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Klosterman SJ, Clark KJ, Anchieta AG, Kandel SL, Mou B, McGrath MT, Correll JC, Shishkoff N. Transmission of Spinach Downy Mildew via Seed and Infested Leaf Debris. PLANT DISEASE 2024; 108:951-959. [PMID: 37840290 DOI: 10.1094/pdis-06-23-1225-re] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/17/2023]
Abstract
Spinach downy mildew, caused by the obligate oomycete pathogen Peronospora effusa, is a worldwide constraint on spinach production. The role of airborne sporangia in the disease cycle of P. effusa is well established, but the role of the sexual oospores in the epidemiology of P. effusa is less clear and has been a major challenge to examine experimentally. To evaluate seed transmission of spinach downy mildew via oospores in this study, isolated glass chambers were employed in two independent experiments to grow out oospore-infested spinach seed and noninfested seeds mixed with oospore-infested crop debris. Downy mildew diseased spinach plants were observed 37 and 34 days after planting in the two isolator experiments, respectively, in the chambers that contained one of two oospore-infested seed lots or seeds coated with oospore-infested leaves. Spinach plants in isolated glass chambers initiated from seeds without oospores did not show downy mildew symptoms. Similar findings were obtained using the same seed lot samples in a third experiment conducted in a growth chamber. In direct grow out tests to examine oospore infection on seedlings performed in a containment greenhouse with oospore-infested seed of two different cultivars, characteristic Peronospora sporangiophores were observed growing from a seedling of each cultivar. The frequency of seedlings developing symptoms from 82 of these oospore-infested seed indicated that approximately 2.4% of seedlings from infested seed developed symptoms, and 0.55% of seedlings from total seeds assayed developed symptoms. The results provide evidence that oospores can serve as a source of inoculum for downy mildew and provide further evidence of direct seed transmission of the downy mildew pathogen to seedlings in spinach via seedborne oospores.
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Affiliation(s)
| | - Kelley J Clark
- Crop Improvement and Protection Research Unit, USDA-ARS, Salinas, CA
| | - Amy G Anchieta
- Crop Improvement and Protection Research Unit, USDA-ARS, Salinas, CA
| | | | - Beiquan Mou
- Crop Improvement and Protection Research Unit, USDA-ARS, Salinas, CA
| | - Margaret T McGrath
- Long Island Horticultural Research and Extension Center, Plant Pathology and Plant-Microbe Biology Section, School of Integrative Plant Science, Cornell University, Riverhead, NY
| | - James C Correll
- Department of Plant Pathology and Entomology, University of Arkansas, Fayetteville, AR
| | - Nina Shishkoff
- Foreign Disease-Weed Science Research Unit, USDA-ARS, Ft. Detrick, MD
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Dhillon B, Feng C, Villarroel-Zeballos MI, Castroagudin VL, Bhattarai G, Klosterman SJ, Correll JC. Sporangiospore Viability and Oospore Production in the Spinach Downy Mildew Pathogen, Peronospora effusa. PLANT DISEASE 2020; 104:2634-2641. [PMID: 32787734 DOI: 10.1094/pdis-02-20-0334-re] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Downy mildew of spinach, caused by the obligate pathogen Peronospora effusa, remains the most important constraint in the major spinach production areas in the United States. This disease can potentially be initiated by asexual sporangiospores via "green bridges", sexually derived oospores from seed or soil, or dormant mycelium. However, the relative importance of the various types of primary inoculum is not well known. The ability of P. effusa sporangiospores to withstand abiotic stress, such as desiccation, and remain viable during short- and long-distance dispersal and the ability of oospores to germinate and infect seedlings remain unclear. Thus, the primary objectives of this research were to evaluate the impact of desiccation on sporangiospore survival and infection efficiency and examine occurrence, production, and germination of oospores. Results indicate that desiccation significantly reduces sporangiospore viability as well as infection potential. Leaf wetness duration of 4 h was needed for disease establishment by spinach downy mildew sporangiospores. Oospores were observed in leaves of numerous commercial spinach cultivars grown in California in 2018 and Arizona in 2019. Frequency of occurrence varied between the two states-years. The presence of opposite mating types in spinach production areas in the United States was demonstrated by pairing isolates in controlled crosses and producing oospores on detached leaves as well as intact plants. Information from the study of variables that affect sporangiospore viability and oospore production will help in improving our understanding of the epidemiology of this important pathogen, which has implications for management of spinach downy mildew.
