Investigation of Using Hyperspectral Vegetation Indices to Assess Brassica Downy Mildew.

Downy mildew caused by Hyaloperonospora brassicae is a severe disease in Brassica oleracea that significantly reduces crop yield and marketability. This study aims to evaluate different vegetation indices to assess different downy mildew infection levels in the Brassica variety Mildis using hyperspectral data. Artificial inoculation using H. brassicae sporangia suspension was conducted to induce different levels of downy mildew disease. Spectral measurements, spanning 350 nm to 1050 nm, were conducted on the leaves using an environmentally controlled setup, and the reflectance data were acquired and processed. The Successive Projections Algorithm (SPA) and signal sensitivity calculation were used to extract the most informative wavelengths that could be used to develop downy mildew indices (DMI). A total of 37 existing vegetation indices and three proposed DMIs were evaluated to indicate downy mildew (DM) infection levels. The results showed that the classification using a support vector machine achieved accuracies of 71.3%, 80.7%, and 85.3% for distinguishing healthy leaves from DM1 (early infection), DM2 (progressed infection), and DM3 (severe infection) leaves using the proposed downy mildew index. The proposed new downy mildew index potentially enables the development of an automated DM monitoring system and resistance profiling in Brassica breeding lines.

Rapid Detection of a Downy Mildew Pathogen, Peronospora destructor, in Infected Onion Tissues and Soils by Loop-Mediated Isothermal Amplification.

Downy mildew of onion caused by a soil-inhabiting water mold, Peronospora destructor, is one of the most devastating diseases that can destroy entire onion fields in a matter of days. In this study, we developed a loop-mediated isothermal amplification (LAMP) assay that allows for rapid detection of P. destructor by visual inspection. The internal transcribed spacer 2 region of P. destructor was used to design primer sets for LAMP reactions. The optimal temperature and incubation time were determined for the most efficient primer set. In the optimized condition, the LAMP assay exhibited at least 100 times more sensitivity than conventional PCR, detecting femtogram levels of P. destructor genomic DNA (gDNA). Detection of the pathogen from a small number of spores without gDNA extraction further confirmed the high sensitivity of the assay. For specificity, the LAMP assay was negative for gDNA of other fungal pathogens that cause various diseases on onion and oomycetes, whereas the assay was positive for gDNA extracted from onion tissues showing the typical downy mildew symptoms. Finally, we examined the efficacy of the LAMP assay in detection of P. destructor in soils. Soils collected from onion fields that had been contaminated with P. destructor were solarized for 60 days. Whereas the LAMP assay was negative for the solarized soils, we were able to detect P. destructor that oversummers in fields. The LAMP assay developed in this study enables rapid detection and diagnosis of downy mildew of onion in infected tissues and in soil.

Effect of Cucurbit Host, Production Region, and Season on the Population Structure of Pseudoperonospora cubensis in Florida.

Pseudoperonospora cubensis, the causal agent of Cucurbit downy mildew (CDM), is one of the most important diseases affecting cucurbit production in the United States. This disease is especially damaging to Florida production areas, as the state is a top producer of many cucurbit species. In addition, winter production in central and south Florida likely serves as a likely source of P. cubensis inoculum for spring and summer cucurbit production throughout the eastern United States, where CDM is unable to overwinter in the absence of a living host. Over 2 years (2017 and 2018) and four seasons (spring 2017, spring 2018, fall 2017, and fall 2018), 274 P. cubensis isolates were collected from cucurbit hosts at production sites in south, central, and north Florida. The isolates were analyzed with 10 simple sequence repeat (SSR) markers to establish population structure and genetic diversity and further assigned to a clade based on a qPCR assay. Results of population structure and genetic diversity analyses differentiated isolates based on cucurbit host and clade (1 or 2). Of the isolates assigned to clade by qPCR, butternut squash, watermelon, and zucchini were dominated by clade 1 isolates, whereas cucumber isolates were split 34 and 59% between clades 1 and 2, respectively. Clade assignments agreed with isolate clustering observed within discriminant analysis of principal components (DAPC) based on SSR markers, although watermelon isolates formed a group distinct from the other clade 1 isolates. For seasonal collections from cucumber at each location, isolates were typically skewed to one clade or the other and varied across locations and seasons within each year of the study. This variable population structure of cucumber isolates could have consequences for regional disease management. This is the first study to characterize P. cubensis populations in Florida and evaluate the effect of cucurbit host and clade-type on isolate diversity and population structure, with implications for CDM management in Florida and other United States cucurbit production areas.

