Search Results (10)

This study examined the effectiveness of neem leaf extract (NLE) and azadirachtin (AZA) against two isolates of Plasmopara halstedii, which causes downy mildew in sunflowers. We also explored their physiological and histopathological effects. The pre-inoculation treatments included 10% and 20% NLE and 0.01% and 0.1% AZA, compared to a mefenoxam-treated control and a non-treated control. All treatments significantly reduced the disease rate compared to the inoculated controls (which had a 73–76% disease rate). The 10% NLE treatment showed the strongest effect against isolate 1 (0% damping-off), while the 20% NLE treatment was most effective against isolate 2 (4% damping-off). Neem treatments also significantly improved plant height; for instance, 20% of NLE-treated plants inoculated with isolate 2 reached approximately 15 cm, compared to about 8 cm in the inoculated controls. Histological analyses indicated limited hyphal spread and low levels of cortical necrosis in neem-treated plants, particularly with 0.1% AZA treatment. This suggests a moderate initial defense response without extensive hypersensitive cell death. Neem treatments were comparable to mefenoxam treatments. These results highlight the potential of neem-derived products, particularly 10% NLE and 0.1% AZA, for the integrated management of sunflower downy mildew through both direct pathogen suppression and enhanced host resistance. Full article

Sunflower is one of the major oil crops in the world. Diseases such as sunflower downy mildew (Plasmopara halstedii (Farl.) Berl. et de Toni) constitute a significant risk factor during sunflower production. Integrated pest management (IPM) is considered an essential tool against sunflower downy mildew; however, the pathogen variability repeatedly affects the efficacy of control measures. This article evaluates some vital elements of the management of sunflower downy mildew disease and analyzes current challenges. In addition, we outlined the options for the future integration of recent research and achievements related to sunflower downy mildew to achieve more sustainable sunflower production. Finally, a SWOT analysis was performed to consider internal factors, such as strengths (S) and weaknesses (W), and external factors, such as opportunities (O) and threats (T) connected to the topic. Full article

Downy mildew caused by Plasmopara halstedii is responsible for significant economic losses in cultivated sunflowers. Field isolates of sunflower downy mildew resistant to mefenoxam, a previously effective active ingredient against the pathogen, have been found across Europe. The main goal of this study was to assess the sensitivity of P. halstedii isolates to mefenoxam through host responses to infection, such as symptoms measured by disease severity and growth reduction, and host tissue reactions, such as hypersensitive reaction and necrosis of invaded cells. Sunflower seeds were treated with Apron XL 350 FS at the European registered rate (3 mg/kg seeds). Seedlings were inoculated using the soil drench method with eight Hungarian P. halstedii isolates. Disease rates and plant heights were measured twice. Histological examinations of cross-sections of sunflower hypocotyls were performed using a fluorescence microscope. In our study, cluster analyses of sunflowers based on macroscopic and microscopic variables showed differentiation of groups of mefenoxam-treated sunflowers inoculated with different P. halstedii isolates. We first revealed a clear difference in host responses of mefenoxam-treated susceptible sunflowers. In addition, examining tissue reactions (e.g., hypersensitive reaction, necrosis) seems more accurate to estimate the sensitivity of P. halstedii isolates to mefenoxam than macroscopic symptoms. Full article

Yield losses in sunflower crops caused by Plasmopara halstedii can be up to 100%, depending on the cultivar susceptibility, environmental conditions, and virulence of the pathogen population. The aim of this study was to investigate the genetic and phenotypic structure of a sunflower downy mildew agent at the field scale. The genetic diversity of 250 P. halstedii isolates collected from one field in southern France was assessed using single-nucleotide polymorphisms (SNPs) and single sequence repeats (SSR). A total of 109 multilocus genotypes (MLG) were identified among the 250 isolates collected in the field. Four genotypes were repeated more than 20 times and spatially spread over the field. Estimates of genetic relationships among P. halstedii isolates using principal component analysis and a Bayesian clustering approach demonstrated that the isolates are grouped into two main genetic clusters. A high level of genetic differentiation among clusters was detected (F_ST = 0.35), indicating overall limited exchange between them, but our results also suggest that recombination between individuals of these groups is not rare. Genetic clusters were highly related to pathotypes, as previously described for this pathogen species. Eight different races were identified (100, 300, 304, 307, 703, 704, 707, and 714), with race 304 being predominant and present at most of the sites. The co-existence of multiple races at the field level is a new finding that could have important implications for the management of sunflower downy mildew. These data provide the first population-wide picture of the genetic structure of P. halstedii at a fine spatial scale. Full article

Downy mildew of sunflower, caused by Plasmopara halstedii (Farl.) Berl. et de Toni, is a relevant disease of this crop. High virulent pathotypes have been identified in several countries, while there are few data on the spread of P. halstedii pathotypes in some important sunflower-growing areas of Europe. The goal of this study was to give up-to-date information on the pathotype structure of P. halstedii in Hungary and provide some actual data on the virulence phenotype of the pathogen for six European countries. Infected leaves of different sunflower hybrids and volunteers were collected in seven countries (Hungary, Bulgaria, Serbia, Turkey, Greece, Romania, and Italy) between 2012 and 2019. A universally accepted nomenclature was used with a standardized set of sunflower differential lines for pathotype characterization of isolates. The virulence pattern of the isolates was determined by a three-digit code (coded virulence formula, CVF). A total of 109 P. halstedii isolates were characterized. As a result of our survey, 18 new P. halstedii pathotypes were identified in Europe. Two out of the eighteen pathotypes were detected from the Asian part of Turkey. The detailed distribution of pathotypes in Hungary is also discussed. Full article

