Susceptibility To Downy Mildew Research Articles

Anatomic and genetic factors associated with the susceptibility of grapevine cultivars to Plasmopara viticola clade aestivalis and clade riparia

The susceptibility of commonly grown grapevine cultivars in the province of Quebec to Plasmopara viticola (Berk. & M.A. Curtis) Berl. & De Toni clades riparia and aestivalis was compared. The relationship between anatomic (density and size of stomata and type of leaf hairs) and genetic composition (percentages of Vitis vinifera L., Vitis riparia Michx. and Vitis aestivalis Michx.) factors of the grapevine cultivars and their susceptibility to the two clades was investigated. The grapevine cultivars were classified according to their susceptibility to each clade. The aggressiveness of the clade riparia was positively correlated with stomata size and negatively correlated with type of leaf hairs. However, the aggressiveness of clade aestivalis was positively correlated with stomata size and density, estimated percentage of V. vinifera ancestry, and the published downy mildew susceptibility of grapevine cultivars; and negatively correlated with estimated percentage of V. riparia ancestry. Furthermore, the grapevine cultivar classification showed that, for P. viticola clade riparia, 44.4%, 44.4%, and 11.1% of the grapevine cultivars were classified as minimally susceptible, moderately susceptible, and highly susceptible, respectively. Alternatively, for P. viticola clade aestivalis, 11.1%, 22.2%, and 66.7% of the grapevine cultivars were classified as minimally susceptible, moderately susceptible, and highly susceptible, respectively. Although some grapevine cultivars fell in the same susceptibility groups for both clades, 78% of grapevine cultivars were classified in different susceptibility groups. The findings of this study provide new information on grapevine and P. viticola interactions, and highlight the importance of knowing which clade of P. viticola is present so that downy mildew control measures can be adapted accordingly.

Susceptibility of Hop Cultivars and Rootstock to Downy Mildew Caused by Pseudoperonospora humuli

Annual downy mildew [Pseudoperonospora humuli (Miyabe & Takah.) G.W. Wilson, (1914)] epidemics threaten hop (Humulus lupulus L. var. lupulus) production throughout the eastern United States. Rootstock rot complicates foliar disease assessments because dormant buds may rot before producing a symptomatic basal shoot and noncolonized buds produce healthy shoots. We selected 12 cultivars to evaluate downy mildew susceptibility (2016 and 2017) and examined the rhizomes of a subset of six cultivars (2018) that showed clear differences in foliar disease ratings to determine rootstock rot susceptibility. Trials were conducted on nontrellised hop yards established at two research farms in 2016 and managed without fungicides. The relative area under the disease progress curve (rAUDPC) values for foliar disease severity and density ratings were consistently higher for ‘Cascade’, ‘Centennial’, and ‘Nugget’ than for ‘Newport’, ‘Tahoma’, and ‘Columbia’. Only ‘Centennial’ had a higher wet-rot cortex discoloration (CD) incidence (71.5%) and severity (31.9%) in its rhizomes than the other cultivars. Among cultivars with low foliar disease levels, Columbia was less vigorous (shoots/plant) than ‘Tahoma’, but had similar wet-rot CD (incidence, 46.4% to 47.2%; severity, 12.5% to 17.7%). The levels of wet-rot CD for ‘Tahoma’, ‘Newport’, and ‘Columbia’ were comparable to those of some cultivars with more severe foliar disease symptoms (‘Nugget’ and ‘Cascade’). Differences in foliar disease among cultivars with similar levels of rootstock rot suggest a resistance defense mechanism to P. humuli. Additional work is needed to clarify rootstock infection sites and characterize defense responses to aid future breeding efforts.

Mining Grapevine Downy Mildew Susceptibility Genes: A Resource for Genomics-Based Breeding and Tailored Gene Editing.

Several pathogens continuously threaten viticulture worldwide. Until now, the investigation on resistance loci has been the main trend to understand the interaction between grapevine and the mildew causal agents. Dominantly inherited gene-based resistance has shown to be race-specific in some cases, to confer partial immunity, and to be potentially overcome within a few years since its introgression. Recently, on the footprint of research conducted in Arabidopsis, putative genes associated with downy mildew susceptibility have been discovered also in the grapevine genome. In this work, we deep-sequenced four putative susceptibility genes—namely VvDMR6.1, VvDMR6.2, VvDLO1, VvDLO2—in 190 genetically diverse grapevine genotypes to discover new sources of broad-spectrum and recessively inherited resistance. Identified Single Nucleotide Polymorphisms were screened in a bottleneck analysis from the genetic sequence to their impact on protein structure. Fifty-five genotypes showed at least one impacting mutation in one or more of the scouted genes. Haplotypes were inferred for each gene and two of them at the VvDMR6.2 gene were found significantly more represented in downy mildew resistant genotypes. The current results provide a resource for grapevine and plant genetics and could corroborate genomic-assisted breeding programs as well as tailored gene editing approaches for resistance to biotic stresses.