Advances in understanding grapevine downy mildew: From pathogen infection to disease management.

Plasmopara viticola is geographically widespread in grapevine-growing regions. Grapevine downy mildew disease, caused by this biotrophic pathogen, leads to considerable yield losses in viticulture annually. Because of the great significance of grapevine production and wine quality, research on this disease has been widely performed since its emergence in the 19th century. Here, we review and discuss recent understanding of this pathogen from multiple aspects, including its infection cycle, disease symptoms, genome decoding, effector biology, and management and control strategies. We highlight the identification and characterization of effector proteins with their biological roles in host-pathogen interaction, with a focus on sustainable control methods against P. viticola, especially the use of biocontrol agents and environmentally friendly compounds.

Comparative Analysis of Transcriptomics and Metabolomics Reveals Defense Mechanisms in Melon Cultivars against Pseudoperonospora cubensis Infection.

Melon (Cucumis melo L.) represents an agriculturally significant horticultural crop that is widely grown for its flavorful fruits. Downy mildew (DM), a pervasive foliar disease, poses a significant threat to global melon production. Although several quantitative trait loci related to DM resistance have been identified, the comprehensive genetic underpinnings of this resistance remain largely uncharted. In this study, we utilized integrative transcriptomics and metabolomics approaches to identify potential resistance-associated genes and delineate the strategies involved in the defense against DM in two melon cultivars: the resistant 'PI442177' ('K10-1') and the susceptible 'Huangdanzi' ('K10-9'), post-P. cubensis infection. Even in the absence of the pathogen, there were distinctive differentially expressed genes (DEGs) between 'K10-1' and 'K10-9'. When P. cubensis was infected, certain genes, including flavin-containing monooxygenase (FMO), receptor-like protein kinase FERONIA (FER), and the HD-ZIP transcription factor member, AtHB7, displayed pronounced expression differences between the cultivars. Notably, our data suggest that following P. cubensis infection, both cultivars suppressed flavonoid biosynthesis via the down-regulation of associated genes whilst concurrently promoting lignin production. The complex interplay of transcriptomic and metabolic responses elucidated by this study provides foundational insights into melon's defense mechanisms against DM. The robust resilience of 'K10-1' to DM is attributed to the synergistic interaction of its inherent transcriptomic and metabolic reactions.

Comparative Transcriptome Analysis between Resistant and Susceptible Pakchoi Cultivars in Response to Downy Mildew.

Downy mildew caused by the obligate parasite Hyaloperonospora brassicae is a devastating disease for Brassica species. Infection of Hyaloperonospora brassicae often leads to yellow spots on leaves, which significantly impacts quality and yield of pakchoi. In the present study, we conducted a comparative transcriptome between the resistant and susceptible pakchoi cultivars in response to Hyaloperonospora brassicae infection. A total of 1073 disease-resistance-related differentially expressed genes were identified using a Venn diagram. The Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that these genes were mainly involved in plant-pathogen interaction, plant hormone signal transduction, and other photosynthesis-related metabolic processes. Analysis of the phytohormone content revealed that salicylic acid increased significantly in the resistant material after inoculation with Hyaloperonospora brassicae, whereas the contents of jasmonic acid, abscisic acid, and 1-aminocyclopropane-1-carboxylic acid decreased. Exogenous salicylic acid treatment also significantly upregulated Hyaloperonospora brassicae-induced genes, which further confirmed a crucial role of salicylic acid during pakchoi defense against Hyaloperonospora brassicae. Based on these findings, we suggest that the salicylic-acid-mediated signal transduction contributes to the resistance of pakchoi to downy mildew, and PAL1, ICS1, NPR1, PR1, PR5, WRKY70, WRKY33, CML43, CNGC9, and CDPK15 were involved in this responsive process. Our findings evidently contribute to revealing the molecular mechanism of pakchoi defense against Hyaloperonospora brassicae.

Activity of the new OSBP inhibitor Y18501 against Pseudoperonospora cubensis and its application for the control of cucumber downy mildew.

