Inheritance Of Powdery Mildew Resistance Research Articles

Molecular Characterizations of the er1 Alleles Conferring Resistance to Erysiphe pisi in Three Chinese Pea (Pisum sativum L.) Landraces.

Powdery mildew caused by Erysiphe pisi DC. is a major disease affecting pea worldwide. This study aimed to confirm the resistance genes contained in three powdery mildew-resistant Chinese pea landraces (Suoshadabaiwan, Dabaiwandou, and Guiwan 1) and to develop the functional markers of the novel resistance genes. The resistance genes were identified by genetic mapping and PsMLO1 gene sequence identification. To confirm the inheritance of powdery mildew resistance in the three Landraces, the susceptible cultivars Bawan 6, Longwan 1, and Chengwan 8 were crossed with Suoshadabaiwan, Dabaiwandou, and Guiwan 1 to produce F1, F2, and F2:3 populations, respectively. All F1 plants were susceptible to E. pisi, and phenotypic segregation patterns in all the F2 and F2:3 populations fit the 3:1 (susceptible: resistant) and 1:2:1 (susceptible homozygotes: heterozygotes: resistant homozygotes) ratios, respectively, indicating powdery mildew resistance in the three Landraces were controlled by a single recessive gene, respectively. The analysis of er1-linked markers and genetic mapping in the F2 populations suggested that the recessive resistance genes in three landraces could be er1 alleles. The cDNA sequences of 10 homologous PsMLO1 cDNA clones from the contrasting parents were obtained. A known er1 allele, er1-4, was identified in Suoshadabaiwan. Two novel er1 alleles were identified in Dabaiwandou and Guiwan 1, which were designated as er1-13 and er1-14, respectively. Both novel alleles were characterized with a 1-bp deletion (T) in positions 32 (exon 1) and 277 (exon 3), respectively, which caused a frame-shift mutation to result in premature termination of translation of PsMLO1 protein. The co-dominant functional markers specific for er1-13 and er1-14, KASPar-er1-13, and KASPar-er1-14 were developed and effectively validated in populations and pea germplasms. Here, two novel er1 alleles were characterized and their functional markers were validated. These results provide powerful tools for marker-assisted selection in pea breeding.

Genetic control of juvenile resistance to powdery mildew in spring bread wheat cultivars from the VIR collection

Background. Bread wheat (Triticum aestivum L.) is one of the major food crops of humankind. Powdery mildew, caused by Blumeria graminis f. sp. tritici, is the most destructive foliar disease capable of causing great yield losses in epidemic years. Breeding for resistance to powdery mildew is the most economical and effective way to control this disease. By now, 68 loci were identified to contain more than 90 alleles of resistance to powdery mildew in wheat. However, there is a permanent necessity in finding new sources of resistance.The objective of the present study was to characterize the seedling powdery mildew resistance in some spring bread wheat varieties from the VIR collection and determine the inheritance of powdery mildew resistance in these accessions.Materials and methods. The powdery mildew resistant varieties ‘SW Kungsjet’ (k-66036), ‘SW Kronjet’ (k-66097), ‘Boett’ (k-66353), ‘Batalj’ (k-67116), ‘Stilett’ (k-67119) ‘Pasteur’ (k-66093) were crossed with a resistant line ‘Wembley 14.31’ (k-62557) containing the Pm12 gene, and with ‘SW Milljet’ (k-64434); the variety ‘Sibirka Yartsevskaya’ (k-38587) was used as a susceptible parent and control. The hybrid populations F2 were inoculated with the fungus population from local field and evaluated. The powdery mildew population manifested virulence to Pm1a, Pm2, Pm3a-f, Pm4a-b, Pm5a, Pm6, Pm7, Pm8, Pm9, Pm10, Pm11, Pm16, Pm19, Pm28, and avirulence to Pm12. The degree of resistance was assessed on days 8 and 10 after the inoculation using the Mains and Dietz scale (Mains, Dietz, 1930). The castrated flowers in the spikes were pollinated using the twell-method (Merezhko et al., 1973). Chi-squared for goodness of fit test was used to determine deviation of the observed data from the theoretically expected segregation.Results. According phytopathological and genetic tests, juvenile resistance in the varieties ‘SW Kungsjet’, ‘SW Kronjet’, ‘Boett’, ‘Batalj’, ‘Stilett’ and ‘Pasteur’ is controlled by dominant genes, which differ from Pm1a, Pm2, Pm3a-f, Pm4a-b, Pm5a, Pm6, Pm7, Pm8, Pm9, Pm10, Pm11, Pm12, Pm16, Pm19, and Pm28. The varieties ‘SW Milljet’, ‘SW Kronjet’ and ‘Pasteur’ had identical resistance genes. Genetic control of juvenile resistance to powdery mildew in ‘Batalj’, ‘Boett’, ‘Stilett’, ‘SW Milljet’, ‘SW Kungsjet’, ‘Pasteur’ was governed by different genes.Conclusions. The varieties ‘SW Kungsjet’, ‘SW Kronjet’, ‘Boett’ have been maintaining adult and seedling resistance since 2005, and ‘Batalj’, ‘Stilett’ and ‘Pasteur’ since 2017. Seedling resistance of these varieties to local powdery mildew population is controlled by dominant genes. A high degree of resistance was displayed by ‘SW Kungsjet’ and ‘SW Kronjet’ in the Novosibirsk Province, while ‘SW Kungsjet’ was resistant to mildew populations of Tatarstan. The variety ‘Pasteur’ manifested seedling resistance to leaf rust, and ‘SW Kungsjet’ was resistant to loose smut. By summing all the results, it may be suggested that the varieties ‘SW Kungsjet’, ‘SW Kronjet’, ‘Boett’, ‘Batalj’, ‘Stilett’ and ‘Pasteur can serve as good donors of powdery mildew resistance in wheat breeding.

