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Insight into pathogen population dynamics provides a key input for effective disease management of the potato late blight pathogen Phytophthora infestans. Phytophthora infestans populations vary from genetically complex to more simple with a few clonal lineages. The presence or absence of certain strains of P. infestans may impact the efficacy of fungicides or host resistance. Current evidence indicates that genetically, the Irish populations of P. infestans are relatively simple with a few clonal lineages. In this study, P. infestans populations were genetically characterized based on samples collected at the national centre for potato breeding during the period 2012–16. The dominance of clonal lineages within this P. infestans population was confirmed and the potential selection pressure of fungicide treatment (2013–15) and host resistance (2016) on this clonal P. infestans population was then investigated. It was found that fungicide products did not notably affect the genetic structure of sampled populations relative to samples from untreated control plants. In contrast, samples taken from several resistant potato genotypes were found to be more often of the EU_13_A2 lineage than those taken from control King Edward plants or potato genotypes with low resistance ratings. Resistant potato varieties Sarpo Mira and Bionica, containing characterized R genes, were found to strongly select for EU_13_A2 strains. [ABSTRACT FROM AUTHOR]

In this study peroxynitrite (ONOO−) is proposed as an important player in defence responses during the interaction of potato ( Solanum tuberosum) and the oomycete pathogen Phytophthora infestans. The potato- avr P. infestans model system exhibited a transient programme of boosted ONOO− formation correlated in time with the burst of nitric oxide ( NO) and superoxide during the first 6 h post-inoculation (hpi). The early ONOO− over-accumulation was not accompanied by TPx gene expression. In contrast, the compatible interaction revealed a 24 h delay of ONOO− formation; however, an enhanced level of NO and superoxide correlated with TPx up-regulation was recorded within the earlier stages of pathogen infection. Peroxynitrite over-accumulation in the susceptible potato coincided with an enhanced level of protein tyrosine nitration starting from 24 hpi. Surprisingly, the nitroproteome profile of the resistant potato did not show any visible difference after inoculation, apart from one band containing subtilisin-like protease-like proteins, which appeared 48 h after pathogen attack. An additional pharmacological approach showed that treatment of the susceptible genotype with ONOO− followed by inoculation with P. infestans contributed to slowing down of the colonization of host tissues by the pathogen via a faster and stronger up-regulation of the key defence markers, including the PR-1 gene. Taken together, the results obtained indicate that a precise control of emitted NO and superoxide in cooperation with thioredoxin-dependent redox sensors in sites of pathogen ingress could generate a sufficient threshold of ONOO−, triggering defence responses. [ABSTRACT FROM AUTHOR]

Quantitative resistance may depend on the effectiveness of PAMP-triggered immunity. This study highlights the diversity of mechanisms involved in the quantitative resistance of potato to Phytophthora infestans. The investigation focused on the implication of the hormone signalling pathways induced in four potato genotypes by a concentrated culture filtrate ( CCF) of P. infestans. The genotypes were ranked according to their level of resistance to P. infestans and discriminant analysis of gene expression profiles separated the most resistant genotype from the three others, particularly because of a strong induction of the salicylic acid ( SA) pathway. In this genotype, transcripts involved in the SA pathway, EDS1, WRKY1, PR-1 and PR-2, were induced by CCF. SA pathway involvement was confirmed by a peak of SA accumulation 12 h after elicitation and by the induction of jaz1 ( jasmonate Zim domain protein 1) transcripts, which inhibit defence responses mediated by jasmonic acid ( JA). By contrast, neither a significant induction of SA-mediated responses nor an accumulation of free SA and PR-2 were observed in the other resistant or two susceptible cultivars. Expression of genes in the ET and JA pathways was either not, or weakly, induced by CCF in potato. Finally, the involvement of signalling pathways was genotype dependent rather than correlating with resistance level. [ABSTRACT FROM AUTHOR]

