Mildew In Wheat Research Articles

Unveiling resistance expression profile to powdery mildew in wheat via Bulked Segregant RNA-Seq

Unveiling resistance expression profile to powdery mildew in wheat via Bulked Segregant RNA-Seq

Multiple routes to fungicide resistance: Interaction of Cyp51 gene sequences, copy number and expression.

We examined the molecular basis of triazole resistance in Blumeria graminis f. sp. tritici (wheat mildew, Bgt), a model organism among powdery mildews. Four genetic models for responses to triazole fungicides were identified among US and UK isolates, involving multiple genetic mechanisms. Firstly, only two amino acid substitutions in CYP51B lanosterol demethylase, the target of triazoles, were associated with resistance, Y136F and S509T (homologous to Y137F and S524T in the reference fungus Zymoseptoria tritici). As sequence variation did not explain the wide range of resistance, we also investigated Cyp51B copy number and expression, the latter using both reverse transcription-quantitative PCR and RNA-seq. The second model for resistance involved higher copy number and expression in isolates with a resistance allele; thirdly, however, moderate resistance was associated with higher copy number of wild-type Cyp51B in some US isolates. A fourth mechanism was heteroallelism with multiple alleles of Cyp51B. UK isolates, with significantly higher mean resistance than their US counterparts, had higher mean copy number, a high frequency of the S509T substitution, which was absent from the United States, and in the most resistant isolates, heteroallelism involving both sensitivity residues Y136+S509 and resistance residues F136+T509. Some US isolates were heteroallelic for Y136+S509 and F136+S509, but this was not associated with higher resistance. The obligate biotrophy of Bgt may constrain the tertiary structure and thus the sequence of CYP51B, so other variation that increases resistance may have a selective advantage. We describe a process by which heteroallelism may be adaptive when Bgt is intermittently exposed to triazoles.

Temporal dynamics of N-hydroxypipecolic acid and salicylic acid pathways in the disease response to powdery mildew in wheat

Wheat (Triticum aestivum) is a major staple crop worldwide, and its yields are significantly threatened by wheat powdery mildew (Blumeria graminis f. sp. tritici). Enhancing disease resistance in wheat is crucial for meeting global food demand. This study investigated the disease response in wheat, focusing on the bioactive small molecules salicylic acid (SA), pipecolic acid (Pip), and N-hydroxypipecolic acid (NHP), to provide new insights for molecular breeding. We found that endogenous levels of SA, Pip, and NHP significantly increased in infected plants, with Pip and NHP levels rising earlier than those of SA. Notably, the rate of increase of NHP was substantially higher than that of SA. The gene expression levels of SARD1 and CBP60g, which are transcription factors for SA, Pip, and NHP biosynthesis, increased significantly during the early stages of infection. We also found that during the later stages of infection, the expression of ALD1, SARD4, and FMO1, which encode enzymes for Pip and NHP biosynthesis, dramatically increased. Additionally, ICS1, which encodes a key enzyme involved in SA biosynthesis, also showed increased expression during the later stages of infection. The temporal changes in ICS1 transcription closely mirrored the behavior of endogenous SA levels, suggesting that the ICS pathway is the primary route for SA biosynthesis in wheat. In conclusion, our results suggest that the early accumulation of Pip and NHP cooperates with SA in the disease response against wheat powdery mildew infection.

Evaluating the efficacy of microbial antagonists in inducing resistance, promoting growth, and providing biological control against powdery mildew in wheat.

This study evaluates the biocontrol efficacy of three bacterial strains (Pseudomonas fluorescens DTPF-3, Bacillus amyloliquefaciens DTBA-11, and Bacillus subtilis DTBS-5) and two fungal strains (Trichoderma harzianum Pusa-5SD and Aspergillus niger An-27) antagonists, along with their combinations at varying doses (5.0, 7.5, and 10.0 g/kg of seeds), against wheat powdery mildew. The most effective dose (10 g/kg seeds) was further analyzed for its impact on induced resistance and plant growth promotion under greenhouse conditions. The study measured defense enzyme activities, biochemical changes, and post-infection plant growth metrics. All tested microbial antagonists at 10 g/kg significantly reduced PM severity, with B. subtilis strain DTBS-5 outperforming others in reducing PM severity and achieving the highest biocontrol efficacy. It was followed by B. amyloliquefaciens strain DTBA-11 and P. fluorescens strain DTPF-3, with the fungal antagonists showing no significant effect. Wheat crops treated with B. subtilis strain DTBS-5 exhibited substantial increases in defense-related enzyme activities and biochemicals, suggesting an induced resistance mechanism. The study found a 45% increase in peroxidase (POD) activity, a 50% increase in catalase (CAT) activity, a 30% increase in phenolic content, and a 25% increase in soluble protein content in the wheat plants treated with microbial antagonists. The study highlights the effectiveness of microbial antagonists, particularly B. subtilis strain DTBS-5, in managing wheat PM through biocontrol, induced resistance, and enhanced plant growth, offering a sustainable alternative to chemical treatments.

