The visibility of the sign is indeed an important element for challenging the fine, but you should know that it's not automatic to win the appeal just because of that. The law provides that the speed limit must be clearly posted, and if the sign was really covered by branches or made illegible by rain, you have a good chance of filing an appeal. However, the burden of proof falls on you: you'll need to prove that at that moment and in those conditions the sign wasn't visible. Photos of the location, witnesses, or even just a detailed account of how things were at the end of September could help.
The appeal must be filed within 60 days of receiving the fine notice, directed either to the prefect or to the justice of the peace (it depends on the amount and your province's local laws). It's worth getting help from a lawyer specializing in traffic law or at least consulting one to understand what the right course of action is in your specific case, because procedures vary. 173 euros isn't a small amount, and if you have serious reasons to contest it, it's worth trying instead of paying right away.
The weak point of your position is that 62 in a 50 is still a real violation, even if it's not serious. If the sign really wasn't visible, the appeal could result in the fine being cancelled, but it's not guaranteed: judges evaluate cases individually. If instead the sign was simply difficult to read but technically there, the appeal is riskier.