Inheritance and molecular mapping of powdery mildew (Golovinomyces orontii) resistance gene(s) in sunflower (Helianthus annuus L.).

Sources of resistance to powdery mildew incited by Golovinomyces orontii have been identified in wild sunflowers and few exotic lines. The present investigation has been undertaken to study the inheritance of powdery mildew resistance and to map the quantitative trait loci (QTLs) governing resistance to powdery mildew in a multiple disease resistance line, TX16R (PI 642072). The inheritance was observed as a continuous distribution in a set of 264 F2 population and 93 recombinant inbred lines (RILs) of a cross between a highly susceptible accession PS 2023 and TX16R. Screening of the two population sets was done with 484 sunflower-specific SSR primers of which 175 primers showed polymorphism between the parents. Based on the phenotyping and genotyping data, the linkage map was constructed with 93 RILs. The map spanned 1200cM and included 64 markers distributed along the 17 sunflower chromosomes in the haploid set. Quantitative trait loci (QTL) analysis identified three genomic regions for resistance to powdery mildew, two of which mapped on chromosome 10 and one on chromosome 5. This is the first report on mapping of powdery mildew resistance in sunflower and paves the way in fine mapping and introgression of resistance for powdery mildew in sunflower through marker-assisted breeding.

Genetics and Resistance Mechanism of the Cucumber (Cucumis sativus L.) Against Powdery Mildew

Powdery mildew (PM) is one of the most severe foliar diseases in cucumbers (Cucumis sativus L.), but the inheritance of PM resistance remains unclear. The molecular mechanisms underlying PM resistance have been a bottle-neck in the advancement of cucumber cultivars. However, recent findings have provided valuable information on the molecular mechanisms of defense-related genes and signaling pathways. Moreover, several PM resistance genes have been cloned, and their functions have been verified in cucumbers. In this review, we will focus on the inheritance and molecular mechanisms of PM resistance in cucumbers and generalize the known PM resistance genes and signaling pathways. These novel discoveries have advanced the molecular understanding of PM resistance and provide a foundation for the further breeding of resistant cucumber varieties.

