Foliar aggressiveness of Northern Ireland isolates of Phytophthora infestans on detached leaflets of three potato cultivars

Carlisle D J, Cooke L R, Watson S and Brown A E. 2002. Plant Pathology 51:424-434.

The aggressiveness of 20 Northern Ireland single-lesion isolates of Phytophthora infestans was compared following their inoculation onto detached leaflets of three potato cultivars chosen on the basis of their differing levels of race-nonspecific resistance to late blight: Bintje (highly susceptible); Cara (moderately resistant); and Stirling (more resistant). Five isolates from outside Northern Ireland were included for comparative purposes: two from the Republic of Ireland; two from the USA (representing the US-i and US-8 clonal lineages); and one from Mexico. To control the variation between tests, a balanced incomplete block design was used, as opposed to either a complete block design or the method of inclusion of standard isolates, where such variation would have increased the error. Highly significant variation for disease para-meters, including latent period, infection frequency, area under the lesion expansion curve (AULEC) and sporulation capa-city, was found between isolates. These differences were much more marked on the cultivars exhibiting higher levels of race-nonspecific resistance. There was a significant interaction between isolate and cultivar for all parameters assessed and, overall, no one isolate was the most aggressive across all three potato cultivars. However, a group comprising seven of the 20 Northern Ireland isolates was consistently found to exhibit the highest levels of aggression towards all three cultivars for each of the disease parameters. These results demonstrate that significant variation for foliar aggressiveness exists within the Northern Ireland population of P. infestans, and indicate the importance of selecting appropriately aggressive isolates for evaluation of host resistance to late blight within breeding programmes.

Corresponding author: D J Carlisle

email: d.Carlisle(at)qub.ac.uk

Phytophthora infestans's 10-year truce with Holland: a long-term analysis of potato late-blight epidemics in the Netherlands

Zwankhuizen M J and Zadoks J C. 2002. Plant Pathology 51:413–423.

A sequence of 47 potato late-blight (Phytophthora infestans) epidemics in the Netherlands, from 1950 to 1996, was analysed using agronomic and meteorological variables. The intensity of annual epidemics was characterized by an index of disease intensity (DI, 0 = absence of late blight; 4 = severe epidemic). Three periods were identified, with average DIs of 2·9, 0 and 2, respectively. Period I (1950—68) had relatively regular epidemics; period 11(1969—78) was virtually blight free; and period III (1979—96) showed large variations in disease intensity. Disease-enhancing factors were number of days with precipitation, and number of hours with temperatures between 10 and 27 °C and relative humidity >90% during the growing season. Limiting factors were number of hours with temperatures >27 °C, and amount of global radiation. Linear discriminant analysis of DI using the blight status of the previous year and meteorological variables correctly classified up to 40 years out of 47 (87.0%), with five out of the six incorrectly classified years falling in period III. Blight status of the previous year and number of days with precipitation were important discriminating variables.

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

Corresponding author: J C Zadoks

email: jczadoks(at)euronet.nl

Genetic diversity in diploid and tetraploid late blight resistant potato germplasm

Bisognin D A and Douches D S. 2002. HortScience 37:178–183.

An understanding of the genetic relationship within potato germplasm is important to establish a broad genetic base for breeding purposes. The objective of this study was to assess the genetic diversity of potato (Solanum tuberosum subsp. tuberosurn Hawkes) germplasm that can be used in the development of cultivars with resistance to late blight caused by Phytophthora infestans (Mont.) de Bary. Thirty-three diploid and 27 tetraploid late blight resistant potato clones were evaluated for their genetic diversity based on 11 isozyme loci and nine microsatellites. A total of 35 allozymes and 42 polymorphic microsatellite fragments was scored for presence or absence. The germplasm was clustered based on the matrix of genetic similarities and the unweighted pair group means analysis of the isozyme and microsatellite data, which were used to construct a dendrogram using NTSYS-pc version 1.7. Twenty-three allozymes and DNA fragments were unique to the wild species. The diploid Solanum species S. berthaultii Hawkes and S. microdontum Bitter formed two distinct phenetic groups. Within S. microdontum, three subgroups were observed. The tetraploid germplasm formed another group, with S. sucrense Hawkes in one subgroup and the cultivated potato and Russian hybrids in another subgroup. Based upon the genetic diversity and the level of late blight resistance, S. microdonturn and S. sucrense offer the best choice for strong late blight resistance from genetically diverse sources. This potato germplasm with reported late blight resistance should be introgressed into the potato gene pool to broaden the genetic base to achieve stronger and more durable resistance.

