Quantitative Resistance Genes Research Articles

Secreted in Xylem 6 (SIX6) Mediates Fusarium oxysporum f. sp. fragariae Race 1 Avirulence on FW1-Resistant Strawberry Cultivars.

Fusarium oxysporum f. sp. fragariae (Fof) race 1 is avirulent on cultivars with the dominant resistance gene FW1, while Fof race 2 is virulent on FW1-resistant cultivars. We hypothesized there was a gene-for-gene interaction between a gene at the FW1 locus and an avirulence gene (AvrFW1) in Fof race 1. To identify a candidate AvrFW1, we compared genomes of 24 Fof race 1 and three Fof race 2 isolates. We found one candidate gene that was present in race 1, was absent in race 2, was highly expressed in planta, and was homologous to a known effector, secreted in xylem 6 (SIX6). We knocked out SIX6 in two Fof race 1 isolates by homologous recombination. All SIX6 knockout transformants (ΔSIX6) gained virulence on FW1/fw1 cultivars, whereas ectopic transformants and the wildtype isolates remained avirulent. ΔSIX6 isolates were quantitatively less virulent on FW1/fw1 cultivars Fronteras and San Andreas than fw1/fw1 cultivars. Seedlings from an FW1/fw1 × fw1/fw1 population were genotyped for FW1 and tested for susceptibility to a SIX6 knockout isolate. Results suggested that additional minor-effect quantitative resistance genes could be present at the FW1 locus. This work demonstrates that SIX6 acts as an avirulence factor interacting with a resistance gene at the FW1 locus. The identification of AvrFW1 enables surveillance for Fof race 2 and provides insight into the mechanisms of FW1-mediated resistance. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A genome sequence for the threatened whitebark pine.

Whitebark pine (WBP, Pinus albicaulis) is a white pine of subalpine regions in the Western contiguous United States and Canada. WBP has become critically threatened throughout a significant part of its natural range due to mortality from the introduced fungal pathogen white pine blister rust (WPBR, Cronartium ribicola) and additional threats from mountain pine beetle (Dendroctonus ponderosae), wildfire, and maladaptation due to changing climate. Vast acreages of WBP have suffered nearly complete mortality. Genomic technologies can contribute to a faster, more cost-effective approach to the traditional practices of identifying disease-resistant, climate-adapted seed sources for restoration. With deep-coverage Illumina short reads of haploid megagametophyte tissue and Oxford Nanopore long reads of diploid needle tissue, followed by a hybrid, multistep assembly approach, we produced a final assembly containing 27.6 Gb of sequence in 92,740 contigs (N50 537,007 bp) and 34,716 scaffolds (N50 2.0 Gb). Approximately 87.2% (24.0 Gb) of total sequence was placed on the 12 WBP chromosomes. Annotation yielded 25,362 protein-coding genes, and over 77% of the genome was characterized as repeats. WBP has demonstrated the greatest variation in resistance to WPBR among the North American white pines. Candidate genes for quantitative resistance include disease resistance genes known as nucleotide-binding leucine-rich repeat receptors (NLRs). A combination of protein domain alignments and direct genome scanning was employed to fully describe the 3 subclasses of NLRs. Our high-quality reference sequence and annotation provide a marked improvement in NLR identification compared to previous assessments that leveraged de novo-assembled transcriptomes.

Approaches to Reduce Rice Blast Disease Using Knowledge from Host Resistance and Pathogen Pathogenicity

Rice is one of the staple foods for the majority of the global population that depends directly or indirectly on it. The yield of this important crop is constantly challenged by various biotic stresses. Rice blast, caused by Magnaporthe oryzae (M. oryzae), is a devastating rice disease causing severe yield losses annually and threatening rice production globally. The development of a resistant variety is one of the most effective and economical approaches to control rice blast. Researchers in the past few decades have witnessed the characterization of several qualitative resistance (R) and quantitative resistance (qR) genes to blast disease as well as several avirulence (Avr) genes from the pathogen. These provide great help for either breeders to develop a resistant variety or pathologists to monitor the dynamics of pathogenic isolates, and ultimately to control the disease. Here, we summarize the current status of the isolation of R, qR and Avr genes in the rice-M. oryzae interaction system, and review the progresses and problems of these genes utilized in practice for reducing rice blast disease. Research perspectives towards better managing blast disease by developing a broad-spectrum and durable blast resistance variety and new fungicides are also discussed.

