If copies were not made before cell division, than I believe both cells would only have half the proper amount of Chromosomes, as I don't think it's possible for one to have all chromosomes, and another cell to not have any.
The xylem transports water to the plant, so if the fungus blocks the xylem, the plant would eventually dehydrate, affecting the role of the xylem.
The phloem transports glucose, nutrients and such, therefore the fungi may try to steal the nutrients for the plant, affecting its role.
One reason why it's important to collect evidence from many fields is that the methodology of each field can introduce a bias through its measurement methods.
For example, imagine that we find out that our method of sequencing DNA is proven to be wrong - if we collect information from many different fields, then this will not invalidate the evolutionary theory, as evidence from other fields still supports it.
The guidelines suggest 3 to 5 servings of vegetables a day