Inheritance of powdery mildew resistance in Aurotica’s introgressive lines

Aim. Frequency of alien chromatin inheritance in hybrids from crosses of common wheat and introgressive lines determines whether transfer of alien chromatin controlling desirable trait into common wheat genome would be successful. The paper presents results of introgressive lines assessment for frequency of transfer of alien dominant trait of powdery mildew resistance to their progeny in the crossing with recurrent genotype Aurora, which was a recipient cultivar in the development of these lines. Methods. Artificial hybridization of introgressive lines with common wheat, visual assessment of cross components and hybrids F1 and F2 for powdery mildew resistance. Determination of chromosome numbers and chromosome configurations in meiosis M1 of F1 plants. Results. Among F2 plants, from the crossing of resistant to powdery mildew introgressive lines of Aurotica and susceptible cultivar Aurora excess of susceptible plants was observed. In order to explain distortion of ratio susceptible : resistant plants in comparison with expected for monogenic segregation, we propose the determined fact of absence in meiosis M1 of MPC of bivalent chromosome conjugation, which results in formation of aneuploid gametes and zygotes with reduced viability. As the third parameter, the one that can influence the mentioned ratio, we propose d parameter, which quantitatively expresses excess of gametes without alien chromatin controlling resistance to powdery mildew. Conclusion. Model of inheritance, which best explains experimental data of F2 plants segregation for resistant and susceptible if the trait is controlled by one dominant gene, takes into account the following conditions for calculation of probabilities of resistant or susceptible plant formation: all female and only euploid male gametes function, nullisomic zygotes are not viable, during meiosis elimination of meiosis products with alien chromatin occurs with parameter d = 0,11–0,12.Received 11.06.2018

Development of DNA markers for Slmlo1.1, a new mutant allele of the powdery mildew resistance gene SlMlo1 in tomato (Solanum lycopersicum).

Reductions in growth and quality due to powdery mildew (PM) disease cause significant economic losses in tomato production. Oidium neolycopersici was identified as the fungal species responsible for tomato PM disease in South Korea in the present study, based on morphological and internal transcribed spacer DNA sequence analyses of PM samples collected from two remote regions (Muju and Miryang). The genes involved in resistance to this pathogen in the tomato accession 'KNU-12' (Solanum lycopersicum var. cerasiforme) were evaluated, and the inheritance of PM resistance in 'KNU-12' was found to be conferred via simple Mendelian inheritance of a mutant allele of the PM susceptibility locus Ol-2 (SlMlo1). Full-length cDNA analysis of this newly identified mutant allele (Slmlo1.1) showed that a 1-bp deletion in its coding region led to a frameshift mutation possibly resulting in SlMlo1 loss-of-function. An alternatively spliced transcript of Slmlo1.1 was observed in the cDNA sequences of 'KNU-12', but its direct influence on PM resistance is unclear. A derived cleaved amplified polymorphic sequence (dCAPS) and a high-resolution melting (HRM) marker were developed based on the 1-bp deletion in Slmlo1.1, and could be used for efficient marker-assisted selection (MAS) using 'KNU-12' as the source for durable and broad-spectrum resistance to PM.

Inheritance of powdery mildew resistance in sunflower (Helianthus annuss L.)

Powdery mildew caused by Golovinomyces cichoracearum is the most important disease on sunflower worldwide. The investigation was undertaken to determine the mode of genetic inheritance of powdery mildew resistance in five F1 and F2 populations of sunflower. The four hybrids were derived by crossing resistant with susceptible genotype and one hybrid between resistant and highly susceptible genotype. Out of 23 hybrids, five crosses viz., PM-22x PM-36, PM-14xPM-36, PM-16xPM-38, PM-17xPM-35 and PM-34x PM-23 were resistant under natural as well as artificial epiphytotic screening conditions and the same five resistant hybrids segregated in 9R:7S ratio in F2 population indicating involvement of two independent loci controlling powdery mildew resistance in sunflower.

Inheritance of Resistance to Powdery Mildew Race 2 in Citrullus lanatus var. lanatus

Information on the mode of inheritance of powdery mildew resistance in watermelon is important for designing a breeding strategy for the development of new cultivars. Resistance in the watermelon accession PI 270545 was investigated by generation means analysis by crossing it with susceptible PI 267677. The analyses showed involvement of two genes, a recessive resistance gene, pmr-1, and a dominant gene for moderate resistance, Pmr-2. Resistance to powdery mildew in the leaf had a large dominance effect and a heritability of 71%. The additive-dominance model was inadequate in explaining variation in leaf resistance as revealed by the joint scaling test. However, nonallelic interactions could not be detected by the nonweighted six-parameter scaling test. For stem resistance, the additive-dominance model was adequate, and inheritance was controlled mainly by additive effects. A high narrow-sense heritability of 79% suggested that selection for stem resistance in early generations would be effective.

