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.

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.

Sporangiospore Viability and Oospore Production in the Spinach Downy Mildew Pathogen, Peronospora effusa

Plant Disease ◽  
2020 ◽  
Vol 104 (10) ◽  
pp. 2634-2641 ◽  
Author(s):  
Braham Dhillon ◽  
Chunda Feng ◽  
Maria Isabel Villarroel-Zeballos ◽  
Vanina Lilian Castroagudin ◽  
Gehendra Bhattarai ◽  
...  
Keyword(s):  
United States ◽  
Downy Mildew ◽  
The United States ◽  
Long Distance ◽  
Important Constraint ◽  
Detached Leaves ◽  
Spinach Downy Mildew ◽  
Obligate Pathogen ◽  
Production Areas ◽  
The Impact

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.

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.

Neurocysticercosis: Report of Unusual Pediatric Cases

PEDIATRICS ◽  
1996 ◽  
Vol 98 (5) ◽  
pp. 974-977
Author(s):  
Julie Kim Stamos ◽  
Anne H. Rowley ◽  
Yoon S. Hahn ◽  
Ellen Gould Chadwick ◽  
Peter M. Schsntz ◽  
...  
Keyword(s):  
United States ◽  
Latin America ◽  
Young Children ◽  
Incubation Period ◽  
Endemic Area ◽  
Close Contact ◽  
The United States ◽  
Prolonged Incubation ◽  
Endemic Areas ◽  
Infants And Young Children

Cysticercosis is widely endemic in Latin America, Asia, and Africa. The incidence of cysticercosis has been increasing in the United States during the last decade.1 Although an infection still seen primarily in immigrants, it has been reported in increasing numbers in individuals who have close contact with persons who have resided in endemic areas.2 Only 6 cases of cysticercosis in children born in the United States have been reported; in 3 of these cases, the parents were from or had traveled to an endemic area and Taenia ova were recovered from the stools of the parent(s).1,3-6 Because of the prolonged incubation period, cases are rarely seen in infants and young children.4

scholarly journals First Report of Impatiens Downy Mildew Outbreaks Caused by Plasmopara obducens Throughout the Hawai'ian Islands

Plant Disease ◽  
2014 ◽  
Vol 98 (5) ◽  
pp. 696-696 ◽  
Author(s):  
J. A. Crouch ◽  
M. P. Ko ◽  
J. M. McKemy
Keyword(s):  
United States ◽  
Downy Mildew ◽  
The United States ◽  
Molecular Data ◽  
Economic Losses ◽  
Voucher Specimen ◽  
First Report ◽  
Plastic Bags ◽  
Leaf Surfaces ◽  
Impatiens Walleriana

