scholarly journals Resurgence of cucurbit downy mildew in the United States: Insights from comparative genomic analysis of Pseudoperonospora cubensis

2017 ◽  
Vol 7 (16) ◽  
pp. 6231-6246 ◽  
Author(s):  
Anna Thomas ◽  
Ignazio Carbone ◽  
Kisurb Choe ◽  
Lina M. Quesada-Ocampo ◽  
Peter S. Ojiambo
Keyword(s):  
United States ◽  
Downy Mildew ◽  
Genomic Analysis ◽  
The United States ◽  
Comparative Genomic Analysis ◽  
Comparative Genomic ◽  
Pseudoperonospora Cubensis ◽  
Cucurbit Downy Mildew

scholarly journals Occurrence and Distribution of Mating Types of Pseudoperonospora cubensis in the United States

2017 ◽  
Vol 107 (3) ◽  
pp. 313-321 ◽  
Author(s):  
Anna Thomas ◽  
Ignazio Carbone ◽  
Yigal Cohen ◽  
Peter S. Ojiambo
Keyword(s):  
United States ◽  
South Carolina ◽  
Downy Mildew ◽  
Mating Type ◽  
Severe Disease ◽  
The United States ◽  
Pseudoperonospora Cubensis ◽  
Mating Types ◽  
Cucurbit Downy Mildew ◽  
Golden Yellow

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.

Efficacy of Fungicides for Pseudoperonospora cubensis Determined using Bioassays over Multiple Years in the Mid-Atlantic and Northeastern United States

2021 ◽  
Author(s):  
Jake Gardner Jones ◽  
Kathryne L. Everts ◽  
Margaret Tuttle McGrath ◽  
Beth K. Gugino
Keyword(s):  
United States ◽  
New York ◽  
Downy Mildew ◽  
The United States ◽  
Pathogen Resistance ◽  
Pseudoperonospora Cubensis ◽  
Cucurbit Downy Mildew ◽  
Fungicide Efficacy ◽  
Repeated Applications ◽  
Primary Management

In the United States, fungicides are the primary management option for cucumber growers to protect their crops from Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew. Pathogen resistance to some fungicides can quickly develop with the repeated applications needed to protect yield. In order to determine fungicide efficacy and monitor it over time, bioassays were conducted from 2016-2019 in Delaware, Maryland, Pennsylvania, and New York. Potted cucumber plants were either sprayed with fungicides or not treated, placed next to field-grown plants with cucurbit downy mildew for up to two days, then kept in a greenhouse until symptoms developed. Severity of symptoms or number of lesions on leaves was recorded 6-14 days after exposure started and used to determine fungicide efficacy. Quadris (azoxystrobin) was ineffective in seven of the nine bioassays, while Revus (mandipropamid) was ineffective in six of seven bioassays. Forum (dimethomorph) and Presidio (fluopicolide) were ineffective in three of eight and four of nine bioassays, respectively. The most effective fungicides were Bravo (chlorothalonil), Zing! (zoxamide + chlorothalonil), and Orondis (oxathiapiprolin), all of which consistently suppressed disease severity more than 90% when compared with the untreated control. Previcur Flex (propamocarb hydrochloride) and Ranman (cyazofamid) were also effective in every bioassay.

Population Analyses Reveal Two Host-Adapted Clades of Pseudoperonospora cubensis, the Causal Agent of Cucurbit Downy Mildew, on Commercial and Wild Cucurbits

2020 ◽  
Vol 110 (9) ◽  
pp. 1578-1587 ◽  
Author(s):  
E. C. Wallace ◽  
K. N. D’Arcangelo ◽  
L. M. Quesada-Ocampo
Keyword(s):  
United States ◽  
North Carolina ◽  
Downy Mildew ◽  
Random Mating ◽  
The United States ◽  
Causal Agent ◽  
Host Susceptibility ◽  
Pseudoperonospora Cubensis ◽  
Cucurbit Downy Mildew ◽  
Population Analyses

Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew, is an airborne, obligate oomycete pathogen that re-emerged in 2004 and causes foliar disease and yield losses in all major cucurbit crops in the United States. Approximately 60 species in the family Cucurbitaceae have been reported as hosts of P. cubensis. Commercial hosts including cucumber, cantaloupe, pumpkin, squash, and watermelon are grown in North Carolina and many host species occur in the wild as weeds. Little is known about the contribution of wild cucurbits to the yearly epidemic; thus, this study aimed to determine the role of commercial and wild cucurbits in the structuring of P. cubensis populations in North Carolina, a region with high pathogen diversity. Ten microsatellite markers were used to analyze 385 isolates from six commercial and four wild cucurbits from three locations representing different growing regions across North Carolina. Population analyses revealed that wild and commercial cucurbits are hosts of P. cubensis in the United States, that host is the main factor structuring P. cubensis populations, and that P. cubensis has two distinct, host-adapted clades at the cucurbit species level, with clade 1 showing random mating and evidence of recombination and clade 2 showing nonrandom mating and no evidence of recombination. Our findings have implications for disease management because clade-specific factors such as host susceptibility and inoculum availability of each clade by region may influence P. cubensis outbreaks in different commercial cucurbits, timing of fungicide applications, and phenotyping for breeding efforts.

scholarly journals Resurgence of Cucurbit Downy Mildew in the United States: A Watershed Event for Research and Extension

Plant Disease ◽  
2015 ◽  
Vol 99 (4) ◽  
pp. 428-441 ◽  
Author(s):  
Gerald J. Holmes ◽  
Peter S. Ojiambo ◽  
Mary K. Hausbeck ◽  
Lina Quesada-Ocampo ◽  
Anthony P. Keinath
Keyword(s):  
United States ◽  
North Carolina ◽  
Downy Mildew ◽  
The United States ◽  
Crop Loss ◽  
Eastern United States ◽  
Cucumis Sativus L ◽  
Cucurbit Downy Mildew ◽  
Control Programs ◽  
Rapid Spread

In 2004, an outbreak of cucurbit downy mildew (CDM) caused by the oomycete Pseudoperonospora cubensis (Berk. & M. A. Curtis) Rostovzev resulted in an epidemic that stunned the cucumber (Cucumis sativus L.) industry in the eastern United States. The disease affects all major cucurbit crops, including cucumber, muskmelon, squashes, and watermelon. Although the 2004 epidemic began in North Carolina, the cucumber crop from Florida to the northern growing regions in the United States was devastated, resulting in complete crop loss in several areas. Many cucumber fields were abandoned prior to harvest. The rapid spread of the disease coupled with the failure of fungicide control programs surprised growers, crop consultants, and extension specialists. The epidemic raised several fundamental questions about the potential causes for the resurgence of the disease. Some of these questions revolved around whether the epidemic would recur in subsequent years and the possible roles that changes in the host, pathogen, and environment may have played in the epidemic.

scholarly journals First Report of Pseudoperonospora cubensis Causing Downy Mildew on Momordica balsamina and M. charantia in North Carolina

Plant Disease ◽  
2014 ◽  
Vol 98 (9) ◽  
pp. 1279-1279 ◽  
Author(s):  
E. Wallace ◽  
M. Adams ◽  
K. Ivors ◽  
P. S. Ojiambo ◽  
L. M. Quesada-Ocampo
Keyword(s):  
United States ◽  
North Carolina ◽  
Downy Mildew ◽  
Leaf Surface ◽  
The United States ◽  
Pseudoperonospora Cubensis ◽  
Post Inoculation ◽  
Bitter Melon ◽  
First Report ◽  
C Cycle