QTL identification for downy mildew resistance in cucumber using genetic linkage map based on SSR markers

Fourteen cucumber lines were tested for genetic homozygosity and performed pairwise comparison to identify a pair with the highest DNA polymorphic level. Cucumber accessions CSL0067 and CSL0139 were selected to generate 315 F2 populations. The genetic linkage map based on 66 polymorphic SSR markers was constructed. It composed of eight linkage groups (LGs) spanning 474.4 cM. Downy mildew disease reaction was evaluated in cotyledons, first and second true leaf on 7, 10, and 14 day after inoculation. The results showed that downy mildew resistance was controlled by multiple recessive genes. The susceptible to resistant ratio of F2 progenies fit 9:7 susceptible/resistant segregation types corresponding to duplicate recessive epistasis. Fourteen QTLs were detected. The phenotypic variance ranged from 5.0 to 12.5%, while LOD values ranged from 3.538 to 9.165. Two major QTLs and two QTL hotspots were identified. Moreover, the additive effects data explained that these QTL reduced downy mildew susceptibility.

Comparative Transcriptome Analysis between a Resistant and a Susceptible Wild Tomato Accession in Response to Phytophthora parasitica

Phytophthora parasitica is one of the most widespread Phytophthora species, which is known to cause multiple diseases in tomato and is capable of infecting almost all plant parts. Our current understanding of tomato-Phytophthora parasitica interaction is very limited and currently nothing is known at the whole genome or transcriptome level. In this study, we have analyzed and compared the transcriptome of a resistant and a susceptible wild tomato accession in response to P. parasitica infection using the RNA-seq technology. We have identified 2657 and 3079 differentially expressed genes (DEGs) in treatment vs control comparison of resistant (Sp-R) and susceptible (Sp-S) samples respectively. Functional annotation of DEGs revealed substantial transcriptional reprogramming of diverse physiological and cellular processes, particularly the biotic stress responses in both Sp-R and Sp-S upon P. parasitica treatment. However, subtle expression differences among some core plant defense related genes were identified and their possible role in resistance development against P. parasitica is discussed. Our results revealed 1173 genes that were differentially expressed only in Sp-R accession upon P. parasitica inoculation. These exclusively found DEGs in Sp-R accession included some core plant defense genes, for example, several protease inhibitors, chitinases, defensin, PR-1, a downy mildew susceptibility factor, and so on, were all highly induced. Whereas, several R genes, WRKY transcriptions factors and a powdery mildew susceptibility gene (Mlo) were highly repressed during the resistance outcome. Analysis reported here lays out a strong foundation for future studies aimed at improving genetic resistance of tomato cultivars against to Phytopphthora species.

A first linkage map and downy mildew resistance QTL discovery for sweet basil (Ocimum basilicum) facilitated by double digestion restriction site associated DNA sequencing (ddRADseq).

Limited understanding of sweet basil (Ocimum basilicum L.) genetics and genome structure has reduced efficiency of breeding strategies. This is evidenced by the rapid, worldwide dissemination of basil downy mildew (Peronospora belbahrii) in the absence of resistant cultivars. In an effort to improve available genetic resources, expressed sequence tag simple sequence repeat (EST-SSR) and single nucleotide polymorphism (SNP) markers were developed and used to genotype the MRI x SB22 F2 mapping population, which segregates for response to downy mildew. SNP markers were generated from genomic sequences derived from double digestion restriction site associated DNA sequencing (ddRADseq). Disomic segregation was observed in both SNP and EST-SSR markers providing evidence of an O. basilicum allotetraploid genome structure and allowing for subsequent analysis of the mapping population as a diploid intercross. A dense linkage map was constructed using 42 EST-SSR and 1,847 SNP markers spanning 3,030.9 cM. Multiple quantitative trait loci (QTL) model (MQM) analysis identified three QTL that explained 37–55% of phenotypic variance associated with downy mildew response across three environments. A single major QTL, dm11.1 explained 21–28% of phenotypic variance and demonstrated dominant gene action. Two minor QTL dm9.1 and dm14.1 explained 5–16% and 4–18% of phenotypic variance, respectively. Evidence is provided for an additive effect between the two minor QTL and the major QTL dm11.1 increasing downy mildew susceptibility. Results indicate that ddRADseq-facilitated SNP and SSR marker genotyping is an effective approach for mapping the sweet basil genome.