First report of strobilurin resistance in field samples of Blumeria graminis f. sp. tritici, causal agent of powdery mildew in wheat, in Paraguay

First report of strobilurin resistance in field samples of Blumeria graminis f. sp. tritici, causal agent of powdery mildew in wheat, in Paraguay

Detection Strategies for Volatile Fragrance Released from Agricultural Products: Progress and Prospects

AbstractThe volatile aroma released from agricultural products is closely related to the health status and quality of their growth, thus endowing the related detection with great significance. For example, the dynamic variation of the volatile chemical composition of a banana during the growth process can reflect its ripeness. Also important for quality monitoring and storage is to precisely and swiftly identify volatile compounds produced by mildew in rice and wheat. In this endeavor, the current detection technologies such as gas chromatography‐mass spectrometry method (GC‐MS) cannot meet the pressing needs of smart agriculture in terms of real‐time monitoring, cost‐effectiveness, sensitivity, and detection speed, thereby necessitating alternative strategies to simultaneously satisfy these requirements. Aiming to provide an overall development trend in this field, this paper summarizes the existing detection technologies including GC‐MS, E‐nose, and sensory analysis with their respective shortcomings and challenges, and then proposes the application prospects. This work can effectively enrich the reliable monitoring methods for volatile agricultural fragrance while promoting the long‐run development of smart agriculture.

The wheat powdery mildew resistance gene Pm4 also confers resistance to wheat blast

Wheat blast, caused by the fungus Magnaporthe oryzae, threatens global cereal production since its emergence in Brazil in 1985 and recently spread to Bangladesh and Zambia. Here we demonstrate that the AVR-Rmg8 effector, common in wheat-infecting isolates, is recognized by the gene Pm4, previously shown to confer resistance to specific races of Blumeria graminis f. sp. tritici, the cause of powdery mildew of wheat. We show that Pm4 alleles differ in their recognition of different AVR-Rmg8 alleles, and some confer resistance only in seedling leaves but not spikes, making it important to select for those alleles that function in both tissues. This study has identified a gene recognizing an important virulence factor present in wheat blast isolates in Bangladesh and Zambia and represents an important first step towards developing durably resistant wheat cultivars for these regions.

Nano protective membrane coated wheat to resist powdery mildew.

The plant pathogenic fungus Blumeria graminis f. sp. tritici infects wheat and reduces its yield. The policy of reducing fertilizer and biocide use in sustainable agriculture has prompted researchers to develop more green and efficient management strategies. In this study, a novel nanoprotective membrane (kaolin-nano titanium dioxide-liquid paraffin, referred to as KTP) that could effectively prevent powdery mildew of wheat was prepared by using 1 g/L kaolin, 2 g/L nanotitanium dioxide and 8% (v/v) liquid paraffin. The prevention and control effects of KTP spraying in advance in the pot and field experiments were 98.45% and 83.04%, respectively. More importantly, the weight of 1000 grains of wheat pretreated with KTP was 2.56 g higher than that of wheat infected with powdery mildew, significantly improving wheat yield. KTP delayed the germination of powdery mildew spores on the leaf surface, and inhibited the formation of mycelia. In addition, KTP did not affect the growth of wheat or the survival of earthworms. KTP nanoprotective membrane are a green and safe prevention and control materials that are which is expected to be widely used in agriculture to control wheat powdery mildew.

