There is cause and effect and they can be hard to separate.
People also attribute the decline of the mezzogiorno to organised crime.
In my view they have the cart in front of the horse. If the only game in town is organised crime, and the only one you can turn to for money or work is organised crime, it is bound to proliferate in desparate areas. Protection rackets deter outside investors.
The tiny incomes and massive taxes forced by balancing budgets on a massive national debt makes the black market proliferate (as it does elsewhere eg in Portugal everything done "in cash". That inevitably feeds organised crime even more. It is why salvini wants proper benefits, so that people are not forced into supporting black market ventures.
Becasue of the massive debts as well, payment terms are very slow. That too is a dissincentive for outside investment to set up companies there.
In summary: I think it is a consequence not a cause of the lack of growth in italy, so massive decline in real terms. People are desparate, so that the black market and crime flourishes. One of the responses of socialist governments to unemployment is beefing up labour laws. It can be the worst response that contributes to (but did not create) the problem.
The upshot of all of this, is if a country can make itself attractive to outside investment, offer sufficient unemployment benefit to allow labour laws to be relaxed then inward investment starts and a few years later revenues flow. But that requires a deficit budget. So they are trapped by the Euro.
It is easier to see the cause and effect in spain. There it was noticeable that increasing the burden of employment dramatically reduced the number of permanent jobs, and guarantee entrepreneurs dont want to set up there. For spain it was self inflicted. Restrictive labour is also the reason the french trains are the worst performing rail system on cost metrics in europe. Sadly Corbyn wants to copy them. We saw what happened when Macron wanted to dismantle some of the restrictive practices!
All of this can self regulate with floating currencies. The euro was the big mistake.
The reason they tried alignment, is the EU wanted a superstate. It got the chicken and egg back to front.
What I want is the euro to disappear. For a new EU to be a cooperative trading zone each with own currency, that allows the currency shifts to stop the positive feedback, that makes the rich richer and the poorer countries poorer. In such a zone the prices of german currency would rise until their products are so expensive, portugal can compete. And when it does it can lower its taxes!
Joint standards can be helpful so long as they are optional. Mandatory only where safety is involved. Joint programmes used only where necessary eg police sharing and security.
The idea of the superstate run from Brussels, rather than trading club was the problem.