Dear Elaheh,

from the photos, they look like Downy mildew produced by the fungus Pseudoperonospora cubensis. However, you must be sure that it is caused by P. cubensis. For that you have to observe sporangia and sporangiophores of the pathogen. Sporangia and sporangiophores are most noticeable during humid conditions on the underside of the leaf. In severe infections, sporulation can also occur on the upper leaf surface.

Most likely yes but you have to make slides of the fungus to be sure.

Yes, downy mildew

Anthracnose, Colletotrichum orbiculare?

For identification of foliar pathogens, it can be very helpful to put the leaf tissue in a moist chamber. A simple moist chamber can be made with a plastic bag and a moist (not dripping wet) piece of paper towel. Place the leaf in the bag with the moist towel, but with the towel not in contact with the leaf. Incubate at an appropriate temperature for your organism of interest. Observe the tissue after 24-48 hours for pathogen development. For downy mildew, sporodochia form, particularly on the abaxial side of the leaf. For Colletotrichum, acervuli will form. Both can be seen with a good hand lens, but can be easier to identify with a microscope.

Some references that can help include http://www.omafra.gov.on.ca/english/crops/facts/10-065.htm, http://www.ct.gov/caes/lib/caes/documents/publications/fact_sheets/plant_pathology_and_ecology/anthracnose_of_cucumber_01-30-14.pdf, http://vegetablemdonline.ppath.cornell.edu/factsheets/Cucurbit_Anthracnose.htm

Note, downy mildew is not a fungus. It is an oomycete which is in a different kingdom than fungi.

Linda,

downy mildew forms sporophores with sporangia, not sporodochia😊.

The symptoms indicating wilt of some type

Downy mildew 😄

On your images fruit bodies are clearly visible. So it should not be too difficult, to figure out what pathogen is present on those leaves.

All you need is a compound microscope, a glass slide, mounting medium, and a glass cover slip and a probe . First, take the probe and carefully scrape off one or more of these tiny, pimple-like protrusions (fruit bodies). Then place them into a droplet of mounting medium (water may be used), and put a cover slip onto the droplet. Because you inserted a whole fruit body, you will most likely have to apply a little pressure on the cover slip right above where the specimen is located, in order to flatten it out a bit and allow the liquid (e.g. water, lactophenol, glycerin)to become distributed evenly under the cover slip. A little bit of heating the underside of the slide may also help with that task. It also does drive out any air bubbles that may be present.

Now you are ready to examine the specimen under the compound microscope.

You should be able to see spores and the spore-bearing structures, which will then give you the answer about what pathogen is causing those lesions.

i think its Downy mildew

Best Regard

Pictures are showing, its downy mildew (Erysiphe cichoracearum) of cucurbits

Dear Bahadur

Erysiphe cichoracearum cause Pwodery mildew

Best Regard

Hi dear, Elaheh Gerami ,,

According to the symtoms appeared in the images, it may be downy mildew.

I agree with Dr. Birol Akbaş, that Downy mildew on cucumber caused by the fungus Pseudoperonospora cubensis.

Hi dear Bahadur Singh Bamaniya,, I am sorry, I dont agree with you, that downy mildew on cucumber caused by Erysiphe cichoracearum , but it caused by Pseudoperonospora cubensis.

Good luck

Dear, Rekha Arya,, I dont agree with you, the symptoms which appeared in the pictures not represent wilt disease, it seems to be downy mildew on cucumber.

With my deeply apologizing.

Yes dear Honorable mate Prof. Mohammed,, Erysiphe cichoracearum and Sphaerotheca fuligiginea are consedered as causal agents of powdery mildew for cucurbits crops.

With highly appreciation.

See also for Colletotrichum orbiculare.

I guess I see downy mildew, prepare a slide and you should be seeing most likely, Pseudoperonospora cubensis

Downy mildew

http://www.apsnet.org/publications/imageresources/Pages/fi000258.aspx

In those pics. we see the sign( sporangium & sporangiospores) as a greyish spots under the leaves, that they belong to Pseudopronospora cubensis causal agent of the Downy mildew.

It depends on the host plant.

The causing agents of downey mildew on cucurbits is Pseudoperonospora cubensis

We could debate all day long, whether this fungus is a powdery or downy mildew. The easiest and fastest way to find out which pathogen is involved, is to prepare a microscopic slide and look at the fungal structures (conidiophores and conidia) under a compound microscope. The whole process takes only a few minutes.

@ Elaheh Gerami

Please have a look at the following links.

http://www.pestnet.org/fact_sheets/cucumber_downy_mildew_143.pdf

https://johnston.ces.ncsu.edu/wp-content/uploads/2013/07/Cucurbit_downy_mildew_eng_2013.pdf?fwd=no

Thanks!

Confused between powdery and downy mildew, any way you must check the causal agent under a microscope.. it is so simple