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Affiliation(s)
- Braham Dhillon
- Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701
| | - Chunda Feng
- Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701
| | | | | | - Gehendra Bhattarai
- Department of Horticulture, University of Arkansas, Fayetteville, AR 72701
| | | | - James C Correll
- Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701
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Kandel SL, Hulse-Kemp AM, Stoffel K, Koike ST, Shi A, Mou B, Van Deynze A, Klosterman SJ. Transcriptional analyses of differential cultivars during resistant and susceptible interactions with Peronospora effusa, the causal agent of spinach downy mildew. Sci Rep 2020; 10:6719. [PMID: 32317662 PMCID: PMC7174412 DOI: 10.1038/s41598-020-63668-3] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/03/2019] [Accepted: 03/03/2020] [Indexed: 12/28/2022] Open
Abstract
Downy mildew of spinach is caused by the obligate oomycete pathogen, Peronospora effusa. The disease causes significant economic losses, especially in the organic sector of the industry where the use of synthetic fungicides is not permitted for disease control. New pathotypes of this pathogen are increasingly reported which are capable of breaking resistance. In this study, we took advantage of new spinach genome resources to conduct RNA-seq analyses of transcriptomic changes in leaf tissue of resistant and susceptible spinach cultivars Solomon and Viroflay, respectively, at an early stage of pathogen establishment (48 hours post inoculation, hpi) to a late stage of symptom expression and pathogen sporulation (168 hpi). Fold change differences in gene expression were recorded between the two cultivars to identify candidate genes for resistance. In Solomon, the hypersensitive inducible genes such as pathogenesis-related gene PR-1, glutathione-S-transferase, phospholipid hydroperoxide glutathione peroxidase and peroxidase were significantly up-regulated uniquely at 48 hpi and genes involved in zinc finger CCCH protein, glycosyltransferase, 1-aminocyclopropane-1-carboxylate oxidase homologs, receptor-like protein kinases were expressed at 48 hpi through 168 hpi. The types of genes significantly up-regulated in Solomon in response to the pathogen suggests that salicylic acid and ethylene signaling pathways mediate resistance. Furthermore, many genes involved in the flavonoid and phenylpropanoid pathways were highly expressed in Viroflay compared to Solomon at 168 hpi. As anticipated, an abundance of significantly down-regulated genes was apparent at 168 hpi, reflecting symptom development and sporulation in cultivar Viroflay, but not at 48 hpi. In the pathogen, genes encoding RxLR-type effectors were expressed during early colonization of cultivar Viroflay while crinkler-type effector genes were expressed at the late stage of the colonization. Our results provide insights on gene expression in resistant and susceptible spinach-P. effusa interactions, which can guide future studies to assess candidate genes necessary for downy mildew resistance in spinach.
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Affiliation(s)
- Shyam L Kandel
- USDA-ARS, Crop Improvement and Protection Research Unit, Salinas, CA, 93905, USA
| | - Amanda M Hulse-Kemp
- Department of Plant Sciences, University of California, Davis, CA, 95616, USA
- USDA-ARS, Genomics and Bioinformatics Research Unit, Raleigh, NC, 27695, USA
| | - Kevin Stoffel
- Department of Plant Sciences, University of California, Davis, CA, 95616, USA
| | | | - Ainong Shi
- Department of Horticulture, University of Arkansas, Fayetteville, AR, 72701, USA
| | - Beiquan Mou
- USDA-ARS, Crop Improvement and Protection Research Unit, Salinas, CA, 93905, USA
| | - Allen Van Deynze
- Department of Plant Sciences, University of California, Davis, CA, 95616, USA.
| | - Steven J Klosterman
- USDA-ARS, Crop Improvement and Protection Research Unit, Salinas, CA, 93905, USA.