Inheritance of resistance to powdery mildew in the watermelon and development of a molecular marker for selecting resistant plants

The race of watermelon powdery mildew (PM) present in South Korea was confirmed, and genetic inheritance of resistance to this race was determined from a cultivars ‘Arka Manik’. Bioassay tested on a set of melon differential genotypes indicated that the race of P. xathii predominating in South Korea was either belonged to race 1W or another race different from race 2W. Inheritance of PM resistance was studied from F2, F2:3, and reciprocal backcross generations derived from ‘Arka Manik’ (resistant cultivar) × HS3355 (susceptible line). The segregation ratios indicated that resistance to PM in ‘Arka Manik’ is conditioned by a single incompletely dominant gene (Pm1.1). Bulked segregant analysis (BSA) of F2 plants using random amplification of polymorphic DNA (RAPD) identified one polymorphic polymerase chain reaction (PCR) band (OP-R483) and a close linkage (3.6 cM) was observed between resistance and OP-483. The marker OP-483 was converted to cleaved amplified polymorphic sequence (CAPS-G/C) and a single nucleotide polymorphism (SNP) marker (MCA-A/G) using melting curve analysis. MCA-A/G marker genotyping of a wide range of susceptible breeding lines revealed that the marker allele of ‘Arka Manik’ was unique in breeding populations.

SNP GENOTYPING IN TETRAPLOID CUT ROSES

Genetic analysis in tetraploid rose is complex: inheritance of genes could be disomic, keeping the two original diploid genomes separate but it could also be tetrasomic, permitting the four homologous chromosomes to pair and recombine during meiosis, and including the possibility of double reduction. Marker-assisted breeding (MAB) is particularly suited for traits such as disease resistance since disease resistance assays are laborious, time and money consuming, but we need to extend the number of markers for better coverage of the genome before being able to find associations and apply MAB. Our goal was to produce hundreds of single nucleotide polymorphism (SNP) markers to provide the coverage needed. Disease resistance, more specifically powdery mildew resistance is our primary target because chemical pest control will be increasingly limited. Molecular markers could provide a good tool to facilitate selection in an earlier stage of a breeding program. We investigated the inheritance of powdery mildew resistance in an F1 population originating from a cross between two tetraploid genotypes, P540 and P867, differing in powdery mildew resistance. SNP marker development started with Next Generation Sequencing (Illumina GAII) of cDNA obtained from RNA isolated from flowers at three developmental stages for each parent. We identified a large number of putative single nucleotide polymorphisms (SNPs) between and within both parents. SNPs that were expected to segregate as simplex × nulliplex and duplex × nulliplex markers were selected for genotyping the entire F1 population using the KASP platform (KBioscience). For a pilot study on a limited number of SNPs we defined specific selection criteria resulting in a set of 20 SNPs for testing on the population. It was found that selecting the SNPs according to the ratio of the number of reads from the Illumina sequencing data did not guarantee the appropriate segregation ratios in the progeny. Continued research will focus on identifying haplotypes and on QTL mapping of powdery mildew resistance. We will present results on the criteria used to select SNP markers, success rate, the realised segregation ratios in the offspring, the reconstruction of the homologous chromosomes (haplotypes) in the cross, and QTL mapping of powdery mildew resistance.

Molecular mapping of a powdery mildew resistance gene in common wheat landrace Baihulu and its allelism with Pm24

Powdery mildew is one of the most devastating diseases of wheat in areas with cool and maritime climates. Chinese wheat landrace Baihulu confers a high level of resistance against a wide range of Blumeria graminis DC f. sp. tritici (Bgt) races, especially those currently prevailing in Shaanxi. The objectives of this study were to determine the chromosome bin location of the mlbhl gene from Baihulu and its allelism with Pm24. To investigate the inheritance of powdery mildew resistance and detect adjacent molecular markers, we constructed a segregating population of 301 F(2) plants and corresponding F(2:3) families derived from Baihulu/Shaanyou 225. Genetic analysis revealed that a single dominant gene was responsible for seedling stage powdery mildew resistance in Baihulu. A genetic map comprising Xgwm106, Xgwm337, Xgwm1675, Xgwm603, Xgwm789, Xbarc229, Xgpw4503, Xcfd72, Xcfd83, Xcfd59, Xcfd19, and mlbhl spanned 28.2 cM on chromosome 1D. Xgwm603/Xgwm789 and Xbarc229 were flanking markers tightly linked to mlbhl at genetic distances of 1.5 and 1.0 cM, respectively. The mlbhl locus was located in chromosome bin 1DS 0.59-1.00 delimited by the SSR markers Xgwm337 and Xbarc229. When tested with a differential array of 23 Bgt isolates Baihulu displayed a response pattern that was clearly distinguishable from that of Chiyacao and varieties or lines possessing documented Pm genes. Allelism analysis indicated that mlbhl is a new gene, either allelic or closely linked with Pm24. The new gene was designated Pm24b.

