Abstract
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/cm 2 ) 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.
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Claims 20
Claim · model-written summary
Single old or new pathotype isolates produced no oospores on detached cucurbit leaves.
- Taxon
- cucurbitse0
- Outcome
- no oosporese0
- Qualifier
- 15 or 20°Ce0
- Setting
- growth chamberse0
- Interactor role
- pathogene0
Not reportedInteractor
Verbatim
e0“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”
Claim · model-written summary
Mixed isolate pairs produced abundant oospores in cucurbit mesophyll within 6 to 11 days.
Verbatim
e0“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/cm 2 ) were formed in the mesophyll within 6 to 11 days”
t0“Cucurbits”
Claim · model-written summary
Oospores formed at 12.5°C.
Verbatim
e0“Oospores were also formed at 12.5°C”
t0“Cucurbits”
Claim · model-written summary
Oospores did not form at 25°C.
Verbatim
e0“but not at 25°C”
t0“Cucurbits”
Claim · model-written summary
Oospores developed on intact plants under saturated humidity at 15 or 20°C.
Verbatim
e0“Oospores developed in intact plants when kept at 15 or 20°C under a humidity-saturated atmosphere during disease development.”
t0“Cucurbits”
Claim · model-written summary
Oospores were round, brown and approximately 40 μm across.
Verbatim
e0“Oospores were round, light brown to brown with an average diameter of ~40 μm.”
t0“Cucurbits”
Claim · model-written summary
Oospores were produced in Cucumis sativum.
Verbatim
e0“Oospores were produced in Cucumis sativum (cvs. Nadiojni and Dalila)”
Claim · model-written summary
Oospores were produced in Cucumis melo.
- Taxon
- Cucumis melo
.jpg?width=60)
Cucumis meloe0 - Material
- Ananas-Yokneam and Ein-Dore0
- Outcome
- Cucumis meloe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
.jpg?width=500)
Verbatim
e0“Cucumis melo (cvs. Ananas-Yokneam and Ein-Dor)”
Claim · model-written summary
Oospores were absent in Cucurbita pepo.
- Taxon
- Cucurbita pepo
Cucurbita pepoe0 - Material
- Arlika, Beirutie0
- Outcome
- not in Cucurbita pepoe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“but not in Cucurbita pepo (cv. Arlika, Beiruti)”
Claim · model-written summary
Oospores were absent in C. moschata.
- Taxon
- C. moschata
Cucurbita moschatae0 - Material
- Dalorite0
- Outcome
- C. moschatae0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“C. moschata (cv. Dalorit)”
Claim · model-written summary
Oospores were absent in C. maxima.
- Taxon
- C. maxima
Cucurbita maximae0 - Material
- Tripolie0
- Outcome
- C. maximae0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“C. maxima (cv. Tripoli)”
Claim · model-written summary
Oospore inoculum caused sporulating downy mildew lesions in Cucumis sativum.
Verbatim
e0“Downy mildew lesions carrying sporangia appeared within 7 to 20 days in leaves of Cucumis sativum”
Claim · model-written summary
Oospore inoculum caused sporulating downy mildew lesions in Cucumis melo.
- Taxon
- Cucumis meloCucumis meloe0
- Outcome
- Downy mildew lesions carrying sporangiae0
- Qualifier
- within 7 to 20 dayse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Downy mildew lesions carrying sporangia appeared within 7 to 20 days in leaves of Cucumis sativum, Cucumis melo”
Claim · model-written summary
Oospore inoculum caused sporulating downy mildew lesions in C. moschata.
- Taxon
- C. moschataCucurbita moschatae0
- Outcome
- Downy mildew lesions carrying sporangiae0
- Qualifier
- within 7 to 20 dayse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Downy mildew lesions carrying sporangia appeared within 7 to 20 days in leaves of Cucumis sativum, Cucumis melo, and C. moschata”
Claim · model-written summary
Oospore inoculum did not produce lesions in C. pepo.
- Taxon
- C. pepoCucurbita pepoe0
- Outcome
- not in C. pepoe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“but not in C. pepo or C. maxima”
Claim · model-written summary
Oospore inoculum did not produce lesions in C. maxima.
- Taxon
- C. maximaCucurbita maximae0
- Outcome
- not in C. pepo or C. maximae0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“but not in C. pepo or C. maxima”
Claim · model-written summary
Molecular and fungicide analyses confirmed recombinant F1 offspring.
Not reportedInteractor
Verbatim
e0“The recombinant origin of the F1 offspring isolates was confirmed by mefenoxam sensitivity tests, random amplified polymorphic DNA, and simple sequence repeat analyses.”
t0“Cucurbits”
Claim · model-written summary
Some F1 progeny lost pathogenicity to C. moschata or C. maxima.
- Taxon
- C. moschata or C. maximaCucurbita maximaCucurbita moschatae0
- Outcome
- lost pathogenicitye0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“F1 progeny isolates of some crosses lost pathogenicity to C. moschata or C. maxima”
Claim · model-written summary
Other F1 progeny gained pathogenicity to Luffa cylindrica or Citrullus lanatus.
Verbatim
e0“others gained pathogenicity to Luffa cylindrica or Citrullus lanatus”
Claim · model-written summary
F1 progeny showed altered fungicide sensitivity or host range relative to parents.
Verbatim
e0“F1 progeny isolates show altered sensitivity to fungicides or altered host range relative to their parents”
t0“Cucurbits”