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Shishkoff N. The oospore stage of Plasmopara obducens, impatiens downy mildew. Mycologia 2019; 111:632-646. [PMID: 31136264 DOI: 10.1080/00275514.2019.1601986] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Impatiens downy mildew is caused by Plasmopara obducens, a pathogen known in the United States for over a hundred years, but newly attacking ornamental Impatiens walleriana in production and in the landscape. Little is known about the life cycle of P. obducens; thus, in this study an attempt was made to determine whether the pathogen is homothallic or heterothallic. Fourteen single-sporangium isolates and three single-zoospore isolates were used in single and dual inoculations of stem tissue to see whether the pathogen was homothallic or heterothallic; all isolates tested were able to produce oospores when inoculated singly, suggesting homothally. Cold treatment at 0 C for at least 1 mo induced oospores to germinate and produce primary sporangia. Inoculation of plant tissue with germinating oospores resulted in infection. Other incubation temperatures (-10, 10, and 20 C) did not induce germination, but fluctuating temperatures (between -10 and 0 C, or 0 and 10 C) induced some germination. Spores incubated at -10 C had significantly thicker walls than spores incubated at other temperatures. Evidence suggests that oospores can serve as an overwintering stage.
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Affiliation(s)
- Nina Shishkoff
- a Foreign Disease-Weed Science Research Unit, Agricultural Research Service, United States Department of Agriculture , Ft. Detrick , 1301 Ditto Ave, Frederick, Maryland , 21702
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Kandel SL, Mou B, Shishkoff N, Shi A, Subbarao KV, Klosterman SJ. Spinach Downy Mildew: Advances in Our Understanding of the Disease Cycle and Prospects for Disease Management. PLANT DISEASE 2019; 103:791-803. [PMID: 30939071 DOI: 10.1094/pdis-10-18-1720-fe] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Downy mildew on spinach is caused by Peronospora effusa, an oomycete pathogen that poses a challenge to spinach production worldwide, especially in organic production. Following infection, P. effusa produces abundant amounts of asexual sporangia. Sporangia become windborne and initiate new infections locally or distantly, leading to widespread epidemics. Oospores produced from the union of opposite mating types have been observed within infected leaves and seeds and may remain viable for many years. Sexual reproduction increases the genetic diversity of P. effusa through sexual recombination, and thus, the movement of oospores on seed has likely fueled the rapid explosion of new pathotypes in different regions of the world over the past 20 years. This review summarizes recent advances in spinach downy mildew research, especially in light of the findings of oospores in contemporary commercial spinach seed lots as well as their germination. Knowledge of the role of the oospores and other aspects of the disease cycle can directly translate into new and effective disease management strategies.
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Affiliation(s)
- Shyam L Kandel
- 1 USDA-ARS Crop Improvement and Protection Research Unit, Salinas, CA 93905
| | - Beiquan Mou
- 1 USDA-ARS Crop Improvement and Protection Research Unit, Salinas, CA 93905
| | - Nina Shishkoff
- 2 USDA-ARS Foreign Disease Weed Science Research Unit, Frederick, MD 21702
| | - Ainong Shi
- 3 Department of Horticulture, University of Arkansas, Fayetteville, AR; and
| | - Krishna V Subbarao
- 4 Department of Plant Pathology, University of California-Davis, Salinas, CA
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Fletcher K, Klosterman SJ, Derevnina L, Martin F, Bertier LD, Koike S, Reyes-Chin-Wo S, Mou B, Michelmore R. Comparative genomics of downy mildews reveals potential adaptations to biotrophy. BMC Genomics 2018; 19:851. [PMID: 30486780 PMCID: PMC6264045 DOI: 10.1186/s12864-018-5214-8] [Citation(s) in RCA: 32] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2018] [Accepted: 10/31/2018] [Indexed: 12/05/2022] Open
Abstract
BACKGROUND Spinach downy mildew caused by the oomycete Peronospora effusa is a significant burden on the expanding spinach production industry, especially for organic farms where synthetic fungicides cannot be deployed to control the pathogen. P. effusa is highly variable and 15 new races have been recognized in the past 30 years. RESULTS We virulence phenotyped, sequenced, and assembled two isolates of P. effusa from the Salinas Valley, California, U.S.A. that were identified as race 13 and 14. These assemblies are high quality in comparison to assemblies of other downy mildews having low total scaffold count (784 & 880), high contig N50s (48 kb & 52 kb), high BUSCO completion and low BUSCO duplication scores and share many syntenic blocks with Phytophthora species. Comparative analysis of four downy mildew and three Phytophthora species revealed parallel absences of genes encoding conserved domains linked to transporters, pathogenesis, and carbohydrate activity in the biotrophic species. Downy mildews surveyed that have lost the ability to produce zoospores have a common loss of flagella/motor and calcium domain encoding genes. Our phylogenomic data support multiple origins of downy mildews from hemibiotrophic progenitors and suggest that common gene losses in these downy mildews may be of genes involved in the necrotrophic stages of Phytophthora spp. CONCLUSIONS We present a high-quality draft genome of Peronospora effusa that will serve as a reference for Peronospora spp. We identified several Pfam domains as under-represented in the downy mildews consistent with the loss of zoosporegenesis and necrotrophy. Phylogenomics provides further support for a polyphyletic origin of downy mildews.