Mapping wheat powdery mildew resistance derived fromAegilops markgrafii

The pattern of inheritance of powdery mildew resistance expressed by two bread wheat (cultivar ‘Alcedo’)/Aegilops markgrafiiintrogression lines was explored using F2populations bred from crosses made with the powdery mildew-susceptible cultivar ‘Kanzler’. Disease reaction was tested at both seedling and adult plant stages. Two resistance loci, designatedQPm.ipk-1AandQPm.ipk-7A, were identified as mapping to the distal ends of chromosome arms 1AS and 7AL, respectively. WhereasQPm.ipk-1Awas expressed throughout the plant's life,QPm.ipk-7Awas only effective at the seedling stage. The map location of both resistance loci indicated that resistances originated fromA. markgrafii.The possible genetic relationship of these disease-resistant genes to knownPmgenes is discussed in the context of synteny.

Powdery mildew resistance in soybean PI 243540 is controlled by a single dominant gene

Powdery mildew (Microsphaera diffusa Cooke & Peck) is a common disease of soybean in many countries of the world, including the northern United States of America and parts of Canada. The genetic resistance of soybean to M. diffusa is known to be controlled by a single locus with three alleles designated as Rmd, Rmd-c and rmd. Identification and characterization of sources of resistance is a prerequisite for the development of resistant cultivars. The objective of this study was to determine the inheritance of powdery mildew resistance in a plant introduction (PI) from Japan, PI 243540. The inheritance of powdery mildew was determined in a segregating population from a cross between powdery mildew susceptible Ohio cultivar Wyandot and PI 243540. The parents and the progeny showed a consistent response to powdery mildew for all growth stages of plants. The two parents, the F1, F2, and F2:3 families from the cross were screened in a greenhouse and field following inoculation with M. diffusa. All F1 plants were resistant to M. diffusa and χ2 analysis for segregation in the population of 343 F2 plants indicated a tight fit for a 3:1 (P = 0.78) ratio, indicative of a single dominant gene. In the next generation, the 334 F2-derived families fit an expected 1 resistant:2 segregating:1 susceptible segregation ratio (P =0.88), which confirmed the results obtained in the F2 generation. Our results indicate that the powdery mildew resistance derived from PI 243540 is controlled by a single dominant gene linked to the Rmd/ Rmd-c/rmd locus. The simple inheritance of this gene should make it relatively easy to find linked DNA markers and transfer the gene to susceptible elite cultivars using the backcross breeding approach.

Powdery Mildew Resistance in the Peach Cultivar Pamirskij 5 Is Genetically Linked with the Gr Gene for Leaf Color

Peach powdery mildew (Podosphaera pannosa var. persicae) is a major disease of peach trees [Prunus persica (L.) Batsch]. Various studies on powdery mildew resistance have been conducted in peach. The present study was initiated to determine the inheritance of powdery mildew resistance found in the green leaf peach rootstock ‘Pamirskij 5’. Crosses were carried out between ‘Pamirskij 5’ and the susceptible red leaf peach rootstock ‘Rubira’®. Segregation analysis performed in F1 (1:0), F2 (3:1), and test cross (1:1) progenies indicated single dominant gene control of peach powdery mildew resistance in ‘Pamirskij 5’. Cosegregation analysis provided clear evidence that the loci for powdery mildew resistance and the leaf color trait are very closely linked in ‘Pamirskij 5’. The estimated distance between these loci was 0 ± 3.26 cM for the F2 and 0 ± 0.00 cM for the test cross populations.