Advances in Durable Resistance to Diseases in Staple Food Crops: A Review

Background: At all stages of their development, plants are in permanent contact with causative agents of various diseases. Mechanisms of disease resistance and its durability in crops largely depend on the pathogen’s lifestyle, namely the nutrition mode and host range. Objective: The objective of this review is to consider the main advances in the production of genotypes with durable disease resistance in the globally important food crops, wheat, rice, and potato, as well as barley. Results: In wheat, durable resistance could be provided by the employment of various adult plant resistance genes against biotrophic pathogens, whose action commonly does not involve hypersensitivity response, as well as major quantitative genes, including mutants of susceptibility alleles, against necrotrophs via marker assisted selection (MAS). In barley, the most prominent example is the gene mlo conferring durable powdery mildew resistance, but it is compromised by higher susceptibility to some necrotrophic fungi. A few genes for broad-spectrum resistance against the rice blast and bacterial blight pathogens confirmed their effectiveness for decades, and they could be combined with effective R genes via MAS. Resistance to late blight of potato is mainly provided by R genes introgressed from wild potato species, which could be pyramided with quantitative trait loci. Genes for extreme resistance to potato viruses derived from related species provide durable and broad-spectrum resistance and could be effectively deployed in potato breeding using MAS. Silencing susceptibility genes by genome editing technologies is the most promising approach to produce plants with durable resistance to many pathogens in the crop species. Genetic transformation with genes for resistance-associated proteins or constructs providing silencing via RNA interference is an effective biotechnological method to generate plants with durable resistance against pathogens, especially viruses. Conclusion: Main advances in the production of crop plants with durable resistance are based on studies of molecular mechanisms of plant immunity and its special features for pathogens with different lifestyles via the use of biotechnological approaches such as MAS for pyramiding of monogenic quantitative resistance genes or qualitative R genes, changes in expression of certain genes associated with resistance, the introduction of transgenes, mutagenesis and genome editing aimed at silencing susceptibility genes.

Classification of 17 species Aegilops using DNA barcoding and SNPs, reveals gene flow among Aegilops biuncialis, Aegilops juvenalis, and Aegilops columnaris.

Rapid changes in agricultural environments caused by global warming pose a major challenge to food production and safety. Common wheat (Triticum aestivum) is a hexaploid plant (AABBDD) that shares large numbers of quantitative traits and resistance genes with B and D genomes of Aegilops species, which are responsible for several metabolic functions and biosynthetic processes, particularly in plant adaptation to biotic as well as abiotic stresses. Comparatively, the abundance of the Aegilops gene pool is much higher than that of Triticum. Therefore, we used four universal DNA barcodes for plants (ITS2, matK, rbcL, and psbM-petN) to construct a phylogenetic tree to classify the genus Aegilops. Fourteen species were distinguished among a total of 17 representative species. Aegilops biuncialis, Aegilops juvenalis, and Aegilops umbellulata could not be grouped into any of the clusters in the phylogenetic tree, indicating that these three species could not be distinguished by four DNA barcodes. Therefore, from 2408 SNPs obtained using genotyping by sequencing (GBS), we manually screened 30 SNPs that could be potentially used to classify these three species. The results of gene flow and genetic differentiation index (Fst) showed that the genetic differentiation among the three species was small, and there was bidirectional horizontal gene transfer between the three species, which was consistent with our results that the three species were difficult to classify by DNA barcode.