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Affiliation(s)
- Kyle Fletcher
- The Genome Center, Genome and Biomedical Sciences Facility, University of California, 451 East Health Sciences Drive, Davis, CA 95616 USA
| | - Steven J. Klosterman
- United States Department of Agriculture, Agricultural Research Service, Salinas, CA 93905 USA
| | - Lida Derevnina
- The Genome Center, Genome and Biomedical Sciences Facility, University of California, 451 East Health Sciences Drive, Davis, CA 95616 USA
- Present Address: The Sainsbury Laboratory, Norwich Research Park, Norwich, NR4 7UH UK
| | - Frank Martin
- United States Department of Agriculture, Agricultural Research Service, Salinas, CA 93905 USA
| | - Lien D. Bertier
- The Genome Center, Genome and Biomedical Sciences Facility, University of California, 451 East Health Sciences Drive, Davis, CA 95616 USA
| | - Steven Koike
- UC Davis Cooperative Extension Monterey County, Salinas, CA 93901 USA
- Present Address: TriCal Diagnostics, Hollister, CA 95023 USA
| | - Sebastian Reyes-Chin-Wo
- The Genome Center, Genome and Biomedical Sciences Facility, University of California, 451 East Health Sciences Drive, Davis, CA 95616 USA
| | - Beiquan Mou
- United States Department of Agriculture, Agricultural Research Service, Salinas, CA 93905 USA
| | - Richard Michelmore
- The Genome Center, Genome and Biomedical Sciences Facility, University of California, 451 East Health Sciences Drive, Davis, CA 95616 USA
- Departments of Plant Sciences, Molecular & Cellular Biology, Medical Microbiology & Immunology, University of California, Davis, 95616 USA
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Thangavel T, Jones S, Scott JB, Livermore M, Wilson CR. Detection of Two Peronospora spp., Responsible for Downy Mildew, in Opium Poppy Seed. PLANT DISEASE 2018; 102:2277-2284. [PMID: 30211657 DOI: 10.1094/pdis-03-18-0503-re] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Downy mildew is a serious threat to opium poppy production globally. In recent years, two pathogen species, Peronospora somniferi and Peronospora meconopsidis, which induce distinct symptoms, have been confirmed in Australia. In order to manage the spread of these pathogens, identifying the sources of inoculum is essential. In this study, we assessed pathogen presence associated with poppy seed. We developed PCR and qPCR assays targeting the coxI and coxII gene regions, for the detection, differentiation, and quantification of P. somniferi and P. meconopsidis in poppy seed. These results were complemented and compared with direct seed histological examination and a seed washing combined with viability staining for oospore detection. The majority of seed lots from all harvest years contained detectable P. meconopsidis, the earliest (1987) predating the first official record of the disease in Tasmania (1996). In contrast, only seed lots harvested in 2012 or later contained P. somniferi, evidence of its more recent introduction. P. meconopsidis contamination was estimated to be as high as 33.04 pg DNA/g of seed and P. somniferi as high as 35.17 pg DNA/g of seed. Incidence of pathogen contamination of seeds, estimated via a group testing protocol, ranged from 0 to 9% (P. meconopsidis) or 0 to 11% (P. somniferi). Mycelia were predominately found external to the seed coat. Seed washing and viability staining demonstrated that putatively viable oospores were present in the majority of seed lots. Transmission testing confirmed both pathogens can be successfully transmitted from infested seed to infected seedling. PCR and qPCR pathogen assays were found to be reliable and offer a routine test for determining pathogen inoculum in poppy seeds.