Downy mildew of impatiens (Impatiens walleriana Hook.f.) was first reported from the continental United States in 2004. In 2011 to 2012, severe and widespread outbreaks were documented across the United States mainland, resulting in considerable economic losses. On May 5, 2013, downy mildew disease symptoms were observed from I. walleriana ‘Super Elfin’ at a retail nursery in Mililani, on the Hawai'ian island of Oahu. Throughout May and June 2013, additional sightings of the disease were documented from the islands of Oahu, Kauai, Maui, and Hawai'i from nurseries, home gardens, and botanical park and landscape plantings. Symptoms of infected plants initially showed downward leaf curl, followed by a stippled chlorotic appearance on the adaxial leaf surfaces. Abaxial leaf surfaces were covered with a layer of white mycelia. Affected plants exhibited defoliation, flower drop, and stem rot as the disease progressed. Based on morphological and molecular data, the organism was identified as Plasmopara obducens (J. Schröt.) J. Schröt. Microscopic observation disclosed coenocytic mycelium and hyaline, thin-walled, tree-like (monopodial branches), straight, 94.0 to 300.0 × 3.2 to 10.8 μm sporangiophores. Ovoid, hyaline sporangia measuring 11.0 to 14.6 × 12.2 to 16.2 (average 13.2 × 14.7) μm were borne on sterigma tips of rigid branchlets (8.0 to 15.0 μm) at right angle to the main axis of the sporangiophores (1,3). Molecular identification of the pathogen was conducted by removing hyphae from the surface of three heavily infected leaves using sterile tweezers, then extracting DNA using the QIAGEN Plant DNA kit (QIAGEN, Gaithersburg, MD). The nuclear rDNA internal transcribed spacer was sequenced from each of the three samples bidirectionally from Illustra EXOStar (GE Healthcare, Piscataway, NJ) purified amplicon generated from primers ITS1-O and LR-0R (4). Resultant sequences (GenBank KF366378 to 80) shared 99 to 100% nucleotide identity with P. obducens accession DQ665666 (4). A voucher specimen (BPI892676) was deposited in the U.S. National Fungus Collections, Beltsville, MD. Pathogenicity tests were performed by spraying 6-week-old impatiens plants (I. walleriana var. Super Elfin) grown singly in 4-inch pots with a suspension of 1 × 104 P. obducens sporangia/ml until runoff using a handheld atomizer. Control plants were sprayed with distilled water. The plants were kept in high humidity by covering with black plastic bags for 48 h at 20°C, and then maintained in the greenhouse (night/day temperature of 20/24°C). The first symptoms (downward curling and chlorotic stippling of leaves) and sporulation of the pathogen on under-leaf surfaces of the inoculated plants appeared at 10 days and 21 days after inoculation, respectively. Control plants remained healthy. Morphological features and measurements matched those of the original inoculum, thus fulfilling Koch's postulates. To our knowledge, this is the first report of downy mildew on I. walleriana in Hawai'i (2). The disease appears to be widespread throughout the islands and is likely to cause considerable losses in Hawai'ian landscapes and production settings. References: (1) O. Constantinescu. Mycologia 83:473, 1991. (2) D. F. Farr and A. Y. Rossman. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ July 16, 2013. (3) P. A. Saccardo. Syllogue Fungorum 7:242, 1888. (4) M. Thines. Fungal Genet Biol 44:199, 2007.

scholarly journals First Occurrence of Downy Mildew of Statice, Caused by Peronospora statices, in California and the Rest of the United States

Plant Disease ◽  
1998 ◽  
Vol 82 (5) ◽  
pp. 591-591 ◽  
Author(s):  
S. T. Koike ◽  
P. A. Nolan ◽  
S. A. Tjosvold ◽  
K. L. Robb
Keyword(s):  
United States ◽  
Downy Mildew ◽  
Fungal Growth ◽  
The United States ◽  
San Diego County ◽  
Leaf Spots ◽  
Initial Symptoms ◽  
First Occurrence ◽  
Humidity Chamber ◽  
Favorable Conditions

In California, hybrid statice (Misty series; Limonium bellidifolium × Limonium latifolium) is grown as a commercial cutflower crop in fields and greenhouses. In 1997, downy mildew was observed on statice plantings in both southern (San Diego County) and central (Monterey and Santa Cruz counties) parts of coastal California. Initial symptoms consisted of light green, irregularly shaped leaf spots that, after a few days, became chlorotic. As disease progressed, chlorotic spots coalesced and turned necrotic, at times resulting in extensive death of leaf tissues. Under favorable conditions, the purple to gray sporulation of the pathogen could be seen on abaxial surfaces of leaves. Conidiophores had main trunks with dichotomous branches and measured 194 to 335 μm in length (mean = 229 μm) from the base to the first branches and 7 to 8 μm across at the widest part. Branch ends were slender with curved tips that measured 5 to 8 μm long. Conidia were ovoid to globose with very short pedicels, and measured 14 to 19 μm × 14 to 17 μm. Conidial surfaces appeared slightly roughened when viewed with a scanning electron microscope. Clearing leaf sections with 10% NaOH (1) revealed the presence of yellow-brown, globose oospores that measured 31 to 47 μm. The pathogen was identified as Peronospora statices (1). Pathogenicity was demonstrated by pressing leaves with abundant sporulation against healthy leaves of test plants (Misty White) and then placing inoculated plants in a humidity chamber. After 10 to 12 days, symptoms similar to those originally observed developed on inoculated plants; after 14 to 16 days, purple fungal growth morphologically similar to the original isolates grew on leaves. Uninoculated control plants did not develop symptoms or signs of downy mildew. This is the first report of downy mildew caused by P. statices on statice in California and the rest of the United States. The disease has also been confirmed on Blue Fantasia (L. bellidifolium × L. perezii). This disease has been reported previously in Italy, The Netherlands, and the United Kingdom (1). Reference: (1) G. S. Hall et al. Eur. J. Plant Pathol. 103:471, 1997.