Advances in wheat breeding for resistance to Fusarium head blight

Fusarium head blight (FHB), mainly caused by Fusarium graminearum, is one of the most devastating diseases of wheat globally. FHB causes an extensive reduction in yield and reduces the grain quality through its contamination with Fusarium toxins such as deoxynivalenol (DON), T2 toxin, HT-2 toxin, nivalenol, and zearalenone. This review provides an overview of updated progress of genetic studies on the resistance to FHB, with an emphasis on the sources of resistance to FHB, resistance gene/quantitative trait loci (QTL) mining, resistance gene cloning, major FHB resistance genes/QTL identification by molecular markers, and resistance mechanisms. The achievements of resistance breeding based on phenotype selection and molecular markers was also summarised. Based on the systematic analysis of breeding limitations and utilisation of FHB resistant materials, the authors put forward three suggestions: First, to toughen the resistance identification of wheat, testing traits such as Fusarium damaged kernel and DON need special attention as visual symptoms are less reliable, resistant varieties should be popularised, and the screening the resistant genes should be strengthened; The second is to use the additive effect of quantitative resistance genes accumulated from existing varieties to reduce the cost of resistance in order to create high yielding resistant varieties. Thirdly, to enhance research and utilization of new genes.

An epi-evolutionary model for predicting the adaptation of spore-producing pathogens to quantitative resistance in heterogeneous environments.

We have modeled the evolutionary epidemiology of spore‐producing plant pathogens in heterogeneous environments sown with several cultivars carrying quantitative resistances. The model explicitly tracks the infection‐age structure and genetic composition of the pathogen population. Each strain is characterized by pathogenicity traits determining its infection efficiency and a time‐varying sporulation curve taking into account lesion aging. We first derived a general expression of the basic reproduction number R0 for fungal pathogens in heterogeneous environments. We show that the evolutionary attractors of the model coincide with local maxima of R0 only if the infection efficiency is the same on all host types. We then studied the contribution of three basic resistance characteristics (the pathogenicity trait targeted, resistance effectiveness, and adaptation cost), in interaction with the deployment strategy (proportion of fields sown with a resistant cultivar), to (i) pathogen diversification at equilibrium and (ii) the shaping of transient dynamics from evolutionary and epidemiological perspectives. We show that quantitative resistance affecting only the sporulation curve will always lead to a monomorphic population, whereas dimorphism (i.e., pathogen diversification) can occur if resistance alters infection efficiency, notably with high adaptation costs and proportions of the resistant cultivar. Accordingly, the choice of the quantitative resistance genes operated by plant breeders is a driver of pathogen diversification. From an evolutionary perspective, the time to emergence of the evolutionary attractor best adapted to the resistant cultivar tends to be shorter when resistance affects infection efficiency than when it affects sporulation. Conversely, from an epidemiological perspective, epidemiological control is always greater when the resistance affects infection efficiency. This highlights the difficulty of defining deployment strategies for quantitative resistance simultaneously maximizing epidemiological and evolutionary outcomes.

Spatiotemporal Changes in Varietal Resistance to Wheat Yellow Rust in France Reveal an Increase in Field Resistance Level During the Period 1985-2018.

Monitoring spatiotemporal changes in varietal resistance and understanding its drivers seem essential to managing plant diseases but require having access to the genetic basis of disease resistance and to its deployment. In this study, we focused on yellow rust (Puccinia striiformis f. sp. tritici) for three decades in France, by using field adult plant resistance levels, Yr race-specific resistance genes of varieties, presence of Puccinia striiformis f. sp. tritici pathotypes and their virulence profiles, and systematic surveys of the acreages of bread wheat varieties available at a yearly survey time and at a district level. Based on these data, we studied spatiotemporal changes in varietal resistance over the period from 1985 to 2018 in 54 French administrative districts (hereafter "departments") by using a set of relevant indicators weighted by the relative acreage proportion of the varieties sown at the department level. Our analyses revealed an increase in varietal resistance over decades that would be due to the accumulation of both quantitative resistance and different race-specific resistance genes. We suggest that, beyond breeders, several actors, including examination offices, agricultural advisory services, and farmers, may have had a substantial influence on these spatiotemporal changes, promoting more resistant varieties and the rapid replacement of newly susceptible varieties by still resistant ones at the beginning of each epidemic.

