My game progress has reached the later stage, with a population of 7500. I have cultivated nearly 40 arable lands to ensure food production, but I am very worried about crop diseases. If I am not careful, the diseases can seriously affect the harvest and crop rotation next year, and even infect neighboring fields. I hope to have a building similar to a mousetrap to eliminate crop diseases.
Welcome to the forum. 
Have a look at the FF wiki to see some crop rotations to help with that.