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Affiliation(s)
- Tamilarasan Thangavel
- Tasmanian Institute of Agriculture (TIA), University of Tasmania, New Town, Tasmania 7008, Australia
| | - Suzanne Jones
- TIA, University of Tasmania, Burnie TAS 7320, Australia
| | - Jason B Scott
- TIA, University of Tasmania, Burnie TAS 7320, Australia
| | - Mark Livermore
- Tasmanian Alkaloids Pty Ltd, P.O. Box 130, Westbury, TAS 7303, Australia
| | - Calum R Wilson
- TIA, University of Tasmania, New Town, Tasmania 7008, Australia
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Subbarao CS, Anchieta A, Ochoa L, Dhar N, Kunjeti SG, Subbarao KV, Klosterman SJ. Detection of Latent Peronospora effusa Infections in Spinach. PLANT DISEASE 2018; 102:1766-1771. [PMID: 30125212 DOI: 10.1094/pdis-12-17-1956-re] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Downy mildew disease of spinach, caused by Peronospora effusa, is managed in conventional fields by a combination of host resistance and scheduled fungicide applications. Fungicides are currently applied to prevent downy mildew epidemics regardless of the infection status of spinach crops. A more streamlined approach would be to develop methods to target either latent infections for fungicide application in conventional production systems or to hasten harvest in organic production. In this study, conventional polymerase chain reaction (PCR) was applied to detect P. effusa DNA in symptomless spinach leaves in three spatially and temporally separated field plots, each containing four 2-m beds, 35 m in length. Spinach leaves were sampled weekly at 3-m intervals at 48 locations throughout each plot. Initial samples were asymptomatic and yet PCR enabled detection of P. effusa DNA extracted from sampled spinach leaves. Detection of latent downy mildew infection in spinach leaves was confirmed by PCR as early as 7 days prior to symptom development. The limit of pathogen DNA detection in spinach leaves was calculated at 10 pg using the conventional PCR approach. Quantitative PCR with TaqMan methodology revealed the presence of inhibitors from spinach leaf DNA extracts and affected amplification efficiencies, but not when diluted, enabling detection of P. effusa DNA at a concentration of <0.1 pg. In conclusion, detection of latent infections may enable management decisions for earlier-than-normal harvest of infected, symptomless organic crops, and for timing fungicide applications on symptomless plants in conventional production.
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Affiliation(s)
| | - Amy Anchieta
- United States Department of Agriculture-Agricultural Research Service (USDA-ARS), Salinas, CA 93905
| | - Lorena Ochoa
- United States Department of Agriculture-Agricultural Research Service (USDA-ARS), Salinas, CA 93905
| | - Nikhilesh Dhar
- Department of Plant Pathology, University of California, Davis, c/o U.S. Agricultural Research Station, Salinas 93905
| | - Sridhara G Kunjeti
- Department of Plant Pathology, University of California, Davis, c/o U.S. Agricultural Research Station, Salinas 93905
| | - Krishna V Subbarao
- Department of Plant Pathology, University of California, Davis, c/o U.S. Agricultural Research Station, Salinas 93905
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Choudhury RA, Garrett KA, Klosterman SJ, Subbarao KV, McRoberts N. A Framework for Optimizing Phytosanitary Thresholds in Seed Systems. PHYTOPATHOLOGY 2017; 107:1219-1228. [PMID: 28726578 DOI: 10.1094/phyto-04-17-0131-fi] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Seedborne pathogens and pests limit production in many agricultural systems. Quarantine programs help prevent the introduction of exotic pathogens into a country, but few regulations directly apply to reducing the reintroduction and spread of endemic pathogens. Use of phytosanitary thresholds helps limit the movement of pathogen inoculum through seed, but the costs associated with rejected seed lots can be prohibitive for voluntary implementation of phytosanitary thresholds. In this paper, we outline a framework to optimize thresholds for seedborne pathogens, balancing the cost of rejected seed lots and benefit of reduced inoculum levels. The method requires relatively small amounts of data, and the accuracy and robustness of the analysis improves over time as data accumulate from seed testing. We demonstrate the method first and illustrate it with a case study of seedborne oospores of Peronospora effusa, the causal agent of spinach downy mildew. A seed lot threshold of 0.23 oospores per seed could reduce the overall number of oospores entering the production system by 90% while removing 8% of seed lots destined for distribution. Alternative mitigation strategies may result in lower economic losses to seed producers, but have uncertain efficacy. We discuss future challenges and prospects for implementing this approach.