Barley powdery mildew control in Western Australia and beyond

AbstractAustralia is one of the largest barley exporters in the world, with Western Australia accounting for some 40% of national production. The crop is predominantly grown in the south and south‐west of the state in winter and spring, where temperate conditions and higher rainfall levels are more suited to barley than northern and eastern regions. Between 2007 and 2013, prolonged outbreaks of barley powdery mildew (BPM) occurred. This was brought about by a combination of the extensive use of susceptible cultivars and an over‐reliance on a small number of single mode‐of‐action demethylation inhibitor fungicides, which select for mutations in the C14α‐demethylase (Cyp51A) gene. This review highlights the steps taken to reduce losses to BPM, breeding efforts to introduce resistance into cultivars and the success of pre‐breeding research to find new and durable resistance genes. We also draw comparisons with powdery mildew in Australian wheat, where similar factors are leading to substantial outbreaks.

Study on the dynamic distribution of spores of powdery mildew pathogen in wheat by rotor airflow of plant protection UAV

Plant protection drones are fast and efficient application machines that are characterised by high application efficiency and no damage to crops. They are particularly suitable for small areas of farmland and mountainous terrain in regions such as Asia and are currently the dominant insecticide application technology in China. The presence of wind is a prerequisite for the spread and dissemination of airborne diseases and it can directly influence the distance and height of ascent of pathogenic spores. This paper investigates the effect of downwash airflow generated by the flight altitude of a plant protection drone on the horizontal distribution, vertical distribution and ground distribution of powdery mildew spores in wheat. Monitoring the changing dynamics of airborne powdery mildew conidia using spore traps. The test results show that: the number of powdery mildew pathogenic spores is related to various factors such as weather, relative humidity and wind speed; the release of spores is greatly influenced by airflow disturbances but has little effect at the early stages of sporulation; the disease is caused by the accumulation process of pathogenic spores and in the control of powdery mildew in wheat, preventive spraying should be carried out within 2–3 days of the germination of pathogenic spores. The study lays the foundation for further in-depth research on the spread of powdery mildew spores and improved pest control, and provides a basis for scientific and rational spraying and control by agricultural drones.

Wheat powdery mildew resistance: from gene identification to immunity deployment.

Powdery mildew is one of the most devastating diseases on wheat and is caused by the obligate biotrophic phytopathogen Blumeria graminis f. sp. tritici (Bgt). Due to the complexity of the large genome of wheat and its close relatives, the identification of powdery mildew resistance genes had been hampered for a long time until recent progress in large-scale sequencing, genomics, and rapid gene isolation techniques. Here, we describe and summarize the current advances in wheat powdery mildew resistance, emphasizing the most recent discoveries about the identification of genes conferring powdery mildew resistance and the similarity, diversity and molecular function of those genes. Multilayered resistance to powdery mildew in wheat could be used for counteracting Bgt, including durable, broad spectrum but partial resistance, as well as race-specific and mostly complete resistance mediated by nucleotide-binding and leucine rich repeat domain (NLR) proteins. In addition to the above mentioned layers, manipulation of susceptibility (S) and negative regulator genes may represent another layer that can be used for durable and broad-spectrum resistance in wheat. We propose that it is promising to develop effective and durable strategies to combat powdery mildew in wheat by simultaneous deployment of multilayered immunity.

Fighting wheat powdery mildew: from genes to fields.

Host resistance conferred by Pm genes provides an effective strategy to control powdery mildew. The study of Pm genes helps modern breeding develop toward more intelligent and customized. Powdery mildew of wheat is one of the most destructive diseases seriously threatening the crop yield and quality worldwide. The genetic research on powdery mildew (Pm) resistance has entered a new era. Many Pm genes from wheat and its wild and domesticated relatives have been mined and cloned. Meanwhile, modern breeding strategies based on high-throughput sequencing and genome editing are emerging and developing toward more intelligent and customized. This review highlights mining and cloning of Pm genes, molecular mechanism studies on the resistance and avirulence genes, and prospects for genomic-assisted breeding for powdery mildew resistance in wheat.

Analysis of defense-related gene expression and leaf metabolome in wheat during the early infection stages of Blumeria graminis f.sp. tritici.