Momordica balsamina (balsam apple) and M. charantia L. (bitter melon/bitter gourd/balsam pear) commonly grow in the wild in Africa and Asia; bitter melon is also cultivated for food and medicinal purposes in Asia (1). In the United States, these cucurbits grow as weeds or ornamentals. Both species are found in southern states and bitter melon is also found in Pennsylvania and Connecticut (3). Cucurbit downy mildew (CDM), caused by the oomycete Pseudoperonospora cubensis, was observed on bitter melon and balsam apple between August and October of 2013 in six North Carolina sentinel plots belonging to the CDM ipmPIPE program (2). Plots were located at research stations in Johnston, Sampson, Lenoir, Henderson, Rowan, and Haywood counties, and contained six different commercial cucurbit species including cucumbers, melons, and squashes in addition to the Momordica spp. Leaves with symptoms typical of CDM were collected from the Momordica spp. and symptoms varied from irregular chlorotic lesions to circular lesions with chlorotic halos on the adaxial leaf surface. Sporulation on the abaxial side of the leaves was observed and a compound microscope revealed sporangiophores (180 to 200 μm height) bearing lemon-shaped, dark sporangia (20 to 35 × 10 to 20 μm diameter) with papilla on one end. Genomic DNA was extracted from lesions and regions of the NADH dehydrogynase subunit 1 (Nad1), NADH dehydrogynase subunit 5 (Nad5), and internal transcribed spacer (ITS) ribosomal RNA genes were amplified and sequenced (4). BLAST analysis revealed 100% identity to P. cubensis Nad1 (HQ636552.1, HQ636551.1), Nad5 (HQ636556.1), and ITS (HQ636491.1) sequences in GenBank. Sequences from a downy mildew isolate from each Momordica spp. were deposited in GenBank as accession nos. KJ496339 through 44. To further confirm host susceptibility, vein junctions on the abaxial leaf surface of five detached leaves of lab-grown balsam apple and bitter melon were either inoculated with a sporangia suspension (10 μl, 104 sporangia/ml) of a P. cubensis isolate from Cucumis sativus (‘Vlaspik' cucumber), or with water as a control. Inoculated leaves were placed in humidity chambers to promote infection and incubated using a 12-h light (21°C) and dark (18°C) cycle. Seven days post inoculation, CDM symptoms and sporulation were observed on inoculated balsam apple and bitter melon leaves. P. cubensis has been reported as a pathogen of both hosts in Iowa (5). To our knowledge, this is the first report of P. cubensis infecting these Momordica spp. in NC in the field. Identifying these Momordica spp. as hosts for P. cubensis is important since these cucurbits may serve as a source of CDM inoculum and potentially an overwintering mechanism for P. cubensis. Further research is needed to establish the role of non-commercial cucurbits in the yearly CDM epidemic, which will aid the efforts of the CDM ipmPIPE to predict disease outbreaks. References: (1) L. K. Bharathi and K. J. John. Momordica Genus in Asia-An Overview. Springer, New Delhi, India, 2013. (2) P. S. Ojiambo et al. Plant Health Prog. doi:10.1094/PHP-2011-0411-01-RV, 2011. (3) PLANTS Database. Natural Resources Conservation Service, USDA. Retrieved from http://plants.usda.gov/ , 7 February 2014. (4) L. M. Quesada-Ocampo et al. Plant Dis. 96:1459, 2012. (5) USDA. Index of Plant Disease in the United States. Agricultural Handbook 165, 1960.

scholarly journals Utility of a Cucumber Plant Bioassay to Assess Fungicide Efficacy Against Pseudoperonospora cubensis

Plant Disease ◽  
2016 ◽  
Vol 100 (2) ◽  
pp. 490-499 ◽  
Author(s):  
Anthony P. Keinath
Keyword(s):  
Downy Mildew ◽  
The United States ◽  
Control Treatment ◽  
Cucumber Plant ◽  
Pseudoperonospora Cubensis ◽  
Water Control ◽  
Cucurbit Downy Mildew ◽  
Potted Plants ◽  
Fungicide Efficacy ◽  
Pathogen Populations

