There are much lesser chances of these symptoms to be viral in origin..

Dear Alex,

I am not a plant pathologist, but still the symptoms appear to me as caused by fungus, similar to that caused by Alternaria, or early blight. Less possibility of being symptoms of a viral infection.

SD

There are much lesser chances of these symptoms to be viral in origin..

it may be virual symptom. but  for better identification require type of leaf spot if any present. 

Some colleagues suggested results of Metribuzin  application before planting, or/and magnesium deficiency.

PS: the plant was infected by PVY+PVS

Саша, это грибы или просто сильный избыток влаги.

@Инна, спасибо! Но там легкая почва, высокое поле, влаги лишней точно не было. Грибы исключены - рядом стоят совсем зеленые растения (защита фунгицидами на высоте). Я поставил выделение на комплекс патогенов. Кажется, что заражение вирусами усиливает проявление дефицита магния (растения с симптомами в основном в начале поля, где обычно мало внесено удобрений).

I maybe herbicide toxicity,

Also, sulfur deficiency can cause this problem.

http://www.haifa-group.com/knowledge_center/crop_guides/potato/nutritional_requirements/

Dear  Alex Ignatov

I'm sure it is not a symptoms of a viral disease on Potato

Similar to this symptoms appear in one of my potato field experiment, When one of the workers used Fertilizer on the leaves instead of the soil

Best Regards

Houda KAWAS

Also

some leaves have necrotic and ring spot as symptoms of the infection with PVY..,   But the symptoms of yellowing with the color of the large brown spots associated with the fall is not a well-known isolation or that the plant reaction as excessive sensitivity or stimulate resistance.

Only tender leaves having the symptom. So it may be due to nutrient deficiency.

it seems like the chlorophyll degradation followed by spot necrosis, which may be due to the nutrients deficiency or even unbalanced sufficiency and sort of things. nonetheless, I also suspected to early blight or alternaria. it is also noteworthy to say that, sometimes, symptoms of disorders overlap and several disorders may be depicted by the same symptom, having that said, the symptoms in your picture is also feasible to be a viral disease symptom such as mosaic virus or etc, to make sure about it further investigation is needed.

Check for alternaria leaf blotch symptoms on lower side of the leaves !!

Appears to be nutrient deficiency - possibly potassium? - does not appear to be virus like especially where 30% of the plants are showing symptoms.

This is a typical symptom of magnesium deficiency.

Katarzyna,

thank you! We found also, that all symptomatic plants were infected by PYV, and symptomless plants nearby were virus-free. Is it expected that virus infection increase micro/macro-element deficiency?

Alex