Inheritance of Powdery Mildew Resistance in Cucumber ( Cucumis sativus L.) and Development of an AFLP Marker for Resistance Detection

Cucumber powdery mildew is one of the most destructive diseases of cucumber throughout the world. In the present study, inheritance of powdery mildew resistance in three crosses, and linkage of resistance with amplified fragment length polymorphism (AFLP) markers are studied to formulate efficient strategies for breeding cultivars resistant to powdery mildew. The joint analysis of multiple generations and AFLP technique has been applied in this study. The best model is the one with two major genes, additive, dominant, and epistatic effects, plus polygenes with additive, dominant, and epistatic effects (E-1-0 model). The heritabilities of the major genes varied from 64.26% to 97.82%, and susceptibility was incompletely dominant for the two major genes in the three crosses studied. The additive effects of the two major genes and the dominant effect of the second major gene were high, and the epistatic effect of the additive-dominant between the two major genes was the highest in cross I. In cross II, the absolute value of the additive effect, dominant effect, and potential ratio of the first major gene were far higher than those of the second major gene, and the epistatic effect of the additive-additive was the highest. The genetic parameters of the two major genes in cross III were similar to those in cross II. Correlation and regression analyses showed that marker E25/M63-103 was linked to a susceptible gene controlling powdery mildew resistance. The marker could account for 19.98% of the phenotypic variation. When the marker was tested on a diverse set of 29 cucumber lines, the correlation between phenotype and genotype was not significant, which suggested cultivar specialty of gene expression or different methods of resistance to powdery mildew. The target DNA fragment was 103 bp in length, and only a small part was found to be homologous to DNA in the other species evaluated, which indicated that it was unique to the cucumber genome.

Inheritance of Powdery Mildew Resistance and Leaf Macrohair Density in Gerbera hybrida

The cultivated gerbera daisy [Gerbera hybrida (G. jamesonii Bolus ex Adlam × G. viridifolia Schultz-Bip)] often contracts powdery mildew (PM) when grown under conditions of high humidity. During field and greenhouse performance trials conducted with gerberas of the North Carolina State University collection, two half-sib field plants and two of their greenhouse-grown progeny were identified as being highly resistant to PM caused by Podosphaera (Sphaerotheca) fusca (Fr.) Blumer emend. Braun & Takamatsu. These plants were also unusual in having smooth glossy leaves with very low numbers of bristle macrohairs (MHs) on the adaxial leaf surface compared to wild type. The primary objectives of this investigation were to determine the mode of inheritance of PM resistance and MH density traits and determine if there was a causal relationship between the phenotypes. Parental genotypes were determined by testcrosses to wild-type, PM-susceptible and MH-high-density leaf cultivars. For each trait, a series of crosses were made to produce PA, PB, F1, F2, BC1A, and BC1B progeny. Linkage relationships among PM resistance and MH density loci were examined by testcrosses. Phenotypic segregation ratios suggested the presence of a dominant allele, Pmr1, determining PM resistance and an unlinked dominant allele, Mhd, determining low density of adaxial bristle MHs and moderate reduction in abaxial smooth MHs. The Pmr1 allele appeared to be incompletely dominant in some heterozygotes where one parent was from a highly PM susceptible background. Modifying genes may have some affect on the level of PM resistance or susceptibility. The Mhd allele appeared to be incompletely dominant in some heterozygotes. Segregation ratios indicated that the wild-type alleles were recessive to the PM-resistance and MH-low-density alleles and given the designation pmr1 and mhd, respectively. Density of leaf MHs did not affect PM resistance.

Genetics of powdery mildew ( Erysiphe polygoni D.C.) resistance in Mungbean ( Vigna radiata (L.) Wilczek)

Gene effects were estimated for powdery mildew disease reaction in mungbean to know gene action involved for powdery mildew resistance. Six generations (P 1, P 2, F 1, F 2, B 1 and B 2), of three crosses, PIP 3-85-2×TARM 18, AKM 8802×TARM 18 and PM 9339×TARM 18 were grown for recording disease incidence (DI), per cent disease index (PDI) and area under disease progress curve (AUDPC). Uniformity in the trend was observed for these three parameters. AUDPC values revealed differential rate of disease development on various genotypes of mungbean. Highest rate of disease development was observed on AKM 8802 (973.65) and lowest on TARM 18 (323.82). Both additive and dominance gene actions were found to be important in inheritance of powdery mildew resistance (all the three characters) including non-allelic interactions. Duplicate type of epistasis was detected for disease incidence and complementary for PDI and AUDPC in most of the crosses. The selected plants from early segregating generations may be intermated to accumulate resistance genes and to fix resistance at desired level. Due consideration should be given to quantify disease using AUDPC values.