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Affiliation(s)
- Robin Alan Choudhury
- First and second authors: Plant Pathology Department, Institute for Sustainable Food Systems, and Emerging Pathogens Institute, University of Florida, Gainesville 32611; third author: U.S. Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas 93905; and first, fourth, and fifth authors: Department of Plant Pathology, University of California, Davis 95616
| | - Karen A Garrett
- First and second authors: Plant Pathology Department, Institute for Sustainable Food Systems, and Emerging Pathogens Institute, University of Florida, Gainesville 32611; third author: U.S. Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas 93905; and first, fourth, and fifth authors: Department of Plant Pathology, University of California, Davis 95616
| | - Steven J Klosterman
- First and second authors: Plant Pathology Department, Institute for Sustainable Food Systems, and Emerging Pathogens Institute, University of Florida, Gainesville 32611; third author: U.S. Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas 93905; and first, fourth, and fifth authors: Department of Plant Pathology, University of California, Davis 95616
| | - Krishna V Subbarao
- First and second authors: Plant Pathology Department, Institute for Sustainable Food Systems, and Emerging Pathogens Institute, University of Florida, Gainesville 32611; third author: U.S. Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas 93905; and first, fourth, and fifth authors: Department of Plant Pathology, University of California, Davis 95616
| | - Neil McRoberts
- First and second authors: Plant Pathology Department, Institute for Sustainable Food Systems, and Emerging Pathogens Institute, University of Florida, Gainesville 32611; third author: U.S. Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas 93905; and first, fourth, and fifth authors: Department of Plant Pathology, University of California, Davis 95616
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Thomas A, Carbone I, Cohen Y, Ojiambo PS. Occurrence and Distribution of Mating Types of Pseudoperonospora cubensis in the United States. PHYTOPATHOLOGY 2017; 107:313-321. [PMID: 27841962 DOI: 10.1094/phyto-06-16-0236-r] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
During the past two decades, a resurgence of cucurbit downy mildew has occurred around the world, resulting in severe disease epidemics. In the United States, resurgence of the disease occurred in 2004 and several hypotheses, including introduction of a new genetic recombinant or pathotype of the pathogen, have been suggested as potential causes for this resurgence. Occurrence and distribution of mating types of Pseudoperonospora cubensis in the United States were investigated using 40 isolates collected from cucurbits across 11 states from 2005 to 2013. Pairing of unknown isolates with known mating-type tester strains on detached leaves of cantaloupe or cucumber resulted in oospore formation 8 to 10 days after inoculation. Isolates differed in their ability to form oospores across all coinoculation pairings, with oospore numbers ranging from 280 to 1,000 oospores/cm2 of leaf tissue. Oospores were hyaline to golden-yellow, spherical, and approximately 36 μm in diameter. Of the 40 isolates tested, 24 were found to be of the A1 mating type, while 16 were of the A2 mating type. Mating type was significantly (P < 0.0001) associated with host type, whereby all isolates collected from cucumber were of the A1 mating type, while isolates from squash and watermelon were of the A2 mating type. Similarly, mating type was significantly (P = 0.0287) associated with geographical region, where isolates from northern-tier states of Michigan, New Jersey, New York, and Ohio were all A1, while isolates belonging to either A1 or A2 mating type were present in equal proportions in southern-tier states of Alabama, Florida, Georgia, North Carolina, South Carolina, and Texas. Viability assays showed that oospores were viable and, on average, approximately 40% of the oospores produced were viable as determined by the plasmolysis method. This study showed that A1 and A2 mating types of P. cubensis are present and the pathogen could potentially reproduce sexually in cucurbits within the United States. In addition, the production of viable oospores reported in this study suggests that oospores could have an important role in the biology of P. cubensis and could potentially influence the epidemiology of cucurbit downy mildew in the United States.