Blumeria graminis f.sp. tritici (Bgt) is an obligate biotrophic fungal pathogen responsible for powdery mildew in bread wheat (Triticum aestivum L.). Upon Bgt infection, the wheat plant activates basal defense mechanisms namely PAMP-triggered immunity (PTI) in the leaves during the first few days. Understanding this early stage of quantitative resistance is crucial for developing new breeding tools and evaluating plant resistance inducers for sustainable agricultural practices. In this sense, we used a combination of transcriptomic and metabolomic approaches to analyze the early steps of the interaction between Bgt and the moderately susceptible wheat cultivar Pakito. Bgt infection resulted in an increasing expression of genes encoding pathogenesis-related proteins (PR-proteins, PR1, PR4, PR5 and PR8), known to target the pathogen, during the first 48 hours post-inoculation. Moreover, RT-qPCR and metabolomic analyses pointed out the importance of the phenylpropanoid pathway in quantitative resistance against Bgt. Among metabolites linked to this pathway, hydroxycinnamic acid amides containing agmatine and putrescine as amine component accumulated from the second to the fourth day after inoculation. This suggests their involvement in quantitative resistance via cross-linking processes in cell wall for reinforcement, what is supported by the up-regulation of PAL (phenylalanine ammonia-lyase), PR15 (encoding an oxalate oxidase) and POX (peroxidase) after inoculation. Finally, pipecolic acid, which is considered as a signal involved in systemic acquired resistance (SAR), accumulated after inoculation. These new insights lead to a better understanding of basal defense in wheat leaves after Bgt infection.

Screening-Capture-Integrated Electrochemiluminescent Aptasensor Based on Mesoporous Silica Nanochannels for the Ultrasensitive Detection of Deoxynivalenol in Wheat.

To prevent the contamination of cereals by mycotoxins, establishing a sensitive and rapid method for the detection of mycotoxins is essential. In this study, a screening-capture-integrated electrochemiluminescence (ECL) aptasensor based on mesoporous silica films (MSFs) was successfully prepared for the ultrasensitive and highly selective detection of deoxynivalenol (DON) in wheat. The narrow nanochannels of MSFs can realize size screening, thereby eliminating the influence of macromolecular substances and providing a pure environment for the signal probe (tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)32+)) to reach the indium tin oxide (ITO) conductive substrate, which significantly improves the anti-interference ability of the screening-capture-integrated ECL sensor. The aptamer (Apt) attached to the surface of the MSFs can specifically capture DON, and the resulting DON-Apt complex has a gated effect on the MSFs, triggering the inhibition of Ru(bpy)32+ in the electrolyte from reaching the ITO surface. Therefore, the ECL intensity of the sensor decreased with increasing DON concentration to achieve a quantitative detection of DON. Under optimized conditions, the linear range of the screening-capture-integrated ECL aptasensor was 0.001-200 μg/kg, and the detection limit was as low as 5.27 × 10-5 μg/kg (S/N = 3). In conclusion, this study developed a screening-capture-integrated ECL aptasensor that combines size screening and specific capture for the detection of DON in wheat, providing a new approach for the early detection of wheat mildew.

Simultaneous analysis of mildew degree and aflatoxin B1 of wheat by a multi-task deep learning strategy based on microwave detection technology

This study proposes a novel qualitative and quantitative analysis method for wheat mildew degree and aflatoxin B1 (AFB1) by microwave detection technology combined with multi-task learning strategy. The team developed a miniaturized microwave detection device with double heterodyne mixing structure, and used this device to obtain the transmission indexes (amplitude and phase) of moldy wheat samples. A multi-task learning model based on convolutional neural network (CNN) structure was designed to realize the self-learning and model calibration of the transmission indexes, and to complete the classification and regression tasks of wheat mildew degree recognition and AFB1 content detection. The obtained results found that in the qualitative analysis, the accuracy, precision, recall, and F1-score of the fusion CNN model were all 100% during the prediction. In the quantitative detection, the RMSEP, RP2 and RPD of the fusion CNN model respectively were 2.0138 μg kg−1, 0.9807 and 7.2849. The overall results reveal that it is feasible to realize simultaneous analysis of the contamination level of wheat mycotoxins by microwave detection technology combined with deep learning, and the application of multi-task learning strategy can effectively avoid the waste of resources in the process of model calibration.

Fungicidal spectrum and biological properties of a new fungicide, pyriofenone.