Corresponding author: D Douches

email: douchesd(at)msu.edu

Co-current introgression of economically important traits in a potato-breeding program

Hayes R J and Thill C A. 2002. American Journal of Potato Research 79:173–181.

Wild potato species contain many traits of economic importance. Late blight (LB) resistance and cold chipping are traits desired in potato cultivars. These traits could be co-currently introgressed if they occurred together in wild potato species. Our research objectives were (1) to determine if variation for cold chipping exists between potato species, accessions within species, and plants within accessions all having foliar LB resistance, and (2) to identify wild potato genotypes combining LB resistance and cold chipping. Materials include 665 genotypes from 43 LB-resistant accessions of 12 potato species having Endosperm Balance Numbers (EBN) of 1, 2, and 4, and 59 LB-resistant genotypes retained from these accessions for breeding. Potato chips were made from greenhouse-grown tubers following storage at 4 C for 6 months. Chip color was scored 1-10, = 4 is acceptable by industry standards. Most of the variation for chip color was due to differences between species. Species ranged in the percentage of acceptably chipping genotypes (0% - 67%) with nine of 12 species having cold-chipping genotypes. Appreciable variation was present within accessions as well. The best chipping accessions were S. verrucosum plant introduction (P1) 161173 - 4.33 / 0.67 (mean / proportion acceptable genotypes), S. stoloniferum P1250510 - 4.36/0.64, S. pin-natisectum P1347766 - 4.65 / 0.35 and 275233 - 4.73 / 0.44, and S. rnegistacrolobum P1 195210 - 5.14 / 0.29. Eleven 1EBN genotypes from S. pinnatisectum and S. trifidum and five 2EBN genotypes from S. verrucosum, S.fendleri, S. stoloniferum, and S. rnicrodontum were identified that combined LB resistance and cold chipping. Co-current introgression would require fewer breeding cycles than other breeding methods to identify hybrid genotypes posessing both traits.

Corresponding author: C Thill

email: thill005(at)umn.edu

Plant defense genes associated with quantitative resistance to potato late blight in Solanum phureja x Dihaploid S. tuberosum hybrids

Trognitz F, Manosalva P, Gysin R, Niño-Liu D, Simon R, Herrera M R, Trognitz B, Ghislain M and Nelson R. 2002. Molecular Plant-Microbe Interactions 15:587–597.

Markers corresponding to 27 plant defense genes were tested for linkage disequilibrium with quantitative resistance to late blight in a diploid potato population that had been used for mapping quantitative trait loci (QTLs) for late blight resistance. Markers were detected by using (i) hybridization probes for plant defense genes, (ii) primer pairs amplifying conserved domains of resistance (R) genes, (iii) primers for defense genes and genes encoding transcriptional regulatory factors, and (iv) primers allowing amplification of sequences flanking plant defense genes by the ligation-mediated polymerase chain reaction. Markers were initially screened by using the most resistant and susceptible individuals of the population, and those markers showing different allele frequencies between the two groups were mapped. Among the 308 segregating bands detected, 24 loci (8%) corresponding to six defense gene families were associated with resistance at x<sup(2)=13, the threshold established using the permutation test at P = 0.05. Loci corresponding to genes related to the phenylpropanoid pathway (phenylalanine ammonium lyase [PAL], chalcone isomerase [CHI], and chalcone synthase [CHS]), loci related to WRKY regulatory genes, and other defense genes (osmotin and a Phytophthora infestans-induced cytochrome P450) were significantly associated with quantitative disease resistance. A subset of markers was tested on the mapping population of 94 individuals. Ten defense-related markers were clustered at a QTL on chromosome III, and three defense-related markers were located at a broad QTL on chromosome XII. The association of candidate genes with QTLs is a step toward understanding the molecular basis of quantitative resistance to an important plant disease.