Over a dozen fungicides are registered in the United States to manage cucurbit downy mildew caused by Pseudoperonospora cubensis. Efficacy varies greatly among them, due, in part, to reduced sensitivity to some fungicides in some pathogen populations. The objective of this study was to determine whether fungicide efficacy could be assessed using fungicide-treated cucumber (Cucumis sativus) exposed to natural inoculum for a brief period. Potted cucumber plants were treated with water or 1 of 13 fungicides registered to control cucurbit downy mildew. One day later, they were placed in a field among cucumber plants that had symptoms and signs of downy mildew. After a 48-h exposure to P. cubensis, potted plants were moved to a growth chamber held at day and night temperatures of 21 and 18°C, respectively, and 50% relative humidity. Severity (leaf area with symptoms) of downy mildew was rated 5 and 7 days later. The assay was done eight times, twice each in July and October 2013 and 2014. Year, season, trial, and interactions among these factors affected downy mildew development. Severity at 7 days on plants treated with mandipropamid, azoxystrobin, dimethomorph, cymoxanil, fluopicolide, and propamocarb was not significantly different from the water control treatment in eight, five, three, three, two, and two of eight bioassays, respectively. Severity on plants treated with cyazofamid, fluazinam, mancozeb + zoxamide, mancozeb, chlorothalonil, and ametoctradin + dimethomorph was less than on plants treated with water in all bioassays. These six fungicides should be effective when applied early in the season to prevent initial infections. Cyazofamid and mancozeb + zoxamide prevented an increase in severity between rating times. In conclusion, the assay consistently detected resistance to mandipropamid and azoxystrobin and demonstrated the efficacy of six other fungicides.

scholarly journals Genetic characterization of Streptococcus equi subspecies zooepidemicus associated with high swine mortality in United States

2019 ◽  
Author(s):  
Xuhua Chen ◽  
Nubia Resende-De-Macedo ◽  
Panchan Sitthicharoenchai ◽  
Orhan Sahin ◽  
Eric Burrough ◽  
...  
Keyword(s):  
United States ◽  
High Mortality ◽  
Genomic Analysis ◽  
The United States ◽  
Genomic Islands ◽  
Comparative Genomic ◽  
Epidemiological Analysis ◽  
Streptococcus Equi ◽  
Swine Population

AbstractHigh mortality events due to Streptococcus equi subspecies zooepidemicus (S. zooepidemicus) in swine have not previously been reported in the United States. In September and October 2019, outbreaks with swine mortality up to 50% due to S. zooepidemicus septicemia were reported in Ohio and Tennessee. Genomic epidemiological analysis revealed that the eight outbreak isolates were clustered together with ATCC 36246, a Chinese strain caused outbreaks with high mortality, also closely related to three isolates from human cases from Virginia, but significantly different from an outbreak-unrelated swine isolate from Arizona and most isolates from other animal species. Comparative genomic analysis on two outbreak isolates and another outbreak-unrelated isolate identified several genomic islands and virulence genes specifically in the outbreak isolates only, which are likely associated with the high mortality observed in the swine population. These findings have implications for understanding, tracking, and possibly preventing diseases caused by S. zooepidemicus in swine.

scholarly journals Formation and Infectivity of Oospores of Pseudoperonospora cubensis, the Causal Agent of Downy Mildew in Cucurbits

Plant Disease ◽  
2011 ◽  
Vol 95 (7) ◽  
pp. 874-874 ◽  
Author(s):  
Y. Cohen ◽  
A. E. Rubin ◽  
M. Galperin
Keyword(s):  
United States ◽  
Downy Mildew ◽  
Cucumis Melo ◽  
The United States ◽  
Pseudoperonospora Cubensis ◽  
Prolonged Incubation ◽  
Field Isolates ◽  
F1 Progeny ◽  
Detached Leaves ◽  
Growth Chambers