Eighty years of gene-for-gene relationship and its applications in identification and utilization of R genes

The gene-for-gene relationship of host-pathogen interaction explained by H. H. Flor in mid of the 20th century set a milestone in understanding the biochemical and genetic basis of plant diseases and several components involved in plant-pathogen interactions. It highlighted the importance of accomplishing differential sets and understanding the pathogen population structure, it further led to the identification and cloning of several resistance (R) genes in plants. These R genes have been deployed and altered for fighting against diseases in a large number of crops using various conventional approaches and biotechnological tools. Identification of R genes and their corresponding Avr genes in many cases played a significant role in understanding of R-Avr gene interactions. Rapid cloning of R genes and editing of susceptible R genes are the other avenues that have broadened the horizon of utilizing R genes in crop improvement programmes. Further, combining R genes with quantitative disease resistance genes has paved the way to develop durable resistance in cultivars. The recent advances in genetics, genomics, bioinformatics and other OMICS tools are now providing greater prospects for deeper understanding of host-pathogen interaction.

Quantitative resistance linked to late effectors.

Quantitative resistance linked to late effectors.

Reciprocal adaptation of rice and Xanthomonas oryzae pv. oryzae: cross-species 2D GWAS reveals the underlying genetics.

A 1D/2D genome-wide association study strategy was adopted to investigate the genetic systems underlying the reciprocal adaptation of rice (Oryza sativa) and its bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo) using the whole-genome sequencing and large-scale phenotyping data of 701 rice accessions and 23 diverse Xoo strains. Forty-seven Xoo virulence-related genes and 318 rice quantitative resistance genes (QR-genes) mainly located in 41 genomic regions, and genome-wide interactions between the detected virulence-related genes and QR genes were identified, including well-known resistance genes/virulence genes plus many previously uncharacterized ones. The relationship between rice and Xoo was characterized by strong differentiation among Xoo races corresponding to the subspecific differentiation of rice, by strong shifts toward increased resistance/virulence of rice/Xoo populations and by rich genetic diversity at the detected rice QR-genes and Xoo virulence genes, and by genome-wide interactions between many rice QR-genes and Xoo virulence genes in a multiple-to-multiple manner, presumably resulting either from direct protein–protein interactions or from genetic epistasis. The observed complex genetic interaction system between rice and Xoo likely exists in other crop–pathogen systems that would maintain high levels of diversity at their QR-loci/virulence-loci, resulting in dynamic coevolutionary consequences during their reciprocal adaptation.

La búsqueda de resistencia al tizón tardío en papas silvestres

Potatoes (Solanum tuberosum L.) are the most cultivated vegetables in the world. The main limiting factor worldwide is late blight, a disease caused by the oomycete Phytophthora infestans, causing up even total production losses. An important part of potato genetic improvement focuses on obtaining cultivars resistant to this disease. However, the pathogen can produce offspring from sexual reproduction, generating new strains that are more virulent, resistant to fungicides and that allow it to overcome genetic resistance in new cultivars. For a long time, the vertical resistance genes of Solanum demissum were used to obtain resistant varieties, but their success have been limited by being race-specific and of short-term duration. Currently there is an interest in horizontal or quantitative resistance genes of various wild species of Solanum, which are difficult to introduce due to ploidy incompatibility and to the Endosperm Balance Number (EBN). Nevertheless, techniques like somatic hybridization, allele mining and marker-assisted breeding have made possible to overcome those barriers. Hence the importance of exploring and identifying new resistance genes in wild species.

