Any sign of sclerotia? Could be Sclerotinia sclerotiorum?

I agree with Jenny Denton. This pathogen is also called" the white mold " which can be observed clearly in the photo No2

Maybe this link is useful forma you.  http://www.potatodiseases.org/whitemold.html

If you couldfind sclerotia around the decaying crop there is no doubt it is sclerotinia sclerotiorum.

Regards

You think it is a pathogen? Maybe they reacted badly to an out-off tune violin warped by the heat and summer thunderstorms ;)  I like the combi in picture three.

To me it also looks like Sclerotinia although 35 degrees C is a bit hot for S. sclerotiorum. Did you also have a lot of humidity in the period leading up to the devastation? Either overhead irrigation or rain?

This looks like Erwinia bacterial infection : see : http://www.apsnet.org/edcenter/intropp/lessons/prokaryotes/Pages/Blacklegpotato.aspx

Thank you Jeremy. I have such hypothesis too. Recently, we have Pectobacterium strains that cause dry rot of plant stems in parenchyma, but does not penetrate xylem/phloem  vessels.  In this case, a large number of secondary pathogens makes analysis more complicated.

Hi Alex,

Just curious, has this area planted potato before, and did this happen before?

@Yuan-Yeu,

It is Black Sea cost, seeds were from different region, first observation of such disease, field was with crop rotation

It looks like fungal infection. Do incubate these into humid chamber. Politene bag sprayed by water. Then look on mycelium in microscope. 

Possibly, you can find spores and sporangia and other sturctures and can idetify the fungus or fungi.

It is killed by damage of vascular tissue inside the stem.

Yes, it looks like Sclerotinia white rot. If we take into account humid and hot weather on the coast. It is possible. I suuport the Jenny and Mohamed.

@Boris, thank you, so far we have not found any sclerotia, and fungi seems to play a minor role in plant death.  The speed of plants collapse has no explanation yet. Next field looks quite healthy.

Looks mores like a bacterial infection by the way the plant is destroyed. Fungal symptoms might have been a secondary infection. Could you give some more details on the first symptoms ? Was it a sudden complete withering of the foliage or did it start more localized on parts of the stem or leaves ?

What about tubers? Have they any symptoms? I think,  it is not soft rot or back leg.

Do isolate fungus on potato-dextrose agar (PDA) with streptomycine 40 mg/L ( to suppress bacteria)  in pure culture.

Do use the same PDA, however, without streptomycine for isolation of bacteria by  incubation at 30C (to suppress fungi). I case of fungal growth at such temperature

  add of fungicides and incubate at 28-30C.

Then. inoculate  fungus and bacteria onto healthy plant separately and wait for the same symptoms development to estimate causal agent according to the Koch's Postulates.

I agree with Jenny Denton and Mohamed Farag

@Boris, thank you for your suggestion. So  far, we confirmed presence of Dickeya solani, Pectobacterium sp. (it is not clear which species), Pseudomonas marginalis, Pseudomonas corrugata. Fungi are absent in inner tissues with symptoms of the disease.

I think the pathogen is white mould it is clear in pic.2 and if there is any small black sclerotium appear on the infected tissue in this case it is the sign of this pathogen 

Thank everybody for helpful ideas. Finally, I have isolated highly aggressive bacteria from those plants, which grows slowly on agar medium, but kills plants fast. Next step - to sequence 16S rRNA, because I have not seen such microorganism before.