Litle kids do not care or have no comprehension of the words 'disgusting', 'spread disease', etc...
It is always best not to draw too much attention to unwanted behaviour, you will just get way more of it.
One Solution: Make it more trouble than it is worth. Have her go and wash her hands everytime she does it. Or sanitize her hands with gel, teach her to do it. Tell her she cannot touch anything until her hands are clean so that other people do not get her germs (don't go overboard with that, do not want to develop an OCD). But do not do this in anger or disgust, just matter-of-factly.
There is a SID the Science Kid episode about germs and how they transfer and what can happen. You can explain what boogers are (mucus trapping of dust in our nose so that it does not get into our lungs) and snot (mucus trapping of germs and fighter T-cells ~ something like that).
Bottom line, she has to want to change. Making her feel bad or guilty about normal behavior that just happens to be socially unacceptable can be damaging to her. And you really don't want to start her caring about what other kids think if you want to raise an individual that thinks for herself and is not a follower.
Find a way to make it fun and informative. Be tolerant and don't worry abot what other people think. Kids's pick noses, she is not the only one.