scholarly journals Host Preference of Mating Type in Pseudoperonospora cubensis, the Downy Mildew Causal Agent of Cucurbits

Plant Disease ◽  
2013 ◽  
Vol 97 (2) ◽  
pp. 292-292 ◽  
Author(s):  
Y. Cohen ◽  
A. E. Rubin ◽  
M. Galperin
Keyword(s):  
Downy Mildew ◽  
Mating Type ◽  
Host Preference ◽  
Practical Implication ◽  
Natural Infection ◽  
Primary Source ◽  
Pseudoperonospora Cubensis ◽  
Close Proximity ◽  
Detached Leaves ◽  
Classical Genetics

The A2 mating type of Pseudoperonospora cubensis was first discovered in Israel in May 2010 on butternut gourd (Cucurbita moschata) (1). We monitored the occurrence of the A2 mating type of P. cubensis in isolates collected during May 2010 through September 2012 from downy mildew-infected cucurbit crops growing along the coastal plain of Israel. Mating type was determined by oospore production in melon leaf discs co-inoculated with sporangia of a test isolate mixed with sporangia of A1 or A2 tester isolates (2). The A1 and A2 tester isolates were maintained at 14°C (14 h light/day) by repeated inoculation of detached leaves of cucumber and pumpkin, respectively. The 29 isolates that were collected from cucumber (Cucumis sativum) were all A1. Of the 33 isolates collected from pumpkin (Cucurbita maxima), squash (C. pepo), or butternut gourd (C. moschata), 88% were A2 and 12% were A1. The host preference of mating type in P. cubensis was monitored at Bar-Ilan University farm during April to July 2012, among about 800 plants of eight cucurbit species (~100 plants per species) that were grown side-by-side in three adjacent net-houses (two 6 × 50 m and one 6 × 100 m) and exposed to natural infection. Downy mildew developed on cucumber, melon, pumpkin, squash, and butternut gourd, but not on watermelon, sponge gourd (Luffa cylindrica), or Momordica balsamina. Three-hundred and three isolates of P. cubensis were collected and tested for mating type: 123 from cucumber, 53 from melon, 30 from pumpkin, 48 from butternut gourd, and 41 from squash. The cucumber isolates expressed A1, A2, and A1A2 at a ratio of 94.3%, 3.3%, and 2.4%, respectively; the melon isolates 58.5%, 26.4%, and 15.1%; the pumpkin isolates 0%, 96.7%, and 3.3%; the butternut isolate 7.3%, 87.3%, and 5.5%; and the squash isolates 2.4%, 97.6%, and 0%, respectively. A1A2 isolates produce oospores when crossed with either A1 or A2 tester isolates. This is the first evidence suggesting a preference of A1 isolates to Cucumis spp. and of A2 isolates to Cucurbita spp. similar preference was recently observed among Chinese isolates of this pathogen (unpublished data). The mechanism(s) controlling this preference is not known. Classical genetics is currently employed to P. cubensis in order to understand if it derives from true linkage. The practical implication for downy mildew management is that growing cucumber/melon in close proximity to pumpkin/squash/butternut gourd should be avoided as it may enhance oospore production in nature. Oospores in soil were recently shown to serve as a primary source of downy mildew infection in cucumber (3). References: (1) Y. Cohen, A. E. Rubin, and M.Galperin. Plant Dis. 95:874, 2011; (2) Y. Cohen and A. E. Rubin. Eur. J. Plant Pathol. 132:577, 2012; (3) Y. J. Zhang et al. J. Phytopathol. 160:469, 2012.

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