Identification of candidate genes for quantitative downy mildew resistance in cucumber – a review

Cucurbit downy mildew (DM), caused by the obligate biotrophic oomycete Pseudoperonospora cubensis, is a major foliar disease of cucumber. Cucumber accession PI197088 was previously shown to be one of the most promising donors for DM resistance. The resistance in PI197088 is controlled by multiple quantitative trait loci (QTL), each with a relatively small effect. We developed a cucumber line with a 12 Mb introgression from PI197088 in a susceptible background. This introgression line was partially resistant to DM. Subsequently, we combined fine mapping data with RNAseq and whole genome resequencing to identify candidate genes for the resistance conferred by this introgression. Interestingly, fine-mapping data suggested that the partial resistance is caused by a combination of multiple genetically linked genes, leading to three subQTL, each with a different effect on the disease phenotype. In one of these subQTL we identified several Receptor Like Kinase genes (RLK). A novel RLK gene appeared to be present in resistant genotypes. In susceptible genotypes, including the reference genotype “Chinese Long 9930”, this novel gene has a 551 base pair deletion, and was therefore not correctly predicted during the annotation of the cucumber genome. In another subQTL, we found a loss-of-function mutation in the novel susceptibility gene (S-gene) CsAAP2A, encoding an amino acid transporter. In the resistant genotype a non-functional allele of this gene is present due to integration of a transposable element in its coding sequence.

Specific Resistance of Barley to Powdery Mildew, Its Use and Beyond. A Concise Critical Review.

Powdery mildew caused by the airborne ascomycete fungus Blumeria graminis f. sp. hordei (Bgh) is one of most common diseases of barley (Hordeum vulgare). This, as with many other plant pathogens, can be efficiently controlled by inexpensive and environmentally-friendly genetic resistance. General requirements for resistance to the pathogens are effectiveness and durability. Resistance of barley to Bgh has been studied intensively, and this review describes recent research and summarizes the specific resistance genes found in barley varieties since the last conspectus. Bgh is extraordinarily adaptable, and some commonly recommended strategies for using genetic resistance, including pyramiding of specific genes, may not be effective because they can only contribute to a limited extent to obtain sufficient resistance durability of widely-grown cultivars. In spring barley, breeding the nonspecific mlo gene is a valuable source of durable resistance. Pyramiding of nonspecific quantitative resistance genes or using introgressions derived from bulbous barley (Hordeum bulbosum) are promising ways for breeding future winter barley cultivars. The utilization of a wide spectrum of nonhost resistances can also be adopted once practical methods have been developed.

De novo Assembly of Transcriptomes From a B73 Maize Line Introgressed With a QTL for Resistance to Gray Leaf Spot Disease Reveals a Candidate Allele of a Lectin Receptor-Like Kinase.

