Characterization of isolates of Phytophthora infestans from Hungary.

Bakonyi J, Laday M, Dula T and Ersek T. 2002. European Journal of Plant Pathology 108:139–146. © Kluwer Academic Publishers 2002.

A total of 36 single-lesion isolates were collected from 9 crops of potato and 13 of tomato in different regions of Hungary in the past decade, particularly in 1998. These were analyzed for mating type, sensitivity to metalaxyl, allozyme genotype at glucose-6-phosphate isomerase and peptidase loci and genotype at 24 loci detected using the multilocus RFLP probe RG57. The ratios of the mating types A1 to A2 were 8:9 and 4:15 among isolates recovered from potato and tomato, respectively. Resistance to metalaxyl was found more frequently among isolates from potato and in the A1 mating type. The populations were not clearly differentiated on the basis of host origin. All isolates were homozygous (100/100) at the locus for glucose-6-phosphate isomerase. Unlike in other European countries, the most common peptidase allele was 96. Genotypes at the peptidase locus were 96/96 (50%), 96/100 (27.7%) and 100/100 (16.6%). In addition, one isolate from 1993 and another from 1998, were defined as 83/96, a genotype that had not been described elsewhere. The 18 RG57 fingerprints that were observed among 36 isolates, with one exception, seem to be unique to Europe. On the basis of combined genotypic traits, 20 multilocus genotypes were designated. These data, which reveal a remarkable variability with unique genotypes of the late blight pathogen, suggest that migration and sexual and/or asexual recombination have a role in the recent evolution of the pathogen in Hungary.

Corresponding author: J. Bakonyi

email: jbak(at)nki.hu

Formation of Phytophthora infestans oospores in nature on tubers in Central Mexico.

Fernández-Pavía S P, Grünwald N J and Fry W E. 2002. Plant Disease 86:73.

Oospore formation by Phytophthora infestans in nature has been detected on potato leaflets in central Mexico (1), but there are no reports of oospore formation on tubers. A severe late blight epidemic occurred in Calimaya, Mexico, in fields where potato cv. Alpha was planted during the summer of 2000. Yield was reduced despite numerous applications of fungicide. Four hundred potato tubers left in the field were collected from the upper 10 cm of soil and examined for late blight symptoms. Tubers with soft and dry rot symptoms were observed, but symptoms of pink rot (Phytophthora erythroseptica) were not found. Four percent of the tubers showed late blight symptoms. Sections of 10 tubers with late blight symptoms were air-dried for 2 weeks in the laboratory and homogenized with a mortar and pestle. Glycerol was added to the homogenized tissue and observed microscopically. Aplerotic oospores (10 to 15 oospores per tuber) with amphyginous antheridia typical of P. infestans were observed. P. mirabilis morphologically similar to P. infestans is present in the area but it does not infect potato tubers. The number of oospores observed in our tuber sample was much lower than the number reported on leaflets (>1 ,000 oospores per leaflet) in the Toluca Valley. Low numbers of oospores have been reported on tubers artificially inoculated with P. infestans under field conditions (2). Infected tubers left in the field may act as a source of primary inoculum. To our knowledge, this is the first report of oospores of P. infestans found on tubers in Mexico under natural field conditions.

(1) Gallegly M E and Galindo J. Phytopathology 48:274, 1958 (2) Levin A el al. Phytopathology 91:579, 2001.

Corresponding author: N J Grünwald

email: njg7(at)cornell.edu

Differential interaction of Phytophthora infestans on tubers of potato cultivars with different levels of blight resistance

Flier W G, Turkensteen L J, van den Bosch G B M, Vereijken P F G and Mulder A. 2001. Plant Pathology 50:292–301.

Differential interactions in tuber blight attack between potato cultivars and Phytophthora infestans isolates were studied using whole tuber and tuber slice assays. Tuber blight incidence and severity were studied in a whole tuber assay, whilst necrosis and mycelium coverage were evaluated in a tuber slice assay. The overall defence reaction of the potato cultivars tested varied considerably. Cultivars like Kartel and Producent showed resistant reactions, whilst Bintje and, to a lesser extent, Astarte reacted more susceptibly after inoculation with aggressive strains of P. infestans. A highly significant cultivar by year interaction was observed when tuber blight incidence was evaluated in two successive years. Differential responses were revealed by changing ranked order of cultivars after exposure to aggressive isolates of P. infestans. The results show that cultivar by isolate interactions existed for all components of tuber blight resistance studied. The quantitative nature of the observed resistance responses suggests the presence of quantitative trait loci governing resistance to tuber blight. The consequences of differential interactions in relation to the stability of tuber resistance are discussed

Corresponding author: W G Flier

email: w.g.flier(at)plant.wag-ur.nl

Comparative analysis of quantitative trait loci for foliage resistance to Phytophthora infestans in tuber-bearing Solanum species.

Simko I. 2002. American Journal of Potato Research 79:125–132.

Previously published data for potato foliage resistance to late blight were examined in a combined analysis to determine how the genetic backgrounds of the host and pathogen affect the occurrence of resistance QTLs. Data from 19 diploid populations and one tetraploid population that originated from at least 12 different tuberbearing Solanum species were included. Comparative analysis across all populations revealed three highly active genomic regions on the distal parts of chromosomes 3, 4, and 5. The region most consistently detected that conferred foliage resistance in Solanum was located on chromosome 5, near marker locus GP21. A previously identified cluster of three race-specific R-genes on chromosome 11 was not associated with polygenic resistance. Statistical examination of active QTLs indicates that congruence among QTLs is significantly affected by both genetic relatedness of mapping populations and race of Phytophthora infestans used for resistance tests. The proportion of congruent QTLs for late blight resistance decreased from 0.52 detected in related Solanum populations tested with the same race of Phytophthora infestans to 0.20 found in unrelated mapping populations tested with different races of Phytophthora infestans. Analysis of resistance mapping tests provided statistical evidence for the occurrence of race-specific QTLs in tuber-bearing Solanum. The highly conserved genomic regions identified in the comparative analysis are likely to be good candidates for gene cloning or marker assisted selection in potato breeding programs.