Pyriofenone is a new fungicide developed by Ishihara Sangyo Kaisha, Ltd. To determine the fungicidal spectrum of pyriofenone, in vivo pot tests and in vitro mycelial growth-inhibition tests were conducted. Pyriofenone showed excellent activity against wheat and cucumber powdery mildew and moderate efficacy against rice blast in the pot tests. In the mycelial growth-inhibition tests, most fungi were not affected by pyriofenone except for Botrytis cinerea, Helminthosporium sacchari, Pseudocercosporella herpotrichoides, Pyricularia oryzae, Rosellinia necatrix, and Verticillium dahliae. The fungicidal properties of pyriofenone on powdery mildew in cucumber and wheat were evaluated precisely. Pyriofenone exhibited excellent preventive and residual activities. It had high rainfastness in the cucumber leaves against powdery mildew. Pyriofenone also showed inhibitory activity on lesion development upon application until 2 days after inoculation, and the lesion expansion and sporulation of the cucumber powdery mildew fungus were effectively controlled. Furthermore, pyriofenone showed translaminar and vapor activities.

Efficiency of different tillage, fertilization systems under short crop rotation and their influence on phytosanitary condition of agrocenoses

The main restraining factor of the widespread introduction of minimization of mechanical tillage into the domestic agricultural practice was and remains the problem of deterioration of the phytosanitary state of agrophytocenoses and the soil environment. Intensive use of pesticides, especially in violation of the regulations of their use, does not solve the problem, and often, on the contrary, aggravates it. Ukraine ranks sixth-seventh in the world in terms of contamination of plant products with pesticide residues. Agricultural practice proves that all pesticides are toxic to humans, and many of them also have mutagenic activity. They also cause great harm to living nature, suppressing the biological activity of soils, destroying pollinating insects, etc. Given this, the value of mechanical cultivation in regulating the phytosanitary state of agrobiocenoses and soil will increase, especially in ecological and organic agriculture. Research (2020– 2022) on the chernozems of the typical experimental field of Bila Ttserkva National Agrarian University established changes in the phytosanitary state of the agrobiocenoses of the field specialized grain five-field crop rotation and its productivity under different systems of the main cultivation of the chernozem soil of a typical low-humus medium loam in the Right Bank Forest Steppe of Ukraine, namely: shelf-disc the treatment has an advantage only in terms of significant limitation of dark brown spotting on winter wheat for sowing after buckwheat. In the remaining cases, it was significantly inferior to control. Indicators of the phytosanitary state of agrobiocenoses significantly worsen with constant soil disking. Discless tillage has an advantage only in terms of significant limitation of dark brown spotting on winter wheat after sowing after buckwheat. In the remaining cases, it was significantly inferior to control. Indicators of the phytosanitary state of agrobiocenoses significantly worsen with constant soil disking. The abundance of weeds is insignificantly lower under differentiated than shelf-disc cultivation. Fertilizer systems did not affect weediness, but worsened other indicators of phytosanitary status. With differentiated cultivation, the population of larvae of the winter weevil and the prevalence of helminth sporous root rot, powdery mildew and septoriosis of winter wheat after buckwheat, as well as the abundance of the meadow butterfly and wireworm in the agrophytocenosis of corn, are significantly reduced. Productivity of crop rotation is almost at the same level for shelf-disc and differentiated tillage. With chisel-disk and disk processing, it is significantly reduced. In a five-field field row crop rotation, differentiated tillage is recommended, in which deep (25–27 cm) plowing is performed only in one field under the row crop, where manure is applied, and in the rest of the fields, plowing and disc tillage to different depths, taking into account biological features of agricultural crops.

The influence of environmentally safe preparations on the yield and infection by winter wheat diseases in the conditions of the Right-Bank Forest Steppe