Corresponding author: R J Nelson

email: <LINK rjn7@cornell.edu>rjn7@cornell.edu

Phospholipase D in Phytophthora infestans and its role in zoospore encystment

Latijnhouwers M, Munnik T and Govers F. 2002. Molecular Plant Microbe Interactions 15:939–946.

We show that differentiation of zoospores of the late blight pathogen Phytophthora infestans into cysts, a process called encystment, was triggered by both phosphatidic acid (PA) and the G-protein activator mastoparan. Mastoparan induced the accumulation of PA, indicating that encystment by mastoparan most likely acts through PA. Likewise, mechanical agitation of zoospores, which often is used to induce synchronized encystment, resulted in increased levels of PA. The levels of diacylglycerolpyrophosphate (DGPP), the phosphorylation product of PA, increased simultaneously. Also in cysts, sporangiospores, and mycelium, mastoparan induced increases in the levels of PA and DGPP. Using an in vivo assay for phospholipase D (PLD) activity, it was shown that the mastoparan-induced increase in PA was due to a stimulation of the activity of this enzyme. Phospholipase C in combination with diacylglycerol (DAG) kinase activity also can generate PA, but activation of these enzymes by mastoparan was not detected under conditions selected to highlight <sup(32)>P-PA production via DAG kinase. Primary and secondary butanol, which, like mastoparan, have been reported to activate G-proteins, also stimulated PLD activity, whereas the inactive tertiary isomer did not. Similarly, encystment was induced by n- and sec-butanol but not by tert-butanol. Together, these results show that Phytophthora infestans contains a mastoparan- and butanol-inducible PLD pathway and strongly indicate that PLD is involved in zoospore encystment. The role of G-proteins in this process is discussed.

Corresponding author: F Govers

email: Francine.Govers(at)wur.nl

Differential expression of G protein alpha and beta subunit genes during development of Phytophthora infestans

Laxalt A M, Latijnhooouwers, van Hulten M and Govers F. 2002. Fungal Genetics and Biology 36:137–146. Academic Press © Elsevier Science 2002.

A G protein alpha subunit gene (pigpa1) and a G protein beta subunit gene (pigpb1) were isolated from the oomycete Phytophthora infestans, the causal agent of potato late blight. Heterotrimeric G proteins are evolutionary conserved GTP-binding proteins that are composed of alpha, beta, and gamma subunits and participate in diverse signal transduction pathways. The deduced amino acid sequence of both pigpa1 and pigpb1, showed the typical conserved motifs present in G alpha or G beta proteins from other eukaryotes. Southern blot analysis revealed no additional copies of G alpha or G beta subunit genes in P. infestans, suggesting that pigpa1 and pigpb1 are single copy genes. By cross-hybridization homologues of pigpa1 and pigpb1 were detected in other Phytophthora species. Expression analyses revealed that both genes are differentially expressed during asexual development, with the highest mRNA levels in sporangia. In mycelium, no pigpa1 mRNA was detected. Western blot analysis using a polyclonal GPA1 antibody confirmed the differential expression of pigpa1. These expression patterns suggest a role for G-protein-mediated signaling during formation and germination of asexual spores of P. infestans, developmental stages representing the initial steps of the infection process.

Corresponding author: F Govers

email: Francine.Govers(at)wur.nl

These abstracts were reprinted with the kind permission of the British Society for Plant Pathology /Blackwell Science  (http://www.blacksci.co.uk/uk/socoth.htm), the American Society for Horticultural Science (http://www.ashs.org/hortscience.asp), the Potato Association of America (www.ume.maine.edu/PAA) ,  the American Phytopathological Society (http://www.apsnet.org/), Elsevier Science (http://www.elsevier.com/homepage/)