage on agar media all significantly affected fatty acid profiles; however, when these conditions were kept constant, profiles were distinct, consistent, and reproducible over time. Isolate profiles were sufficiently specific that individual isolates could be distinctly identified by FAME profiles. In general, individual isolate characteristics were more determinant than genotype group characteristics, although genotype could be determined for most isolates tested. Results indicated that FAME profiles can be an additional tool useful for characterizing isolates and populations of P. infestans.

 Corresponding author: Bob Larkin, Email: bob.larkin(at)ars.usda.gov

These abstracts were reprinted with the kind permission of the National Academy of Sciences, U.S.A. (www.pnas.org), Springer-Verlag GmbH & Co. (www.springeronline.com), the British Society for Plant Pathology /Blackwell Publishing (www.blackwellpublishing.com) the American Phytopathological Society (www.apsnet.org ) and Kluwer Academic Publishers (www.wkap.nl).