Susceptibility of Hop Cultivars to Downy Mildew: Associations with Chemical Characteristics and Region of Origin

Hop downy mildew (caused by Pseudoperonospora humuli) is a yield-limiting disease in many hop-production regions of the world. In this research, 110 cultivars that are or were widely grown in the United States, Europe, or Australasia were evaluated in western Oregon over three years for their reaction to the shoot infection phase of downy mildew and vigor. There was a large range of downy mildew susceptibility and vigor amongst commercial cultivars, with some cultivars possessing a very high level of resistance. Overall, however, disease resistance and vigor were significantly greater in cultivars originating from Europe than those originating from the United States, Japan, and Australia/New Zealand. Amongst a subset of 79 cultivars, vigor was negatively correlated with levels, in cones, of cohumulone, a chemical constitute of bittering acids typically found in germplasm derived from North America. The generally poor vigor observed in cultivars derived outside of Europe likely is indicative of a lack of tolerance to the crown infection phase of the disease. Thus, the best sources of downy mildew resistance seems to be found in cultivars from the United Kingdom and continental Europe, and such cultivars are typically lower yielding and lack distinctive aroma and flavor characteristics presently desired by craft brewers. Introgression of downy mildew resistance into North American germplasm with high yield and desirable brewing characteristics is needed. Accepted for publication 3 March 2016. Published 7 March 2016.

Dm3 is one member of a large constitutively expressed family of nucleotide binding site-leucine-rich repeat encoding genes.

The major cluster of resistance genes in lettuce cv. Diana contains approximately 32 nucleotide binding site-leucine-rich repeat encoding genes. Previous molecular dissection of this complex region had identified a large gene, RGC2B, as a candidate for encoding the downy mildew resistance gene, Dm3. This article describes genetic and transgenic complementation data that demonstrated RGC2B is necessary and sufficient to confer resistance with Dm3 specificity. Ethylmethanesulphonate was used to induce mutations to downy mildew susceptibility in cv. Diana (Dm1, Dm3, Dm7, and Dm8). Nineteen families were identified with a complete loss of resistance in one of the four resistance specificities. Sequencing revealed a variety of point mutations in RGC2B in the six dm3 mutants. Losses of resistance were due to single changes in amino acid sequence or a change in an intron splice site. These mutations did not cluster in any particular region of RGC2B. A full-length genomic copy of RGC2B was isolated from a lambdaphage library and introduced into two genotypes of lettuce. Transgenics expressing RGC2B exhibited resistance to all isolates expressing Avr3 from a wide range of geographical origins. In a wildtype Dm3-expressing genotype, many of the RGC2 family members are expressed at low levels throughout the plant.

Downy mildew incidence of pearl millet hybrids with different male-sterility inducing cytoplasms

The use of different sources of cytoplasmic male sterility (CMS) in hybrid seed production of pearl millet [Pennisetum glaucum (L.) R. Br.] is advocated to avoid possible disease epidemics occurring due to cytoplasmic uniformity. The effects of commercially unexploited, but potentially exploitable, sources of CMS, like A2, A3 and A4, on downy mildew [Sclerospora graminicola (Sacc.) Schroet] incidence were studied by using the disease incidence of isonuclear hybrids with male-sterile and fertile cytoplasm. The mean downy mildew incidence of hybrids carrying different male-sterile cytoplasm was similar to that of hybrids retaining the fertile cytoplasm. The cytoplasm accounted for only 0.6% of the total variation and its effect was non-significant; pollinators could explain most of the variation in determining the disease incidence of hybrids. This suggested that these male-sterile cytoplasms are not linked to downy mildew susceptibility and thus can be exploited commercially to broaden the cytoplasmic base of the male-sterile lines and, ultimately, of hybrids.

Downy Mildew Resistance of Various Pea Genotypes

Abstract The resistance of various vining pea cultivars and breeding lines against downy mildew infestation of leaves and pods was investigated in field trials in southern Sweden. The various pea genotypes differed significantly in severity of downy mildew infection. The rate of resistance of pods and leaves varied independently among pea genotypes. The moderate resistance of the old cultivar ‘Dark Skin Perfection’ was confirmed. Two genotypes had a higher rate of resistance. A correlation between rate of downy mildew susceptibility and proportion of plants with stem-base rot caused by Fusarium spp. was found for different genotypes.