The root system seems health, so I think the symptom on stem related to physiological disorder or phytotoxicity . For sure it is better to investigate the disorder pattern in the greenhouse, check the root and stem and look the cross section of those for vascular discoloration.          

 Hi Dr Fani

I have special thanks in advance for your cooperation. 

I think it is a physiological disorder, perhaps phytotoxicity of some nutrient. Examine the health of the root system and also the stem sure that there is no vascular discoloration.

Hi Mr Athayde 

I wanna know if this problem is not related to physiological disorder, 

Do you think what pathogen  is the most likely?

Dr Athayde, thank you for your answer.

It is not easy to identify a plant disease just by photo. In many cases similar symptoms due to different causal agents. It is necessary to have same extra information such as the extend of the disease problem the conditions under which it was appeared, it is connected with high or low temperactures etc. Any way such symptoms except of physiological or environmental reasons can be caused by the fungus Sclerotinia sclerotiorum. 

Hi,

I can not see the root system clearly in the picture but seems to me healthy. This kind of symptoms can be seen with abiotic factors like phytotoxicity. Did you sprayed anything on plant? Otherwise you need to cut a little part of stem just below the yellow part longitudinally or cross and can see if there is any kind of discoloration. If yes there is chance of Fusarium infection. Very prevalent in cucumber. Also you need to collect more information such as, did you over-watered them, or any kind of spray, added fertilizer and also is there any discoloration/ injury in roots you found. Hope it will help.

 Hi 

I hope you have a good time. I apologize for delay reply.

 I have a special thanks in advance for your cooperation with me.

Thanks a million.