Gray leaf spot (GLS) disease in maize, caused by the fungus Cercospora zeina, is a threat to maize production globally. Understanding the molecular basis for quantitative resistance to GLS is therefore important for food security. We developed a de novo assembly pipeline to identify candidate maize resistance genes. Near-isogenic maize lines with and without a QTL for GLS resistance on chromosome 10 from inbred CML444 were produced in the inbred B73 background. The B73-QTL line showed a 20% reduction in GLS disease symptoms compared to B73 in the field (p = 0.01). B73-QTL leaf samples from this field experiment conducted under GLS disease pressure were RNA sequenced. The reads that did not map to the B73 or C. zeina genomes were expected to contain novel defense genes and were de novo assembled. A total of 141 protein-coding sequences with B73-like or plant annotations were identified from the B73-QTL plants exposed to C. zeina. To determine whether candidate gene expression was induced by C. zeina, the RNAseq reads from C. zeina-challenged and control leaves were mapped to a master assembly of all of the B73-QTL reads, and differential gene expression analysis was conducted. Combining results from both bioinformatics approaches led to the identification of a likely candidate gene, which was a novel allele of a lectin receptor-like kinase named L-RLK-CML that (i) was induced by C. zeina, (ii) was positioned in the QTL region, and (iii) had functional domains for pathogen perception and defense signal transduction. The 817AA L-RLK-CML protein had 53 amino acid differences from its 818AA counterpart in B73. A second “B73-like” allele of L-RLK was expressed at a low level in B73-QTL. Gene copy-specific RT-qPCR confirmed that the l-rlk-cml transcript was the major product induced four-fold by C. zeina. Several other expressed defense-related candidates were identified, including a wall-associated kinase, two glutathione s-transferases, a chitinase, a glucan beta-glucosidase, a plasmodesmata callose-binding protein, several other receptor-like kinases, and components of calcium signaling, vesicular trafficking, and ethylene biosynthesis. This work presents a bioinformatics protocol for gene discovery from de novo assembled transcriptomes and identifies candidate quantitative resistance genes.

Advances and Challenges in Bacterial Spot Resistance Breeding in Tomato (Solanum lycopersicum L.).

Bacterial spot is a serious disease of tomato caused by at least four species of Xanthomonas. These include X. euvesicatoria (race T1), X. vesicatoria (race T2), X. perforans (races T3 and T4), and X. gardneri, with the distinct geographical distribution of each group. Currently, X. gardneri and X. perforans are two major bacterial pathogens of tomato in North America, with X. perforans (race T4) dominating in east-coast while X. gardneri dominating in the Midwest. The disease causes up to 66% yield loss. Management of this disease is challenging due to the lack of useful chemical control measures and commercial resistant cultivars. Although major genes for resistance (R) and quantitative resistance have been identified, breeding tomato for resistance to bacterial spot has been impeded by multiple factors including the emergence of new races of the pathogen that overcome the resistance, multigenic control of the resistance, linkage drag, non-additive components of the resistance and a low correlation between seedling assays and field resistance. Transgenic tomato with Bs2 and EFR genes was effective against multiple races of Xanthomonas. However, it has not been commercialized because of public concerns and complex regulatory processes. The genomics-assisted breeding, effectors-based genomics breeding, and genome editing technology could be novel approaches to achieve durable resistance to bacterial spot in tomato. The main goal of this paper is to understand the current status of bacterial spot of tomato including its distribution and pathogen diversity, challenges in disease management, disease resistance sources, resistance genetics and breeding, and future prospectives with novel breeding approaches.

Marker-Assisted Introgression of Quantitative Resistance Gene pi21 Confers Broad Spectrum Resistance to Rice Blast

The quantitative resistance gene pi21 from Sensho was introgressed to an indica breeding line IR63307-4B-13-2, a pyramiding line IRBB4/5/13/21, and a tropical japonica line Kinandang Patong by marker-assisted backcrossing. A total of 192 improved lines at the BC4F3 and BC4F4 generations were developed and confirmed to have the gene introgression via genotyping using a pi21-specific InDel marker. Thirteen randomly selected improved lines, representing all the three genetic backgrounds, demonstrated resistance against leaf blast composites in the field and a broader spectrum resistance against individual isolates compared to the recurrent parents in the glasshouse. Specifically, the tested lines exhibited pi21-acquired resistance against 11 leaf blast isolates that elicited susceptible reactions from the recurrent parents. All the tested lines maintained a comparative heading date, and similar or improved panicle length, number of primary branches per panicle and number of total grains per panicle relative to the recurrent parents. The physical grain characteristics of the recurrent parents were also maintained in the 13 lines tested, although variability in the amylose content and chalkiness degree was observed. The successful marker-assisted introgression of pi21 in diverse genetic backgrounds and the resulting broader spectrum resistance of improved lines against leaf blast indicate the potential of pi21 for deployment in cultivars grown across other rice growing regions in Asia.