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Affiliation(s)
- Anna Thomas
- First, second, and fourth authors: Center for Integrated Fungal Research, Department of Entomology and Plant Pathology, North Carolina State University, Raleigh 27695; and third author: Faculty of Life Sciences, Bar-Ilan University, Ramat Gan 52100, Israel
| | - Ignazio Carbone
- First, second, and fourth authors: Center for Integrated Fungal Research, Department of Entomology and Plant Pathology, North Carolina State University, Raleigh 27695; and third author: Faculty of Life Sciences, Bar-Ilan University, Ramat Gan 52100, Israel
| | - Yigal Cohen
- First, second, and fourth authors: Center for Integrated Fungal Research, Department of Entomology and Plant Pathology, North Carolina State University, Raleigh 27695; and third author: Faculty of Life Sciences, Bar-Ilan University, Ramat Gan 52100, Israel
| | - Peter S Ojiambo
- First, second, and fourth authors: Center for Integrated Fungal Research, Department of Entomology and Plant Pathology, North Carolina State University, Raleigh 27695; and third author: Faculty of Life Sciences, Bar-Ilan University, Ramat Gan 52100, Israel
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11
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Choudhury RA, Koike ST, Fox AD, Anchieta A, Subbarao KV, Klosterman SJ, McRoberts N. Spatiotemporal Patterns in the Airborne Dispersal of Spinach Downy Mildew. PHYTOPATHOLOGY 2017; 107:50-58. [PMID: 27482627 DOI: 10.1094/phyto-04-16-0162-r] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Downy mildew is the most devastating disease threatening sustainable spinach production, particularly in the organic sector. The disease is caused by the biotrophic oomycete pathogen Peronospora effusa, and the disease results in yellow lesions that render the crop unmarketable. In this study, the levels of DNA from airborne spores of P. effusa were assessed near a field of susceptible plants in Salinas, CA during the winter months of 2013-14 and 2014/15 using rotating-arm impaction spore-trap samplers that were assessed with a species-specific quantitative polymerase chain reaction (qPCR) assay. Low levels of P. effusa DNA were detectable from December through February in both winters but increased during January in both years, in correlation with observed disease incidence; sharp peaks in P. effusa DNA detection were associated with the onset of disease incidence. The incidence of downy mildew in the susceptible field displayed logistic-like dynamics but with considerable interseason variation. Analysis of the area under the disease progress curves suggested that the 2013-14 epidemic was significantly more severe than the 2014-15 epidemic. Spatial analyses indicated that disease incidence was dependent within an average range of 5.6 m, approximately equivalent to the width of three planted beds in a typical production field. The spatial distribution of spores captured during an active epidemic most closely fit a power-law distribution but could also be fit with an exponential distribution. These studies revealed two important results in the epidemiology of spinach downy mildew in California. First, they demonstrated the potential of impaction spore-trap samplers linked with a qPCR assay for indicating periods of high disease risk, as well as the detection of long-distance dispersal of P. effusa spores. Second, at the scale of individual crops, a high degree of spatial aggregation in disease incidence was revealed.
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Affiliation(s)
- R A Choudhury
- First, fifth, and seventh authors: Department of Plant Pathology, University of California, Davis 95616; second author: University of California Cooperative Extension, 1432 Abbott St., Salinas 93901; third author: Fox Weather, LLC, Fortuna, 95540; and fourth and sixth authors: United States Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas, CA 93905
| | - S T Koike
- First, fifth, and seventh authors: Department of Plant Pathology, University of California, Davis 95616; second author: University of California Cooperative Extension, 1432 Abbott St., Salinas 93901; third author: Fox Weather, LLC, Fortuna, 95540; and fourth and sixth authors: United States Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas, CA 93905
| | - A D Fox
- First, fifth, and seventh authors: Department of Plant Pathology, University of California, Davis 95616; second author: University of California Cooperative Extension, 1432 Abbott St., Salinas 93901; third author: Fox Weather, LLC, Fortuna, 95540; and fourth and sixth authors: United States Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas, CA 93905
| | - A Anchieta
- First, fifth, and seventh authors: Department of Plant Pathology, University of California, Davis 95616; second author: University of California Cooperative Extension, 1432 Abbott St., Salinas 93901; third author: Fox Weather, LLC, Fortuna, 95540; and fourth and sixth authors: United States Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas, CA 93905
| | - K V Subbarao
- First, fifth, and seventh authors: Department of Plant Pathology, University of California, Davis 95616; second author: University of California Cooperative Extension, 1432 Abbott St., Salinas 93901; third author: Fox Weather, LLC, Fortuna, 95540; and fourth and sixth authors: United States Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas, CA 93905
| | - S J Klosterman
- First, fifth, and seventh authors: Department of Plant Pathology, University of California, Davis 95616; second author: University of California Cooperative Extension, 1432 Abbott St., Salinas 93901; third author: Fox Weather, LLC, Fortuna, 95540; and fourth and sixth authors: United States Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas, CA 93905
| | - N McRoberts
- First, fifth, and seventh authors: Department of Plant Pathology, University of California, Davis 95616; second author: University of California Cooperative Extension, 1432 Abbott St., Salinas 93901; third author: Fox Weather, LLC, Fortuna, 95540; and fourth and sixth authors: United States Department of Agriculture-Agricultural Research Service, 1636 E. Alisal St., Salinas, CA 93905
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