Y18501 is a new oxysterol-binding protein inhibitor (OSBPI) with a similar structure to oxathiapiprolin. Y18501 showed strong inhibitory activities against Phytophthora spp. and Pseudoperonospora cubensis, with EC50 ranging from 0.0005 to 0.0046 µg/mL. It also had good control efficacy on cucumber downy mildew (CDM) in the green house and in the field, and could effectively inhibit different development stages of P. cubensis, especially for sporangiophore production, sporangial production, mycelium extension, and elongation of germ tube. In addition, Y18501 showed excellent protective and curative activities against P. cubensis. It also had acropetal systemic mobility in the cucumber leaves, and could be taken up and translocated to the upper leaves more effectively from the lower leaves than from the roots. Y18501 had poorer permeability in cucumber leaves compared to oxathiapiprolin. The simultaneous application of Y18501 and chlorothalonil could significantly promote the inhibition of P. cubensis.

Hydroponic versus soil-based cultivation of sweet basil: impact on plants' susceptibility to downy mildew and heat stress, storability and total antioxidant capacity.

BACKGROUND: In recent years, hydroponically cultivated basil has gained extensive popularity over soil-based cultivation. Evidence for potential differences between both cultivation methods, in terms of resistance to biotic and abiotic stress factors, storage properties and shelf-life, is still lacking and the potential effect of cultivation method on the antioxidant capacity has not yet been fully explored. This study aimed to determine which of the two basil cultivation methods produces plants that are more resilient to downy mildew and external heat treatment and that exhibit better storage and shelf-life performance. RESULTS: Hydroponically grown basil was significantly more affected by browning than the soil-grown basil at the end of the storage and end of the shelf-life period. Under both cultivation methods, the extent of browning increased significantly between the end of the storage and end of the shelf-life period, by a factor of 1.4. Moreover, hydroponically grown plants were significantly more sensitive to heat treatment than soil-grown basil. However, the soil-grown basil exhibited significantly greater susceptibility to downy mildew than the hydroponically grown basil. At harvest, and at the end of the storage period, the antioxidant capacity of hydroponically cultivated basil was significantly greater than that of soil-grown basil. CONCLUSIONS: Hydroponically cultivated basil exhibited greater resistance to downy mildew, but less resilience to heat and browning during storage and a shelf-life period, resulting in poorer storage and shelf-life performance as compared to soil-cultivated basil. The greater total antioxidant capacity of the hydroponically cultivated basil seems to be the major cause for the observed phenomena. © 2023 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Cell-specific RNA profiling reveals host genes expressed in Arabidopsis cells haustoriated by downy mildew.

The downy mildew oomycete Hyaloperonospora arabidopsidis, an obligate filamentous pathogen, infects Arabidopsis (Arabidopsis thaliana) by forming structures called haustoria inside host cells. Previous transcriptome analyses have revealed that host genes are specifically induced during infection; however, RNA profiling from whole-infected tissues may fail to capture key transcriptional events occurring exclusively in haustoriated host cells, where the pathogen injects virulence effectors to modulate host immunity. To determine interactions between Arabidopsis and H. arabidopsidis at the cellular level, we devised a translating ribosome affinity purification system using 2 high-affinity binding proteins, colicin E9 and Im9 (immunity protein of colicin E9), applicable to pathogen-responsive promoters, thus enabling haustoriated cell-specific RNA profiling. Among the host genes specifically expressed in H. arabidopsidis-haustoriated cells, we found genes that promote either susceptibility or resistance to the pathogen, providing insights into the Arabidopsis-downy mildew interaction. We propose that our protocol for profiling cell-specific transcripts will apply to several stimulus-specific contexts and other plant-pathogen interactions.

Genome-Enabled Insights into Downy Mildew Biology and Evolution.

Oomycetes that cause downy mildew diseases are highly specialized, obligately biotrophic phytopathogens that can have major impacts on agriculture and natural ecosystems. Deciphering the genome sequence of these organisms provides foundational tools to study and deploy control strategies against downy mildew pathogens (DMPs). The recent telomere-to-telomere genome assembly of the DMP Peronospora effusa revealed high levels of synteny with distantly related DMPs, higher than expected repeat content, and previously undescribed architectures. This provides a road map for generating similar high-quality genome assemblies for other oomycetes. This review discusses biological insights made using this and other assemblies, including ancestral chromosome architecture, modes of sexual and asexual variation, the occurrence of heterokaryosis, candidate gene identification, functional validation, and population dynamics. We also discuss future avenues of research likely to be fruitful in studies of DMPs and highlight resources necessary for advancing our understanding and ability to forecast and control disease outbreaks.

A comparison of transporter gene expression in three species of Peronospora plant pathogens during host infection.