The oomycete Pseudoperonospora cubensis attacks members of the Cucurbitaceae, causing severe foliage damage especially to cucumber and melon. Recently, new pathotypes of this oomycete appeared in Israel (2) and Italy (1) and highly aggressive isolates appeared in the United States (3). Since oospores of P. cubensis were rarely seen and sexual propagation by oospores was never reported (4), it is assumed that it propagates clonally by sporangia. Here we report on sexual reproduction of P. cubensis under controlled conditions in the laboratory. We found that field isolates belonging to the old pathotype 3 or to the new pathotype 6 (2) inoculated singly onto detached leaves of cucurbits in growth chambers at 15 or 20°C produced no oospores, even after prolonged incubation periods. However, when sporangia of some paired field isolates were mixed together at a 1:1 ratio, similarly inoculated onto detached leaves, and incubated at 15 or 20°C, numerous oospores (up to ~300/cm2) were formed in the mesophyll within 6 to 11 days, depending on the isolates pair, the host inoculated, and temperature. Oospores were also formed at 12.5°C but not at 25°C. Oospores developed in intact plants when kept at 15 or 20°C under a humidity-saturated atmosphere during disease development. Oospores were round, light brown to brown with an average diameter of ~40 μm. Oospores were produced in Cucumis sativum (cvs. Nadiojni and Dalila) and Cucumis melo (cvs. Ananas-Yokneam and Ein-Dor) but not in Cucurbita pepo (cv. Arlika, Beiruti), C. moschata (cv. Dalorit), or C. maxima (cv. Tripoli). To verify that oospores are infective, cucumber or melon leaves containing oospores were homogenized in water. The homogenate was twice brought to dryness at 25 to 30°C in petri dishes to differentially kill the vegetative structures of the pathogen (sporangia, cystospores, zoospores, and mycelia), resuspended in water, and inoculated onto detached leaves of various cucurbits in growth chambers at 15 or 20°C. Downy mildew lesions carrying sporangia appeared within 7 to 20 days in leaves of Cucumis sativum, Cucumis melo, and C. moschata but not in C. pepo or C. maxima. The recombinant origin of the F1 offspring isolates was confirmed by mefenoxam sensitivity tests, random amplified polymorphic DNA, and simple sequence repeat analyses. F1 progeny isolates of some crosses lost pathogenicity to C. moschata or C. maxima, toward which one of their parents was pathogenic, while others gained pathogenicity to Luffa cylindrica or Citrullus lanatus toward which neither parent was pathogenic. Data confirmed that isolates of P. cubensis can mate to produce oospores, especially under constant humidity conditions; such oospores are infective to cucurbits and F1 progeny isolates show altered sensitivity to fungicides or altered host range relative to their parents. To our knowledge, this is the first report of oospore formation by P. cubensis in the laboratory and on their pathogenicity to cucurbits. Reasons for the parallel appearance of new pathotypes of P. cubensis in Israel in 2002 (2) and Italy in 2003 (1) and the reemergence of highly aggressive isolates of the pathogen in the United States in 2004 (3) are not known. They may be related to oospore production and sexual recombination in P. cubensis. References: (1) C. Cappelli et al. Plant Dis. 87:449, 2003. (2) Y. Cohen et al. Phytoparasitica 31:458, 2003. (3) G. J. Holmes et al. Am. Veg. Grower. February, 14-15, 2006. (4) A. Lebeda and Y. Cohen. Eur. J. Plant Pathol.129:157, 2011.

Diagnostic Guide for Cucurbit Downy Mildew

2020 ◽  
Vol 21 (3) ◽  
pp. 166-172 ◽  
Author(s):  
Andres Salcedo ◽  
Mary Hausbeck ◽  
Stacey Pigg ◽  
Lina M. Quesada-Ocampo
Keyword(s):  
Downy Mildew ◽  
Host Resistance ◽  
The United States ◽  
Geographic Range ◽  
Pseudoperonospora Cubensis ◽  
Pathogen Identification ◽  
Cucurbit Downy Mildew ◽  
Identification Evaluation ◽  
And Storage ◽  
Symptoms And Signs

Cucurbit downy mildew caused by the oomycete Pseudoperonospora cubensis is the most devastating foliar disease on cultivated cucurbitaceous crops. Failure of host resistance in cucumber and previously effective fungicides has occurred in the last few years in the United States and Europe, making accurate and early diagnosis critical for timely disease management. The objective of this diagnostic guide is to describe the current taxonomy, host, geographic range, symptoms, and signs as well as effective techniques for pathogen identification, evaluation, isolation, and storage for P. cubensis.

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