A total of 23 Scottish and 14 Dutch potato R-gene differentials as well as five Austrian, two Dutch and two German commercial potato cultivars were screened for the R1 allele conferring resistance to Phytophthora infestans carrying Avr1, via PCR amplification and sequencing. A single 1400 bp fragment with complete sequence identity to the corresponding part of the R1 allele, was obtained from genomic DNA of all potato R-gene differential clones whose denomination indicates R1 or a combination of R1 and other major resistance factors. The R1 allele was detected, as expected, in all these clones. This fragment also occurred in one Austrian and one German cultivar. Unexpectedly, the same R1 allele also was detected within all R5, R6 and R9 differentials. [ABSTRACT FROM AUTHOR]

Infection of several commercially important potato cultivars withTobacco rattle virus(TRV) can result in systemic infection with M-type virus, which persists through generations of vegetative propagation, although spraing symptoms in the tubers rarely develop. In field trials of nine such cultivars it was shown that TRV infection can delay plant emergence and retard subsequent growth, reduce tuber yield and mean tuber weight, increase tuber numbers and the proportion of small tubers, increase the incidence of growth cracks and misshapen tubers, diminish dry matter content, and exacerbate after-cooking blackening. Each cultivar was affected differently, showing different combinations and degrees of these effects. Virus concentration in leaf extracts from individual plants was also highly variable. These, and other potato cultivars similarly susceptible to TRV, are grown on large areas of UK and European potato land. The implications of growing such cultivars for the seed, ware and processing sectors of the potato industry are discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The rate of late blight disease was analysed for individuals of a diploid Solanum phureja–Solanum tuberosum dihaploid hybrid population (PD), using three different assessment techniques, in the laboratory, screenhouse, and field. These hybrids expressed low disease rates in the field, comparable to resistance based on intact R genes. However, none of the parents of PD expressed any R genes and the pattern of segregation within the PD population was not indicative of R-gene inheritance. The foliage (or leaflet) area diseased had the largest broad-sense heritability of all criteria analysed, in all tests performed. In the field evaluations, the PD population showed intermediate levels of broad-sense heritability for foliage area diseased, relative to the much larger heritability detected for the group of controls possessing R genes. Resistance in the field of the PD hybrids had very little genotype-environment (G × E) interaction, indicating stability of its expression. All genotypes without R genes exhibited heritable, reduced rates of late blight disease in the field, but they were susceptible with low heritability in screenhouse and laboratory tests. This differential expression of disease indicates that the plants' indirect response to unknown environmental stress in the field may have been measured. The value of the PD hybrids for breeding of late blight resistant potato and the use of the disease rate data for detection of the underlying quantitative trait loci are discussed. [ABSTRACT FROM AUTHOR]

Ability to overcome the resistance of genes H1, H2 and H3 incorporated in potato cultivars was compared in 44 South American and nine European populations of potato cyst nematodes (Globodera spp.) by recording the development of females on rooted stem cuttings grown in pots in a glasshouse. Most of the South American populations were able to multiply on all the potato plants containing resistance genes, whereas the European populations were unable to overcome the resistance of at least one of the genes. Of particular note, however, was a population of Globodera rostochiensis from Italy which overcame resistance conferred by gene H1. Our results suggest that European populations of potato cyst nematodes contain only a fraction of the genetic variability found in South American populations. [ABSTRACT FROM AUTHOR]

Ability to overcome the resistance of genes H1, H2 and H3 incorporated in potato cultivars was compared in 44 South American and nine European populations of potato cyst nematodes (Globodera spp.) by recording the development of females on rooted stem cuttings grown in pots in a glasshouse. Most of the South American populations were able to multiply on all the potato plants containing resistance genes, whereas the European populations were unable to overcome the resistance of at least one of the genes. Of particular note, however, was a population of Globodera rostochiensis from Italy which overcame resistance conferred by gene H1. Our results suggest that European populations of potato cyst nematodes contain only a fraction of the genetic variability found in South American populations. [ABSTRACT FROM AUTHOR]