Protein transporters move essential metabolites across membranes in all living organisms. Downy mildew causing plant pathogens are biotrophic oomycetes that transport essential nutrients from their hosts to grow. Little is known about the functions and gene expression levels of membrane transporters produced by downy mildew causing pathogens during infection of their hosts. Approximately 170-190 nonredundant transporter genes were identified in the genomes of Peronospora belbahrii, Peronospora effusa, and Peronospora tabacina, which are specialized pathogens of basil, spinach, and tobacco, respectively. The largest groups of transporter genes in each species belonged to the major facilitator superfamily, mitochondrial carriers (MC), and the drug/metabolite transporter group. Gene expression of putative Peronospora transporters was measured using RNA sequencing data at two time points following inoculation onto leaves of their hosts. There were 16 transporter genes, seven of which were MCs, expressed in each Peronospora species that were among the top 45 most highly expressed transporter genes 5-7 days after inoculation. Gene transcripts encoding the ADP/ATP translocase and the mitochondrial phosphate carrier protein were the most abundant mRNAs detected in each Peronospora species. This study found a number of Peronospora genes that are likely critical for pathogenesis and which might serve as future targets for control of these devastating plant pathogens.

QTL mapping of resistance to Pseudoperonospora cubensis clade 2, mating type A1, in Cucumis melo and dual-clade marker development.

KEY MESSAGE: This is the first identification of QTLs underlying resistance in Cucumis melo to an isolate of Pseudoperonospora cubensis identified as Clade 2/mating type A1. Pseudoperonospora cubensis, causal organism of cucurbit downy mildew (CDM), causes severe necrosis and defoliation on Cucumis melo (melon). A recombinant inbred line population (N = 169) was screened against an isolate of P. cubensis (Clade 2/mating type A1) in replicated greenhouse and growth chamber experiments. SNPs (n = 5633 bins) identified in the RIL population were used for quantitative trait loci (QTL) mapping. A single major QTL on chromosome 10 (qPcub-10.3-10.4) was consistently associated with resistance across all experiments, while a second major QTL on chromosome 8 (qPcub-8.3) was identified only in greenhouse experiments. These two major QTLs were identified on the same chromosomes (8 and 10) but in different locations as two major QTLs (qPcub-8.2 and qPcub-10.1) previously identified for resistance to P. cubensis Clade 1/mating type A2. Kompetitive allele-specific PCR (KASP) markers were developed for these four major QTLs and validated in the RIL population through QTL mapping. These markers will provide melon breeders a high-throughput genotyping toolkit for development of melon cultivars with broad tolerance to CDM.

Impact of Infection Timing and Autumnal Fungicide Applications on Perennation of Pseudoperonospora humuli and Severity of Hop Downy Mildew.

Pseudoperonospora humuli, causal agent of hop downy mildew, is known to survive winter as systemic mycelium in the crown and developing buds of hop (Humulus lupulus). Field studies were conducted over three growing seasons to quantify the association of infection timing to overwintering of P. humuli and development of downy mildew. Cohorts of potted plants were inoculated sequentially from early summer to autumn, overwintered, and then evaluated for symptoms of systemic downy mildew in emerging shoots. Shoots with systemic P. humuli developed after inoculation at any time in the previous year, with the most severe disease typically resulting from inoculation in August. Independent of the timing of inoculation, diseased shoots emerged coincident with the emergence of healthy shoots, beginning as early as late February and continuing through late May to early June. Surface crown buds on inoculated plants exhibited internal necrosis associated with P. humuli at rates ranging from 0.3 to 1.2%, whereas P. humuli was detected by PCR on 7.8 to 17.0% of asymptomatic buds depending on the timing of inoculation and year. Four experiments were conducted to quantify the impact of foliar fungicides applied in autumn on downy mildew the following spring. There was a small reduction of disease in only one study. Together, these studies indicate that infection by P. humuli that leads to overwintering can occur over a broad period of time, but delaying infection until autumn tends to reduce disease levels in the following year. However, in established plantings, postharvest application of foliar fungicides appeared to have little impact on severity of downy mildew in the ensuring year.

High-throughput time series expression profiling of Plasmopara halstedii infecting Helianthus annuus reveals conserved sequence motifs upstream of co-expressed genes.

Downy mildew disease of sunflower, caused by the obligate biotrophic oomycete Plasmopara halstedii, can have significant economic impact on sunflower cultivation. Using high-throughput whole transcriptome sequencing, four developmental phases in 16 time-points of Pl. halstedii infecting Helianthus annuus were investigated. With the aim of identifying potential functional and regulatory motifs upstream of co-expressed genes, time-series derived gene expression profiles were clustered based on their time-course similarity, and their upstream regulatory gene sequences were analyzed here. Several conserved motifs were found upstream of co-expressed genes, which might be involved in binding specific transcription factors. Such motifs were also found associated with virulence related genes, and could be studied on a genetically tractable model to clarify, if these are involved in regulating different stages of pathogenesis.

