The history of fire blight is very important. In general, if you have had fire blight in the orchard over the past two years, then that is a high-risk situation. Fire blight in orchards around the region the previous year indicates moderate risk. To keep it simple, consider any risk in or near your orchard in the past two years to represent a significant risk. Tree susceptibility is determined by the cultivar and rootstock, the age, and the amount of new growth.
Young trees have relatively more young, new tissue.
Many new cultivar and rootstock are very susceptible to fire blight. In addition, young trees can't absorb much damage from fire blight. Last year's epidemic in southwest Michigan hit Gala and Fuji particularly hard. Honeycrisp and Ginger Gold area quite susceptible, as well as the more established Paula Red, Jersey Mac and Jonagold. Also bear in mind that English cider varieties, and some of the older heirloom varieties like Roxbury Russett are highly susceptible to blight.