Corresponding author: I Simko

email: simkoi(at)ba.ars.usda.gov

Production of phytoalexins, glycoalkaloids and phenolics in leaves and tubers of potato cultivars with different degrees of field resistance after infection with Phytophthora infestans.

Andreu A, Oliva C, Distel S and Daleo G. 2001. Potato Research: 44:1–9.

The kinetics of accumulation of phytoalexins, glycoalkaloids and phenolics was studied in two potato cultivars differing in their degrees of field resistance when infected with Phytophthora infestans. Tuber slices and leaves of cvs Pampeana INTA (high degree of field resistance, free of R genes) and Bintje (susceptible) were infected with race C (complex race 1, 3, 5, 7, 11) of Phytophthora infestans. Phytoalexins and phenolics accumulated in tuber and leaf tissues, which had been inoculated. The levels of these compounds in the susceptible cv. Bintje were relatively low and similar to those found before inoculation. Leaves of cv. Pampeana INTA had a very high glycoalkaloid content, suggesting that glycoalkaloids may play a role in protection of leaves against the fungus. However, we could find no correlation between resistance and glycoalkaloid content of tubers. Our results suggest a major role of phytoalexins, phenolics and glycoalkaloids in the complex mechanisms of field resistance.

Corresponding author: S Distel

email: sdistel(at)tutopia.com

Physical mapping across an avirulence locus of Phytophthora infestans using a highly representative, large-insert bacterial artificial chromosome library

Whisson S C, van der Lee T, Bryan G J, Waugh R, Govers F and Birch P R J. 2001. Molecular Genetics and Genomics 226:289–295. ©Springer-Verlag 2001.

The oomycete plant pathogen Phytophthora infestans is the causal agent of late blight, one of the most devastating diseases of potato worldwide. As part of efforts to clone avirulence genes (Avr) genes and pathogenicity factors from P. infestans, we have constructed a bacterial artificial chromosome (BAC) library from an isolate containing six Avr genes. The BAC library comprises clones with an average insert size of 98 kb and represents an estimated 10 genome equivalents. A three-dimensional pooling strategy was developed to screen the BAC library for amplified fragment length polymorphism (AFLP) markers, as this type of marker has been extensively used in construction of a P. infestans genetic map. Multiple positive clones were identified for each AFLP marker tested. The pools were used to construct a contig of 11 BAC clones in a region of the P. infestans genome containing a cluster of three avirulence genes. The BAC contig is predicted to encompass the Avr11 locus but mapping of the BAC ends will be required to determine if the Avr3 and Avr10 loci are also present in the BAC contig. These results are an important step toward the positional cloning of avirulence genes from P. infestans, and the BAC library represents a valuable resource for large-scale studies of oomycete genome organization and gene content.

Corresponding author: S C Whisson

email: swhiss(at)scri.sari.ac.uk

A ß-glucosidase/xylosidase from the phytopathogenic oomycete Phytophthora infestans.

Brunner F, Wirtz W, Rose J K C, Darvill A G, Govers F, Scheel D and Nürnberger T. 2002. Phytochemistry 59:689–696. © Elsevier Science 2002.

An 85-kDa ß-glucosidase/xylosidase (BGX1) was purified from the axenically grown phytopathogenic oomycete Phytophthora infestans. The bgx1 gene encodes a predicted 61-kDa protein product which, upon removal of a 21 amino acid leader peptide, accumulates in the apoplastic space. Extensive N-mannosylation accounts for part of the observed molecular mass difference. BGX1 belongs to family 30 of the glycoside hydrolases and is the first such oomycete enzyme deposited in public databases. The bgx1 gene was found in various Phytophthora species, but apparently is absent in species of the related genus, Pythium. Despite significant sequence similarity to human and murine lysosomal glucosylceramidases, BGX1 demonstrated neither glucocerebroside nor galactocerebroside-hydrolyzing activity. The native enzyme exhibited glucohydrolytic activity towards 4-methylumbelliferyl (4-MU) ß-D-glucopyranoside and, to a lesser extent, towards 4-MU- ß-D-xylopyranoside, but not towards 4-MU-a-D-glucopyranoside. BGX1 did not hydrolyze carboxymethyl cellulose, cellotetraose, chitosan or xylan, suggesting high substrate specificity and/or specific cofactor requirements for enzymatic activity.

Corresponding author: T Nürnberger

email: tnuernbe(at)ipb-halle.de

These abstracts were reprinted with the kind permission of Kluwer Academic Publishers (www.wkap.nl) the American Phytopathological Society (http://www.apsnet.org ), the British Society for Plant Pathology /Blackwell Science (http://www.blacksci.co.uk/uk/socoth.htm), the Potato Association of America (http://www.ume.maine.edu/PAA), the European Association for Potato Research (http://www.dpw.wageningen-ur.nl/eapr/), Springer-Verlag GmbH & Co. (http://www.springer.de/), Elsevier Science (http://www.elsevier.com/locate/phytochem).