Introgression and targeting of the Pl37 and Pl38 genes for downy mildew resistance from wild Helianthus annuus and H. praecox into cultivated sunflower (Helianthus annuus L.).

KEY MESSAGE: Two new downy mildew resistance genes, Pl37 and Pl38, were introgressed from wild sunflower species into cultivated sunflower and mapped to sunflower chromosomes 4 and 2, respectively Downy mildew (DM), caused by the oomycete pathogen Plasmopara halstedii (Farl.) Berl. & de Toni, is known as the most prevalent disease occurring in global sunflower production areas, especially in North America and Europe. In this study, we report the introgression and molecular mapping of two new DM resistance genes from wild sunflower species, Helianthus annuus and H. praecox, into cultivated sunflower. Two mapping populations were developed from the crosses of HA 89/H. annuus PI 435417 (Pop1) and CMS HA 89/H. praecox PRA-417 (Pop2). The phenotypic evaluation of DM resistance/susceptibility was conducted in the BC1F2-derived BC1F3 populations using P. halstedii race 734. The BC1F2 segregating Pop1 was genotyped using an Optimal GBS AgriSeq™ Panel consisting of 768 mapped SNP markers, while the BC1F2 segregating Pop2 was genotyped using a genotyping-by-sequencing approach. Linkage analysis and subsequent saturation mapping placed the DM resistance gene, designated Pl37, derived from H. annuus PI 435417 in a 1.6 cM genetic interval on sunflower chromosome 4. Pl37 co-segregated with SNP markers SPB0003 and C4_5738736. Similarly, linkage analysis and subsequent saturation mapping placed the DM resistance gene, designated Pl38, derived from H. praecox PRA-417 in a 0.8 cM genetic interval on sunflower chromosome 2. Pl38 co-segregated with seven SNP markers. Multi-pathotype tests revealed that lines with Pl37 or Pl38 are immune to the most prevalent and virulent P. halstedii races tested. Two germplasm lines, HA-DM15 with Pl37 and HA-DM16 with Pl38, were developed for use in sunflower DM-resistance breeding.

Resistance Risk Assessment for the New OSBP Inhibitor Y18501 in Pseudoperonospora cubensis and Point Mutations (G705V, L798W, and I812F) in PscORP1 that Confer Resistance.

Y18501 is a new oxysterol-binding protein inhibitor (OSBPI) that shows strong inhibitory activity against Pseudoperonospora cubensis. In this study, the sensitivities of 159 Ps. cubensis isolates to Y18501 were determined, with EC50 values ranging from 0.001 to 11.785 µg/mL, indicating that a Y18501-resistant subpopulation has appeared in the field. Ten Y18501-resistant mutants were obtained by fungicide adaptation and displayed fitness equal to or stronger than their parental isolates, which suggests that the resistance risk of Ps. cubensis to Y18501 is high. The consecutive applications of Y18501 in the field resulted in the rapid resistance of Ps. cubensis and decreased control efficacy of cucumber downy mildew (CDM), which could be alleviated by compounding with mancozeb. A positive cross-resistance was detected between Y18501 and oxathiapiprolin. The amino acid substitutions G705V, L798W, and I812F in PscORP1 conferred resistance to Y18501 in Ps. cubensis, which was validated by molecular docking and molecular dynamics simulations.

EffectorO: Motif-Independent Prediction of Effectors in Oomycete Genomes Using Machine Learning and Lineage Specificity.

Oomycete plant pathogens cause a wide variety of diseases, including late blight of potato, sudden oak death, and downy mildews of plants. These pathogens are major contributors to loss in numerous food crops. Oomycetes secrete effector proteins to manipulate their hosts to the advantage of the pathogen. Plants have evolved to recognize effectors, resulting in an evolutionary cycle of defense and counter-defense in plant-microbe interactions. This selective pressure results in highly diverse effector sequences that can be difficult to computationally identify using only sequence similarity. We developed a novel effector prediction tool, EffectorO, that uses two complementary approaches to predict effectors in oomycete pathogen genomes: i) a machine learning-based pipeline that predicts effector probability based on the biochemical properties of the N-terminal amino-acid sequence of a protein and ii) a pipeline based on lineage specificity to find proteins that are unique to one species or genus, a sign of evolutionary divergence due to adaptation to the host. We tested EffectorO on Bremia lactucae, which causes lettuce downy mildew, and Phytophthora infestans, which causes late blight of potato and tomato, and predicted many novel effector candidates while recovering the majority of known effector candidates. EffectorO will be useful for discovering novel families of oomycete effectors without relying on sequence similarity to known effectors. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Genome Resource of Streptomyces atratus PY-1, a Broad-Spectrum Antimicrobial Strain in Particular Antagonistic Against Plasmopara viticola.

