Mango Stein

I am wondering why banana cultivations in greenhouses is so troublesome. At least it seems to be for others, I've not tried yet. Diseases seem to afflict the plants, which is odd because I thought the greenhouse is a very controlled and isolated environment (you could keep diseases out) but it seems they show up from somewhere (maybe the soil or food or water?)

Even installing fans for air movement does not help that much, since the air cannot be taken away, rather it just circulates and spreads the disease. Any thoughts much appreciated.