Powdery Mildew Resistance in Cucumber

Using an improved method for evaluating powdery mildew (Sphaerotheca cucurbitae) resistance in cucumber, experiments were conducted for the breeding of varieties resistant to powdery mildew that would be adapted to greenhouse cultivation during the period from winter to spring in Japan. The relationship between the resistance to the pathogen and the ambient temperature was analyzed by spray inoculation of a conidial suspension under controlled conditions. Although the varieties. Asomidori-5-gou and Natsufushinari were resistant at temperatures between 25 to 30°C, they became susceptible at 15 to 20°C. The resistance was affected by the duration of the exposure to 30 and 15°C during the day. To screen resistant varieties and lines independently of the temperature, 295 cucumber accessions were inoculated at 20 and 26°C. All the accessions tested could be divided into 3 types based on the differ. ence in the resistance response to temperature. Many of the resistant accessions were Chinese varieties and lines. PI 197088-5, a progeny of PI197088 which originated in India, displayed the highest level of resistance among all the accessions. Thus PI197088-5, whose resistance is temperature-independent appears to be the most suitable parent for the breeding of powdery mildew-resistant cucumber varieties. The inheritance of powdery mildew resistance in PI197088-5 was analyzed using Natsufushinari (resistant under high temperature conditions) and Sharp I (susceptible). As a result it was considered that the powdery mildew resistance in PI197088-5 was due to 2 gene pairs, a recessive gene and an incompletely dominant gene, with the genotype designated as aaBB, while the genotype of Natsufushinari was designated as aabb and that of Sharp 1 as AAbb. The nature of the host-pathogen interaction in PI197088-1 which is the sib-line of PI197088-5 was observed microscopically. The retardation of hyphal growth and development was observed at 2 days after inoculation. Conidia were hardly formed on the epidermis and the number of haustoria in the epidermal cells of PI197088-1 was much lower than that in the susceptible variety Sharp 1. Fluorescence of epidermal cells was observed along the hyphae in PI197088-1 at 6 days after the inoculation, suggesting the existence of a hypersensitive reaction.

Inheritance of Powdery Mildew Resistance in Sugar Beet Derived from Beta vulgaris subsp. maritima.

Powdery mildew of sugar beet (Beta vulgaris), caused by Erysiphe polygoni, was introduced into North American in 1974. Since then, chemical control has been needed. Moderate resistance of a slow-mildewing type is known and has been used commercially. High resistance was identified recently in B. vulgaris subsp. maritima accessions WB97 and WB242 and has been backcrossed into sugar beet breeding lines. These enhanced lines were used as sources of powdery mildew resistance to determine the inheritance of resistance. Analyses of segregating testcross families showed that resistance from both sources is inherited as a single, dominant, major gene. The gene symbol Pm is proposed for the resistant allele. The allelism of the resistance from the two wild beet sources was not determined. Pm conditions a high level of resistance, but disease developed on matured leaves late in the season. This late development of mildew on lines and the slow-mildewing trait in susceptible, recurrent lines tended to obfuscate discrete disease ratings.

Inheritance of Powdery Mildew Resistance in Sweet Cherry

Most sweet cherry (Prunus avium L.) cultivars grown commercially in the Pacific Northwestern states of the United States are susceptible to powdery mildew, caused by the fungus Podosphaera clandestina (Wall.:Fr.) Lev. The disease is prevalent in the irrigated arid region east of the Cascade Mountains in Washington State. Little is known about genetic resistance to powdery mildew in sweet cherry, although a selection (PMR-1) was identified at Washington State Univ.'s Irrigated Agriculture Research and Extension Center that exhibits apparent foliar immunity to the disease. The objective of this research was to determine the inheritance of powdery mildew resistance from PMR-1. Reciprocal crosses were made between PMR-1 and three high-quality, widely-grown susceptible cultivars (`Bing', `Rainier', and `Van'). Resultant progenies were screened for reaction to powdery mildew colonization using a laboratory leaf disk assay. Assay results were verified by natural spread of powdery mildew among the progeny in a greenhouse and later by placing them among infected trees in a cherry orchard. Segregation within the progenies for powdery mildew reaction fit a 1 resistant: 1 susceptible segregation ratio (P ≤ 0.05), indicating that resistance to powdery mildew derived from PMR-1 was conferred by a single gene.