Streptomyces atratus PY-1 exhibited promising antimicrobial properties; in particular, it is highly inhibitory to Plasmopara viticola, which causes downy mildew of grape. It is very necessary to carry out systematic and in-depth research on the PY-1 strain for the improvement, application, and promotion of biocontrol agents. The PY-1 genome was fully sequenced and assembled. We present the draft genome sequence of PY-1, with a size of 9, 254, and 781 bp. Preliminary analysis on the PY-1 genome sequence shows that at least 35 gene clusters are involved in the biosynthesis of polyketides, terpenes, and nonribosomally synthesized peptides.

Grapevine-Associated Lipid Signalling Is Specifically Activated in an Rpv3 Background in Response to an Aggressive P. viticola Pathovar.

Vitis vinifera L. is highly susceptible to the biotrophic pathogen Plasmopara viticola. To control the downy mildew disease, several phytochemicals are applied every season. Recent European Union requirements to reduce the use of chemicals in viticulture have made it crucial to use alternative and more sustainable approaches to control this disease. Our previous studies pinpoint the role of fatty acids and lipid signalling in the establishment of an incompatible interaction between grapevine and P. viticola. To further understand the mechanisms behind lipid involvement in an effective defence response we have analysed the expression of several genes related to lipid metabolism in three grapevine genotypes: Chardonnay (susceptible); Regent (tolerant), harbouring an Rpv3-1 resistance loci; and Sauvignac (resistant) that harbours a pyramid of Rpv12 and Rpv3-1 resistance loci. A highly aggressive P. viticola isolate was used (NW-10/16). Moreover, we have characterised the grapevine phospholipases C and D gene families and monitored fatty acid modulation during infection. Our results indicate that both susceptible and resistant grapevine hosts did not present wide fatty acid or gene expression modulation. The modulation of genes associated with lipid signalling and fatty acids seems to be specific to Regent, which raises the hypothesis of being specifically linked to the Rpv3 loci. In Sauvignac, the Rpv12 may be dominant concerning the defence response, and, thus, this genotype may present the activation of other pathways rather than lipid signalling.

Efficacy of Organic Fungicides for Downy Mildew in Field-Grown Sweet Basil.

Downy mildew is a common, widespread disease affecting basil leaves. No tolerance for disease symptoms, especially on leaves for fresh consumption, necessitates management. Six replicated experiments were conducted between 2010 and 2016 with field-grown basil of a susceptible cultivar exposed to naturally occurring, wind-dispersed sporangiospores of Peronospora belbahrii to evaluate fungicides approved for use on organically produced crops and products in development. Products tested currently registered for use on basil in the U.S. and labeled for downy mildew were Actinovate (Streptomyces lydicus), Companion (Bacillus subtilis), Cueva (copper octanoate), Double Nickel (Bacillus amyloliquefaciens), Forticept EP #1 (thyme oil), Milagrum Plus (Bacillus subtilis), Organocide (sesame oil), Oso (polyoxin D zinc salt), OxiDate (hydrogen dioxide), Procidic (citric acid), Regalia (Reynoutria sachalinensis extract), Stargus (Bacillus amyloliquefaciens), and Trilogy (neem oil). Most are biopesticides. A conventional fungicide, Revus (mandipropamid), was included in most experiments as a positive control. Applications were made weekly to foliage with a backpack sprayer starting before symptoms were seen, except in 2013 when disease onset was early and 2015 when applications were made twice weekly. Organic treatments tested in 2013 started with a soil drench application around the base of plants two days after transplantation. Fungicide efficacy was assessed based on incidence of symptomatic leaves rather than disease severity because there is no tolerance for disease on fresh-market herbs. None were effective based on weekly severity assessments or AUDPC values, confirming results from other researchers that downy mildew cannot be effectively managed with organic fungicides applied to susceptible basil cultivars.