Goal. To assess the impact of seed and crop treatment with biopreparations, as well as the introduction of a biodestructor into the soil, on the productivity of winter wheat and its damage by diseases. Methods. Field — for registration of diseases of winter wheat; quantity and weight — for determination of the crop structure; mathematical-statistical — for finding out the reliability of the results obtained. Results. It was established that pre-sowing treatment of seeds with ecologically safe preparations (Vympel-K, Organic-balance, MikoHelp) helps to increase the yield of winter wheat by 2.6–8.8% in the case of seed treatment with the Organic-balance preparation and introduction of the Organic-balance biodestructer into the soil. Foliar treatment with Organic-balance helped to increase yield by 5.9–9.2%, and application of the biodestructor Organic-balance to the soil increased by 2.9–6.3%. In addition, seed treatment with biopreparations and a biodestructor reduced the spread of root rot to 6.9–9.6% compared to 14.2% in the control variant. The treatment of crops with the drug Organic-balance restrained the spread of powdery mildew of wheat to 60–64% compared to 77–80% in the control. Conclusions. Application of biologically safe preparations (Organic-balance biodestructor — in the amount of 1.5 l/ha, Vympel-K — 0.5 l/t, Organic-balance — 1.5 l/t, MikoHelp — 3.0 l/t), treatment per leaf (Organic-balance — 0.5 l/ha) contributed to a significant (2.9–9.2%) increase in the productivity of winter wheat and a reduction in its damage by diseases. Preparations for seed treatment showed a higher percentage efficiency — an increase in yield by 5.7–8.8% in areas where the destructor was not applied and foliar treatment was not carried out compared to the background (2.6–4.1%), where these measures did not conduct. Treatment with the biological preparation MikoHelp gives the greatest effect. Its use contributes to an increase in yield by 4.1–8.8%, or 0.24–0.45 t/ha. The specified ecologically safe elements of agricultural technology are recommended to be used to improve ecologically safe technologies for growing winter wheat in agro-formations of various forms of ownership of the Right-Bank Forest Steppe of Ukraine.

Rhein–Amino Acid Ester Conjugates as Potential Antifungal Agents: Synthesis and Biological Evaluation

In the search for crop protectants, amino acid ester conjugates have been widely investigated as potential antifungal agents. In this study, a series of rhein-amino acid ester conjugates were designed and synthesized in good yields, and their structures were confirmed by 1H-NMR, 13C-NMR and HRMS. The bioassay results revealed that most of the conjugates exhibited potent inhibitory activity against R. solani and S. sclerotiorum. In particular, conjugate 3c had the highest antifungal activity against R. solani with an EC50 value of 0.125 mM. For S. sclerotiorum, conjugate 3m showed the highest antifungal activity with an EC50 value of 0.114 mM. Satisfactorily, conjugate 3c exhibited better protective effects than that of the positive control, physcion, against powdery mildew in wheat. This research supports the role of rhein-amino acid ester conjugates as potential antifungal agents for plant fungal diseases.

The broad use of the Pm8 resistance gene in wheat resulted in hypermutation of the AvrPm8 gene in the powdery mildew pathogen

BackgroundWorldwide wheat production is under constant threat by fast-evolving fungal pathogens. In the last decades, wheat breeding for disease resistance heavily relied on the introgression of chromosomal segments from related species as genetic sources of new resistance. The Pm8 resistance gene against the powdery mildew disease has been introgressed from rye into wheat as part of a large 1BL.1RS chromosomal translocation encompassing multiple disease resistance genes and yield components. Due to its high agronomic value, this translocation has seen continuous global use since the 1960s on large growth areas, even after Pm8 resistance was overcome by the powdery mildew pathogen. The long-term use of Pm8 at a global scale provided the unique opportunity to study the consequences of such extensive resistance gene application on pathogen evolution.ResultsUsing genome-wide association studies in a population of wheat mildew isolates, we identified the avirulence effector AvrPm8 specifically recognized by Pm8. Haplovariant mining in a global mildew population covering all major wheat growing areas of the world revealed 17 virulent haplotypes of the AvrPm8 gene that grouped into two functional categories. The first one comprised amino acid polymorphisms at a single position along the AvrPm8 protein, which we confirmed to be crucial for the recognition by Pm8. The second category consisted of numerous destructive mutations to the AvrPm8 open reading frame such as disruptions of the start codon, gene truncations, gene deletions, and interference with mRNA splicing. With the exception of a single, likely ancient, gain-of-virulence mutation found in mildew isolates around the world, all AvrPm8 virulence haplotypes were found in geographically restricted regions, indicating that they occurred recently as a consequence of the frequent Pm8 use.ConclusionsIn this study, we show that the broad and prolonged use of the Pm8 gene in wheat production worldwide resulted in a multitude of gain-of-virulence mechanisms affecting the AvrPm8 gene in the wheat powdery mildew pathogen. Based on our findings, we conclude that both standing genetic variation as well as locally occurring new mutations contributed to the global breakdown of the Pm8 resistance gene introgression.