Genetic and Genomic Tools in Sunflower Breeding for Broomrape Resistance.

Broomrape is a root parasitic plant causing yield losses in sunflower production. Since sunflower is an important oil crop, the development of broomrape-resistant hybrids is the prime breeding objective. Using conventional plant breeding methods, breeders have identified resistant genes and developed a number of hybrids resistant to broomrape, adapted to different growing regions worldwide. However, the spread of broomrape into new countries and the development of new and more virulent races have been noted intensively. Recent advances in sunflower genomics provide additional tools for plant breeders to improve resistance and find durable solutions for broomrape spread and virulence. This review describes the structure and distribution of new, virulent physiological broomrape races, sources of resistance for introduction into susceptible cultivated sunflower, qualitative and quantitative resistance genes along with gene pyramiding and marker assisted selection (MAS) strategies applied in the process of increasing sunflower resistance. In addition, it presents an overview of underutilized biotechnological tools, such as phenotyping, -omics, and genome editing techniques, which need to be introduced in the study of sunflower resistance to broomrape in order to achieve durable resistance.

The Panicle Blast Resistance Mechanism of <i>qPbm11</i> in the Rice Cultivar Miyazaki-mochi is Independent from that of <i>Pb1</i>

Understanding the mechanism of disease resistance is critical for combating rice blast disease that causes the most deleterious effects on rice yield, and for evolving successful blast tolerance breeding programs using blast resistance (R) genes. The qPbm11 locus in the blast resistant cultivar Miyazaki-mochi exerts panicle resistance similar to the well-known quantitative panicle resistance gene Pb1. qPbm11 and Pb1 show similarity in panicle blast resistance, quantitative resistance, and proximity of loci on the rice genome. Pb1 resistance is dependent on the expression of transcription factor WRKY45. To identify the resistance mechanism of the qPbm11 locus, we downregulated WRKY45 in the cultivar Miyazaki-mochi through an Agrobacterium-mediated transformation and surveyed its panicle resistance. WRKY45 RNAi transgenic Miyazaki-mochi plants exhibited similar levels of panicle resistance compared to WT, indicating that the qPbm11 locus is independent of the Pb1 resistance pathway. Our results suggested that cloning the genes responsible for qPbm11-mediated resistance along with Pb1 using pyramiding technology could enhance panicle resistance in rice.

Intra- and inter-species variability of the aggressiveness in four Fusarium head blight species on durum wheat plants detected in an in vitro Petri-dish assay

Aggressiveness assessment of Fusarium head blight (FHB) is crucial for explaining the interaction in FHB–wheat association. Currently, in vitro tools have proved to be very useful in identifying disease responses in wheat to FHB infection. To update our knowledge, intra- and inter-species variability of the aggressiveness was studied in 16 isolates of four FHB species (F. culmorum, F. solani, F. verticillioides and F. equiseti) using an in vitro Petri-dish test. Three aggressiveness criteria, germination rate reduction, standardised area under disease progress curve (AUDPCstandard) and coleoptile length reduction, were evaluated on a durum wheat cultivar showing a moderate level of quantitative resistance. Regarding AUDPCstandard, intra- and inter-species variability was detected. The other two aggressiveness criteria did not distinguish isolates within and among species. The three aggressiveness criteria were not significantly correlated. However, it was not possible to cluster the isolates based on their species origins because of similarity in pathogenic level among the 16 fungal isolates. The results provide evidence that there is a differential interaction between quantitative resistance genes in wheat plants and the 16 tested FHB isolates. The values of AUDPCstandard showed significant correlation with both: disease incidence and disease severity obtained under controlled conditions (r = 0.627** and r = 0.539*, respectively), and disease incidence generated by field conditions (r = 0.525*). AUDPCstandard could be of potential use in evaluating the aggressiveness of FHB in adult wheat plants under controlled and field conditions.