The management of downy mildew (Plasmopara halstedii) in sunflower, is heavily dependent on genetic resistance, whilst entomopathogenic fungi (EF) can reduce other sunflower diseases. In this work, we characterized P. halstedii from Spain and other countries collected in the past few years. Twenty-three races were identified (the most frequent in Spain being 310, 304, 705 and 715), with an increasing proportion of highly virulent races. Five isolates from countries other than Spain overcame the resistance in RHA-340. In addition, we assessed the efficacy of five EF against downy mildew and their effects on sunflower growth in axenic conditions. None of the entomopathogens reduced disease severity, nor did they have any effect on plant growth when applied together with P. halstedii. In contrast, three EF reduced some of the plant growth variables in the absence of the pathogen. Microbiological and molecular diagnostics suggest that the axenic system and the short experimental time used in this study did not favor the successful establishment of EF in the plants or their potential biocontrol effect. Our results show a shift in P. halstedii racial patterns and suggest that soil as a growth substrate and long infection times are needed for EF effectiveness against downy mildew. Full article

Downy mildew (DM) is one of the severe biotic threats to sunflower production worldwide. The inciting pathogen, Plasmopara halstedii, could overwinter in the field for years, creating a persistent threat to sunflower. The dominant genes Pl₁₈ and Pl₂₀ conferring resistance to known DM races have been previously mapped to 1.5 and 1.8 cM intervals on sunflower chromosomes 2 and 8, respectively. Utilizing a whole-genome resequencing strategy combined with reference sequence-based chromosome walking and high-density mapping in the present study, Pl₁₈ was placed in a 0.7 cM interval on chromosome 2. A candidate gene HanXRQChr02g0048181 for Pl₁₈ was identified from the XRQ reference genome and predicted to encode a protein with typical NLR domains for disease resistance. The Pl₂₀ gene was placed in a 0.2 cM interval on chromosome 8. The putative gene with the NLR domain for Pl₂₀, HanXRQChr08g0210051, was identified within the Pl₂₀ interval. SNP markers closely linked to Pl₁₈ and Pl₂₀ were evaluated with 96 diverse sunflower lines, and a total of 13 diagnostic markers for Pl₁₈ and four for Pl₂₀ were identified. These markers will facilitate to transfer these new genes to elite sunflower lines and to pyramid these genes with broad-spectrum DM resistance in sunflower breeding. Full article

Two neem-derived pesticides were examined under in vitro and in vivo conditions to test their efficacy in controlling Plasmopara halstedii pathotype 704, a causal agent of downy mildew in sunflower. All the tested concentrations of neem leaf extract and the highest concentration of commercial neem product significantly reduced the sporangial germination under in vitro conditions. In in vivo experiment, 3-days old pre-treated seedlings with both concentrations of neem leaf extract and the highest concentration of commercial product showed a significant reduction in the infection indicating possible systemic effect of neem. When the seedlings were treated following the infection with P. halstedii (i.e., post-treatment), only the highest concentrations of neem leaf extract and the commercial product showed a significant reduction in the infection indicating curative effect of neem. Possibilities for the control of P. halstedii with neem-derived pesticides are discussed. Full article

Cyclophilins (CYPs), a highly-conserved family of proteins, belong to a subgroup of immunophilins. Ubiquitous in eukaryotes and prokaryotes, CYPs have peptidyl-prolyl cis–trans isomerase (PPIase) activity and have been implicated as virulence factors in plant pathogenesis by oomycetes. We identified 16 CYP orthogroups from 21 diverse oomycetes. Each species was found to encode 15 to 35 CYP genes. Three of these orthogroups contained proteins with signal peptides at the N-terminal end, suggesting a role in secretion. Multidomain analysis revealed five conserved motifs of the CYP domain of oomycetes shared with other eukaryotic PPIases. Expression analysis of CYP proteins in different asexual life stages of the hemibiotrophic Phytophthora infestans and the biotrophic Plasmopara halstedii demonstrated distinct expression profiles between life stages. In addition to providing detailed comparative information on the CYPs in multiple oomycetes, this study identified candidate CYP effectors that could be the foundation for future studies of virulence. Full article

Rust caused by the fungus Puccinia helianthi and downy mildew (DM) caused by the obligate pathogen Plasmopara halstedii are two of the most globally important sunflower diseases. Resistance to rust and DM is controlled by race-specific single dominant genes. The present study aimed at pyramiding rust resistance genes combined with a DM resistance gene, using molecular markers. Four rust resistant lines, HA-R3 (carrying the R₄ gene), HA-R2 (R₅), HA-R8 (R₁₅), and RHA 397 (R_13b), were each crossed with a common line, RHA 464, carrying a rust gene R₁₂ and a DM gene Pl_Arg. An additional cross was made between HA-R8 and RHA 397. Co-dominant simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers linked to the target genes were used to discriminate between homozygotes and heterozygotes in F₂ populations. Five pyramids with different combinations of rust resistance genes were selected in the homozygous condition through marker-assisted selection, and three of them were combined with a DM resistance gene Pl_Arg: R₄/R₁₂/Pl_Arg, R₅/R₁₂/Pl_Arg, R_13b/R₁₂/Pl_Arg, R₁₅/R₁₂, and R_13b/R₁₅. The pyramiding lines with the stacking of two rust and one DM genes were resistant to all known races of North American sunflower rust and all known races of the pathogen causing DM, potentially providing multiple and durable resistance to both rust and DM. A cluster of 12 SNP markers spanning a region of 34.5 Mb on chromosome 1, which co-segregate with Pl_Arg, were tested in four populations. Use of those markers, located in a recombination suppressed region in marker selection, is discussed. Full article