Question

Grey top of the leaves   What could be the problem?!
From my expirience with avocado i had before, this leads to plants death. I dont know should I treta this with sometning??
Leaves turn grey, and then into brown. process starts on the top of the leaf, as shown on the picture.

Answer
Srba:
Based on your photo, I would suspect the origin of stress to a root or soil related issue.  I don't think this is a contagious/infectious situation.   I see the leaf tips affected first, followed by the entire leaf dying.  These types of scenarios are often related.  Sometimes the stress may be water related (either too much or not enough). More often than not it is usually an excess of moisture in the soil.  The soil needs to dry out. Sometimes fertilizer salts may build up in the soil and cause the leaf edges/tips to brown.  Flush water through the soil, let drain, then leave it alone until the soil become quite dry and the plant starts to wilt before watering.

Examine the roots.  They should be whitish or cream colored, not black.  Gently pull on some of the smaller roots.  If they break easily, this may also indicate a root problem too.

Regards

Steve